Dear Dinolist members,
I was trawling through the archives to catch up with some of the discussions in the last 12 months, and found Mike Taylor's question about big fish. I started writing a quick (!) reply, which quickly turned into a long post and has since evolved into a bit of a monster.
I guess good questions should be short but require long answers; but, hey, about half way through this tome (I've been putting it together, on and off, for two weeks) I started wondering whether I should get out more...I suppose the reason for the brain dump is that, for a while now, I've been fascinated by what it is to be a large, aquatic animal. It's become a big secondary question for me (fitting in with my thesis on Kronosaurus). Back in 1996, when Darren Naish was just an undergraduate who'd read too much, he and I put together a bit of a fun poster for the first Secondarily Marine Vertebrates conference in Poitiers, which proposed that there has been a remarkable consistency in the trophic structure of marine ecosystems since the middle Triassic, and offered the Recent Carribean as an ecological analogue to the Oxford Clay. I think that I can recognise in that the start of much of the musing and book reading that has resulted in the email now clogging up your Inbox.
At this stage of my dementia I find it hard to remember what it is that I've read, what has been explained to me very patiently in the pub, and what I think I've figured out for myself. I make no claims that any of the below is original - although I think that some of it might be - but it does represent a synthesis of sorts of the literature trawl I've been doing for the last eight years about large marine animals, and may for that reason be worth skimming through if you're interested in the marine stuff (or if you're bored with dinosaurs, or have given up trying to find a bootleg of Josh Smith on a camel). It might be worth turning into a paper at some later stage, but at this point it has taken up too much of my time so I'm going to send it on its way. I have done some quick checks for spellings and some basic re-edits, but it started as a letter to Mike and finished as a post to the list, so I apologise for inconsistencies in style, grammar, and the fact that it needs a good editor. It's also completely unreferenced, because my library is still in boxes.
Any comments, suggestions, feedback, and criticisms would be welcomed (but please make them constructive, I'm a sensitive soul). And Mickey, I promise never to post something this long again. Honest.
Darren, do you fancy going halves on a presentation (poster or otherwise) to the Cambridge SVPCA? I'm not going to be able to make it over.
By the way, Spain will win the World Cup 3-2 to Argentina. Raul will score the winning goal. Any takers?
Re: Big fish.
I couldn't resist replying to your email about big fish and big swimmers. Here are some thoughts...
>I would like to know where all the big swimmers are in the fossil record?
There are plenty. Some include (off the top of my head);
Fish genera shown in bold. Taxa with specimens attaining mass of 30,000 kg or more in red.
Upper Devonian, North America: _Dunkleosteus_, placoderm, 6 - 7 metres (2,000 - 3,000 kg)
Carboniferous, Scotland: _Rhizodus_, lobe-finned fish, 5 - 6 metres (1,500 - 2,500 kg)
Middle Triassic, North America: _Cymbospondylus_, ichthyosaur, 10 metres, (6,000 - 10,000 kg)
Late Triassic, North America: _Shonisaurus_, ichthyosaur, 15 metres, (15,000 - 20,000 kg)
Late Triassic, North America: 'Sikanni ichthyosaur', 23 metres, (25,000 - 50,000 kg)
Early Jurassic, Europe: _Temnodontosaurus_, ichthyosaur, 9 metres, (6,000 - 10,000 kg)
Mid Jurassic, Europe: _Leedsichthys_, bony fish, 15 - 30 metres, (15,000 - 50,000 kg)
Mid Jurassic, Europe: _Liopleurodon_, plesiosaur, 8 -10 metres, (8,000 10,000 kg)
? Mid Jurassic, Europe: pliosaur indet., plesiosaur, 10 - 15 metres, (10,000 - 20,000 kg)
Late Jurassic, Europe, Russia: _Pliosaurus_, plesiosaur, 8 -10 metres, (8,000 - 10,000 kg)
Late Jurassic, N. America: _Megalneusaurus_, plesiosaur, 8 - 10 metres (8,000 - 10,000 kg)
Early Cretaceous, Australia, South America: _Kronosaurus_, plesiosaur, 8 - 10 metres (8,000 - 10,000 kg)
Early Cretaceous, Australia: _Platypterygius_, ichthyosaur, 5 - 7 metres, (3,000 - 5,000 kg)
Early Cretaceous, Australia: lamniform indet., shark 5 - 7 metres (2,000 - 3,000 kg)
Early Cretaceous, Australia: _Cretachelone_, turtle, ?4 - 6 metres, (?3,000 - 5,000 kg)
Late Cretaceous, North America: _Xiphanctinus_, bony fish, 4 metres (500 - 1,000 kg)
Late Cretaceous, North America: _Brachauchenius_, plesiosaur, 5 - 8 metres, (3,000 - 8,000 kg)
Late Cretaceous, North America: _Cretoxyrhina_, shark, 5 - 7 metres (2,000 - 3,000 kg)
Late Cretaceous, Europe: _Mosasaurus_, mosasaur, 15 metres (8,000 - 10,000 kg)
Mid Tertiary, North America, North Africa: _Basilosaurus_, archaeocete whale, 15 - 20 metres, (10,000 - 20,000 kg)
Mid - Late Tertiary, global: _Carcharodon megalodon_, shark, 18 metres (40,000 - 50,000 kg)
Late Tertiary - Recent: _Carcharodon carcharias_, (white shark), 6 - 7 metres (2,000 - 3,000 kg)
?Mid Tertiary - Recent: _Cetorhinus_ (basking shark), 10 metres (5,000 kg)
?Mid Tertiary - Recent: _Rhincodon_ (whale shark), 12-?18 metres (20,000 - ?30,000 kg)
?? - Recent: _Manta_ (manta ray), 9 metres (wing span), (1,500 kg)
Cretaceous - Recent: _Acipenser_ (sturgeon), bony fish, 4 - 6 metres, (1,000 - 1,500 kg)
?? - Recent: _Thynnus_ (tuna), bony fish, 4 metres, (900 kg)
?? - Recent: _Mola_ (ocean sun fish), bony fish, 3 metres, (2,000 kg)
?? - Recent: _Makaira_ (marlin), bony fish, 3 metres, (800 kg)
Mid Tertiary - Recent: _Dermochelys_ (leatherback turtle), turtle, 2 metres, (900 kg)
Late Tertiary - Recent: _Crocodylus porosus_ (saltwater crocodile), eusuchian, 6 metres, (2,000 kg)
?? - Recent: _Mirounga_ (elephant seal), pinneped, 5 metres, (4,500 kg)
Late Tertiary - Recent: _Hydrodamalis_ (sea cow), sirenian, 8 metres (5,000 - 8,000 kg)
?? - Recent: _Orcinus_ (orca), odontocete whale, 8 - 10 metres, (7,000 - 10,000 kg)
?? - Recent: _Physeter_ (sperm whale), odontocete, 15 - 20 metres, (40,000 - 60,000 kg)
?? - Recent : _Eubalaena_(right whale), mysticete, 15 - 20 metres, 40,000 - 100,000 kg)
?? - Recent: _Balaena_(bowhead whale), mysticete, 15 - 20 metres, (50,000 - 110,000 kg)
?? - Recent: _ Eschrichtius_ (gray whale), mysticete, 12 - 15 metres, (20,000 - 35,000 kg)
?? - Recent: _Megaptera_ (humpback whale), mysticete, 15 - 18 metres, (30,000 - 50,000 kg)
?? - Recent: _Balaenoptera_ (rorquals), mysticete, 7 - 30 metres (7,000 - 160,000 kg)
Please note, I don't have any references with me at the moment, so the length and mass estimates should not be taken as anything like authoritative - I have only supplied these as guesstimates for the purposes of making some broad comparisons as part of an answer to your question. This is by no means a comprehensive list of every marine animal over 100 kg, either!
>It seems odd that in the 300+ million years of vert evolution, we know of no swimmer larger than the extant whales....
>It's a Well Known Fact that it's much easier to be big in water than on land, right?...
>Then how is it that the largest prehistoric swimmers we know of are the big pliosaurs (70 tons?)...
>And -- related -- does anyone have any ideas where there has never been (to my knowledge) a really big fish? I mean,
> pah, whale sharks? Don't make me laugh! Is there something about the fishy body-plan that makes them unscaleable
> past thirty tons or so?
You've raised a few different points here, but they're inter-related, so I'll try to answer them together.
Firstly, my own opinion is that it is no coincidence that the largest animals of all time are with us now - I suspect that this is the first time since the vertebrates appeared where the right conditions are present for truly enormous marine animals.
Giants then and now.
I also have to say that I don't agree that whale sharks aren't giants...and I don't regard anything under 30 tonnes as being unimpressive. If you are to take the view that anything under 30,000 kg is small then you are left with the fact that only eleven families of animal, from four classes, have produced species which have got to that particular threshold.
Three of those families are terrestrial. Only two families of dinosaurs - the brachiosaurids and the titanosaurids - included species which were, by your definition, large. The rhinos, of course, are the only other terrestrial group that managed this, although they only managed one species - Indricotherium - that just pushed the scales up to 30,000kg.
The remaining eight families are marine; four families of mammal (all, of course, whales), a little known bony fish (the giant planktonivorous teleost Leedsichthys, known from some scrappy but nevertheless impressive Oxford Clay specimens), and two families of modern neoselachian sharks. The pointed failure of any marine reptiles to get over the 30 tonne mark has been recently rectified by Betsy Nicholls' discovery of the Sikanni ichthyosaur - although no detailed information is available as yet, the reported size of 23 metres indicates a probable mass somewhere between 30 and 40 tonnes, depending on its exact proportions. Presuming that it was a shastisaurid (same family as the previous record holding marine reptile, Shonisaurus), it would have been quite long and thin - hence the smaller mass compared to the slightly shorter Carcharodon megalodon.
Leedsichthysmust have been an amazing animal, and we eagerly await the discovery of more fossils that will shed more light on it. Estimates of its size vary greatly, between 15 and 30 metres (??15 to 50 tonnes), due to the fragmentary nature of the remains. In many ways it is the missing large planktonivore of the Mesozoic, offering a tantalising insight into a group of basal teleosts that may have occupied the 'baleen whale niche' during the age of giant terrestrial reptiles.
Your original question is correct in noting that there is a dearth of fossil marine taxa that have matched the sizes of the modern giant whales, and apart from Leedsichthys there is only one species of extinct fish that weighed more than 30 tonnes. That animal just happens to be my vote for the scariest predator of all time, the recently extinct big toothed white shark Carcharodon megalodon. At an estimated maximum length of 15 to 18 metres, this giant lamnid would have weighed between 35 and 50 tonnes! As not many people consider its surviving three tonne relative to be especially puny, that is a truly amazing size for a predatory shark.
The living whale shark Rhincodon is the largest fish alive, but may not even belong on this list. The largest specimen on record was a 12 metre individual which weighed 20,400 kg. There are, however, a number of reports of animals reaching 15 to 18 metres; I would expect individuals of this size to weigh between 30 and 50 tonnes.
The other four families containing >30 tonne giants are all modern whales. The sperm whale is the only member of its family and the only toothed whale to get over the 10 tonne mark. In contrast most baleen whale species are well over 30 tonnes - three of the four species of the right whales, family Balaenidae, produce specimens that are well over 30 tonnes and in fact get up to 100 tonnes in weight. Large individuals of grey whale (the only species in the family Eschrichtiidae) just get over 30 tonnes, but the final family, Balaenopteridae, is replete with giant species. Containing six species (one in the genus Megaptera and five in the genus Balaenoptera), three of these species (the humpback, the fin, and the blue) exceed 30 tonnes.
Let's call any animal over 30 tonnes a giant. Of the twelve species of giant marine vertebrates, nine species (one species of shark, and eight species in four families of whale) are extant, while three species in three families (one shark, one bony fish, and a reptile) are extinct. So there are more giant marine taxa alive now - or less taxa in the fossil record - then you would expect if giantism was evenly distributed through time.
However, as I stated above, a threshold of thirty tonnes might not be a biologically relevant threshold to use when discussing large size. More commonly, biologists use a set of size classes based upon a logarithmic scale. For example, you might class animals between 100 kg and 1 tonne as being 'large'; between 1 and 10 tonnes as 'big', and between 10 and 100 tonnes as 'huge'. I suppose you could call anything above 100 tonnes a freak...
Large, big, and huge animals through time...
There's both a temporal pattern and a taxonomic pattern to the history of large body sizes. Taking the temporal perspective first, we notice that there are very few marine Palaeozoic animals that got above 'large'. The two listed above - the placoderm Dunkleosteus and the crossopterygian Rhizodus - are not typical of Palaeozoic marine animals. Interestingly, with the sole exception of Mesosaurus, there were no aquatic tetrapods in the Palaeozoic.
During the Triassic the reptiles invaded the marine realm. The first group to become completely aquatic was probably the ichthyosaurs, and as early as the middle Triassic we see, in Cymbospondylus, a marine animal much larger than any of the fish that had preceded it and which pushes the 10 tonne mark for the first time. Soon after, in the late Triassic, Shonisaurus and the as yet unnamed Sikanni ichthyosaur are well into the 'huge' category, at 15-20 and 25-50 tonnes respectively. So it takes the advent of air-breathing marine animals before we finally see 'huge' swimming creatures.
In the Jurassic, however, the pattern is different. With the possible exception of Temnodontosaurus, the ichthyosaurs fail to produce any further huge species, and although the other prevalent marine reptile group - the plesiosaurs - have plenty of animals at the upper end of 'big' (e.g. Liopleurodon, Pliosaurus), they don't get really huge. Again, there is a possible exception to this - the rumoured 'mega pliosaur' which inspired the 'hugely' inflated estimates for the size of Liopleurodon in Walking With Dinosaurs. I've discussed this at length before (see the archives of the Dinosaur Mailing List, February 2001, at http://www.cmnh.org/dinoarch/2001Feb/msg00891.html ); if such animal does exist then it is unlikely to be any larger than 15 metres, which would make it 20 - 25 tonnes (definitely not 70 tonnes!) The teleost Leedsichthys was, in contrast, most definitely 'huge', and may well have been the only animal of this size class in the Jurassic.
There is a similar story in the Cretaceous. Two taxa - the pliosaur Kronosaurus and the mosasaur Mosasaurus - push the upper end of 'big', but there are no 'huge' marine animals during the entire period. Things change shortly after the advent of the Tertiary and the invasion of the ocean by the mammals, with Basilosaurus tipping the scales past any of the Jurassic and Cretaceous marine reptiles and becoming the first huge marine tetrapod since the Triassic ichthyosaurs. After that being 'huge' becomes more popular - Carcharodon megalodon turns up in the Miocene, and then there is the Late Tertiary radiation of baleen whales, of which nine species from a total of eleven alive today are genuinely huge (and let's not forget that three of those are veritable freaks!).
The overall pattern, then, is one of increasing maximum sizes through time. Aquatic vertebrates are small until the early Devonian, when 'large' species start to appear. During the Late Devonian and to the Carboniferous there are 'big' forms, but these are the exception rather than the rule. In the Triassic, with the advent of air-breathing marine animals, there are 'huge' animals for the first time, but throughout the Mesozoic, the time of the real giants in the terrestrial systems, it is very rare for marine animals to be 'huge', although there are a large number of 'big' taxa. It takes until the advent of the Cainozoic, and the evolution of the marine mammals, for 'huge' to become relatively common, and for the first (and thus far only) freaks to appear. I guess this is a wonderful affirmation of Cope's rule - at the taxonomic level of sub-phylum!
Taxonomic patterns of large size.
Again, this pattern is fairly obvious. If we, for the purposes of this discussion, divide the vertebrates into ten major classes, as follows;
Agnathans (jawless fishes)
Placoderms (armoured fishes)
Acanthodians (spiny finned fishes)
Sarcopterygians (lobe finned fishes)
Actinopterygians (ray finned fishes)
Chondrichthyians (sharks and rays)
then we can look at how each fares in our size classes.
Jawless fishes, acanthodians, amphibians, and birds have produced no marine organisms that get into the 'large' class (100 > x > 1000 kg).
Placoderms only produced one group - the arthrodires - that included large species, and a small number of these got big. But then, they weren't around for very long, so they didn't really get a fair go.
Sarcopterygians produced plenty of large species (including the living coelacanth, which weighs in at about 100 kg), but only produced one species of really big animal (and very scary Rhizodus must have been too).
Actinopterygians are the most diverse of these classes, by a long way, and yet ray finned fishes above 100 kg are very rare. Some herrings, groupers, jewfishes, catfish, gar-pikes, osteoglossids, perchichthyids, and scrombrids get above 100 kg, but only the sturgeons and the mola get into the 'big' size class. Similarly, the fossil record is short on ray-fins of one tonne or more, with the notable exception of Leedsichthys and the honourary mention of Xiphanctinus.
The cartilaginous fishes, the sharks and rays, have a fossil record stretching back to the Devonian, but have only recently (since the Cretaceous) started to produce species that get into the 'big' size class. The exception to this are the six-gilled and seven-gilled sharks (e.g. Hexanchus, Somniosus), which have a fossil record stretching back to the early Mesozoic and which reach reported sizes of 5 - 7 metres (?1000 - 2000 kg). The arrival of the lamniform sharks in the Cretaceous seems to have spurred on the achievement of large sizes in sharks, with 'big' lamniforms appearing in the Lower Cretaceous and remaining common ever since. As we have noted above, one of these became huge. Big lamniforms include predators (e.g. Carcharodon) and planktonivores (Cetorhinus). Other shark orders include species that have got to be big - the ground sharks (Carcharhiniformes) include the tiger shark Galeocerdo and the hammerhead sharks Sphyrna, which each get to 6 metres. The rays also have a ‘big’ species, the manta. The only modern ‘huge’ shark - the whale shark (Rhincodon) - belongs to the Orectolobiformes (carpet sharks).
That leaves the reptiles and mammals. Of the marine reptiles, there are many plenty of 'big' species but few that got to be 'huge', although there are a number of pliosaurs and at least one mosasaur that reached the threshold between these size classes. The huge reptiles - a few Triassic ichthyosaurs and one possible Jurassic pliosaur - are rare. In contrast, the marine mammals, though only recently evolved, include many huge taxa and have the largest number of huge animals of any vertebrate class. As you so rightly ask, why should this be?
Physiological constraints on large size?
As first year biology courses go to great lengths to hammer into students, many aspects of an animal's physiology and anatomy are profoundly affected by the size of the animal. Specialised organs such as lungs, heart, livers, kidneys, and guts are necessary for any animal larger than a flatworm, and this is because of two factors that change with increasing scale. Firstly, the surface area : volume ratio decreases rapidly with increasing body size - hence the need for lungs/gills, livers, kidneys, and guts to increase the surface area available for gas exchange, detoxification, osmoregulation, and digestion/absorption respectively. Additionally the physical distance from the insides of the animal to the epidermis, or to the specialised 'surface area increasing' organs, increases linearly with body size, meaning that most tissues will be too far away from sources of oxygen, water, and nutrients etc., and necessitating some sort of circulatory system.
And as most people on the dinosaur mailing list would be acutely aware, thermal physiology and reproductive physiology are also affected by body size. But there is one aspect of an animal's physiology, so famously fundamental for terrestrial organisms that become large, that has a minimal constraining effect on marine animals - the skeleton. As you point out, it is a Well Known Fact that the aquatic environment results in very few skeletal constraints upon supporting large bodies.
Of course, all of the animals in our list are so much bigger than a flatworm that, whether they are 100 kg or 100 tonnes, they are just about equally challenged with respect to gas exchange, osmoregulation, etc., and most use the same set of specialised organs to deal with these challenges. But if the biomechanical capacity of the skeleton is not the major factor constraining marine animals from reaching large body sizes - as it is with terrestrial organisms - then could it be that the explanation for the taxonomic pattern of large body size lies in subtle but significant scaling effects related to the other aspects of physiology? Perhaps the different systems that the respective vertebrate classes use for circulation or osmoregulation offer some explanation as to why some groups of animals are constrained from reaching large body sizes. Let's consider, in turn; skeleton, locomotion, circulatory system, reproduction, thermal physiology, gas exchange, detoxification, osmoregulation, and digestion.
Skeleton: the fact that reptiles, mammals, bony fish, and sharks have all produced gigantic species doesn't suggest that the muscles and connective tissues of different vertebrates become inefficient at large sizes. Similarly, there doesn't seem to be a problem with using bone, or even cartilage, to build the support structures.
Part of the reason that the skeleton of a large animal is not particularly challenged in water is that most body tissues are about the same density as water, and thus most animals are near to neutrally buoyant in water. Bone is of course an exception to this - it is usually significantly denser than water - and many large marine animals have very low bone density, resulting in them being almost perfectly neutrally buoyant. This strategy does, however, have consequences for the strength of the bone - not a problem whilst in water, but potentially very important if the animal's ecology or lifecycle necessitates occasional forays onto the land.
Thus, whilst the vertebrate skeleton doesn't seem to impose any overall limits upon body size in water, the interactions between the skeleton, locomotion, feeding ecology, and reproductive ecology may impose constraints on an animal's maximum size. For example, an animal that reproduces out of water (such as a turtle, crocodile, or pinniped – see discussion on reproduction below) will require a skeleton capable of supporting and moving it when it is on the land; this will constrain its opportunities for varying bone structure and density, and hence the buoyancy and swimming strategies available to it when it is in the water. It is interesting to note that no 'amphibious' animal alive today exceeds five tonnes (the biggest is the elephant seal Mirounga), and five tonnes is also the upper limit for the extinct, huge turtles (Cretachelone, Archelon, Protostega). Possible exceptions to this limit may be the big Triassic phytosaur Rutiodon (approx. 9 metres, 4,000 - 6,000 kg), and the huge Cretaceous crocs such as Deinosuchus (approx. 12 metres, 10,000 - 14,000 kg); but then these archosaurs have an extra support structure to their skeleton, namely the osteoderms of the paravertebral shield, and in any case may not have been able to support the ‘high walk’ seen in modern crocs that are capable of extensive terrestrial locomotion (see Steve Salisbury's excellent work on this subject).
Over-predicting from skeletal remains – a cautionary tale. The capacity of various skeletal types to support large bodies is sometimes surprising, and statements about the ability of extinct species’ skeletons to support their body weight need to be made with caution. The elephant seal Mirounga is a case in point. Large male elephant seals are huge animals that weigh almost 5 tonnes; like all seals, their limbs are highly specialised for swimming, and when on the land the entire weigh of the animal is supported by the torso. When moving on land, the seals engage in a gait in which they resemble an obese caterpillar. It may not look very elegant, but it is surprisingly effective – bull elephant seals are able to put on a real turn of ‘waddle’ when they need to, and are reputed to be able to outpace humans on a beach! The sight of a bull elephant seal steaming along at full tilt - on its stomach - to defend his part of the beach is surely one of the more intriguing experiences a zoologist can hope for.
When I first started my PhD I remember discussing whether or the plesiosaur body plan would be capable of any form of terrestrial locomotion (in connection with that old chestnut about their probable reproductive mode, of course. I tried to imagine finding the skeleton of a big Mirounga on a desolate beach in somewhere, and being able to predict - without knowing from having observed the living animals - that this animal, so obviously specialised in its skeleton for a life in the ocean, would have been capable of outsprinting me on the beach. A couple of years later I was able to see a skeletal display of elephant seals, and searched through the glass of the display case as best I could for anything in the skeleton that made a concession to terrestrial living. All I could see was a standard seal skeleton magnified a few dozen times – nothing obvious in the backbone, ribcage, or girdles that you could point to and say, wisely, "ah yes, that is a specialised anatomical feature that allows a pinniped the size of a pachyderm to move rapidly on land".
I really think that if we knew of elephant seals only from skeletal remains, the wisdom would be that the large adults were incapable of leaving the water. We may have a whole series of theories to explain this – if we’d been able to work out that the females were far smaller than the males (which they are), we might have imagined that only the females were able to come onto land to give birth, with mating taking place in the water. Or perhaps we might think that the young were precocious and were born at sea. If we’d never seen any seal alive we might think that, unlike their extant pinniped relatives (sea lions and walrus), seals gave birth in the water – and we would have argued this because the largest seals were simply too big to have come onto land. Of course we are lucky enough to be able to observe seals in the flesh, and so can reserve our wanton speculations to totally extinct groups such as plesiosaurs, etc…
Locomotion: Swimming: I can't really think of any constraints that the biomechanics of the different swimming modes, whether axial or paraxial, might impose upon body size. On the contrary, it seems that the bigger you are in the water the easier it is to go very fast. Some of the fastest animals in the ocean include the 10 tonne orca (Orcinus orca; ?60 km/h), the 30 tonne sei whale (Balaenoptera borealis; 60 - 70 km/h), and even the 150 tonne blue (B. musculus; 45 - 55 km/h). Note, however, that the very fastest are probably all in the 100 – 1000 kg ‘large’ category - marlins, makos, tuna, and Commerson's dolphin (Cephalorhynchus), for example.
Diving in air breathing animals, however, is an aspect of aquatic locomotion that is strongly affected by body size. Essentially, larger animals are able to dive for longer (and hence deeper) - the deepest and longest diver is the sperm whale, and the records for diving in seals and turtles are held by the largest members of these groups (elephant seals and leatherbacks respectively). See the discussion below on diving performance for an consideration of why this may be so.
Terrestrial locomotion: with animals whose skeletons are highly specialised towards an aquatic existence, there is probably going to be a threshold of size over which terrestrial locomotion will be impossible. Yet size alone may not be the most important factor – it is possible that locomotory gaits can be a factor in determining whether or not large animals are going to be capable of movement on land.
Most people would agree that the large whales are simply too big to be able to move on land. And yet it is also true that all cetaceans, large and small, are incapable of terrestrial locomotion and will die if they get stranded. (The only exception to this is the orca, which is, in very limited circumstances, capable of partially beaching itself and returning to the water – observation suggests, however, that this is a highly specialised behaviour that that is only performed in a small number of orca populations, that requires a great deal of training and practice, and that is culturally transmitted within these populations). So it is not primarily the enormous size of sperm whales that kills them when they get beached – it is the fact that they are a modern cetacean and as such lack any capability for terrestrial locomotion.
If we look at all the modern marine tetrapods that are capable of terrestrial locomotion, we see that they all have one thing in common - gait. Pinnipeds, penguins, and turtles - which are capable of moving on land - all swim using paraxial (limb based) gaits, whilst cetaceans and sirenians - which are incapable of terrestrial locomotion - swim using only an axial (tail based) gait.
Might this observation allow us to make general predictions about the ability of fossil taxa to move on the land? Can reconstructing the gait used for aquatic locomotion tell us that plesiosaurs were capable of sufficient terrestrial locomotion to lay eggs or to ‘pup’ on land, whilst mosasaurs were unable to come onto land and must therefore have reproduced entirely in the water? Or is size more important than gait?
Circulatory system: One might suppose that there is the potential for scaling problems with this aspect of an animal's physiology - imagine having to pump thousands of litres of blood around a 30 metre animal! - but in reality the vertebrate heart seems to be quite capable of pumping blood around very large organisms. The various classes of vertebrate do have slightly different heart anatomies, but since six of the ten major classes of vertebrate have produced big animals, and four of these have produced huge ones, this doesn't appear to be a major problem.
Of course, air breathing marine tetrapods need to be have a cardiovascular system that can keep pulmonary blood separated from the rest of the system during diving in order to maximise dive times. This does require a heart of a certain level of complexity; either four chambered, as in mammals, or with special valves, such as in crocs. I don't think that this has a direct interaction with constraints upon body size, however.
Reproduction: Potentially, this can exercise a strong constraint on body size if the life-cycle necessitates a return to land, or even navigation of near shore or estuarine aquatic environments. As discussed above (see 'skeleton') the need of many modern groups - crocodiles, turtles, birds, seals, walrus, and seal-lions - to return to land to breed has probably prevented any member of these groups from becoming 'huge' (again, with the possible exception of Deinosuchus and the other ‘supercrocs’). This constraint has even been used to argue that groups such as plesiosaurs and mosasaurs, for which we have little or no direct evidence of reproductive ecology, must have been able to reproduce entirely in the water because they include taxa too large to have been able to return to the land to breed (see for example, Darren Naish’s discussion of this athttp://www.cmnh.org/dinoarch/1998Jun/msg00646.html ). Certainly, groups such as the ichthyosaurs and the whales were only able to attain huge sizes because they had solved this constraint by giving birth to live young in the water.
In theory, though, there would be ways that reproductively-land-bound species could still reach ‘huge’ body sizes. The most obvious is if there is a large size difference between males and females – males could be very large if the females remained small enough (i.e. under 5 tonnes?) to move around on land.
Less obvious, but fun to consider, is the possibility that the eggs are laid underwater. Before I get howled down in protest, there is one animal (Australasian, of course!) that does this – the northern snake-necked turtle Chelodina rugosus. In this species the eggs are laid in the muddy bottom of water holes during the wet season – the young hatch out during the dry season, when the water hole has dried up. The eggs are apparently able to survive because when they are laid they are in a state of metabolic suspension, which lasts for as long as they are in water. Brings to mind a wonderful image of large gravid female plesiosaurs congregating on certain beaches on a king tide, trying to discard their eggs in the shallowest water they can before leaving the eggs to hatch a fortnight later…
The final option open to an egg laying big marine animal is very unlikely – the Pacific salmon tactic of reproducing only once, committing suicide in one glorious reproductive extravaganza! Needless to say, this would be a very unusual breeding tactic for a large animal.
Thermal physiology: When most people think of the relationship between body size and thermal physiology they are usually considering the 'well known fact' that large body size strongly facilitates homeothermy and tachy-metabolic endothermy. In other words, it is much easier for large animal to be 'warm blooded', and there are some clear examples of this - the leatherback turtle is the largest extant turtle and is the only warm blooded member of its group, allowing it to exploit temperate waters too cold for other sea turtles; white sharks, makos, and porbeagles are large sharks which are warm blooded, and can also exploit far colder waters than many other sharks; similarly tuna, marlin, and swordfish are all large members of their families, and are all warm blooded to some degree.
There are, however, some constraints upon thermal physiology that are imposed by large body size, even though they may not be as well known as the converse relationship. Firstly, large, warm-blooded animals tend to have large appetites and must be able to find enough food to fuel themselves - this is a significant issue for all large animals, terrestrial or marine (see below). Secondly, large warm-blooded animals may in fact produce too much heat and might not be able to dump heat rapidly enough in certain environments. Although water has much higher heat conductivity than air, this can still become a problem for really huge marine animals, and may be one factor in keeping blue whales away from tropical waters in the summer time and leaving these environments to other, cold blooded planktonivores such as whale sharks and manta rays.
The overheating issue is probably also part of the explanation for the enormous dorsal fin of adult male killer whales. In Orcinus the males are bigger than the females, but the dorsal is proportionately very large in males, and it was widely assumed that the fin was some indicator of male maturity / sexiness. However there is a story of an adult male at an aquarium in North America whose dorsal fin had to be amputated for some reason - this animal then had real problems trying to thermoregulate and ended up having to be kept in much colder water than usual. It’s only anecdotal , but it does suggest that the dorsal fin of orcas may play a similar thermoregulatory role to the large ears of African elephants.
Gas exchange: As I have hinted at above, I think that this may be the most important physiological trait constraining large body size in fish. Large bodies need large amounts of oxygen, especially if the species in question is endothermic. Water holds a maximum of approximately 4% dissolved oxygen by volume - air, however, has 20%. This large difference probably more than makes up for the fact that the gill is a more efficient basic design for a gas exchange organ. Anyway, I don't think that it is a coincidence that the first huge marine animals (i.e. >10 tonnes) were air breathing reptiles, and that the vast majority (18 genera out of the 23 listed above) of animals that reached 10 tonnes or more were air breathers. Air breathers also out number water breathers in the 'big' class (between 1 and 10 tonnes) - even one of the earliest big animals, the lobe finned fish Rhizodus, would have had the option of breathing air to fuel its activities thanks to its dual lung/gills system.
Why should lungs be better than gills for large aquatic animals? It is possible to construct a number of arguments, of varying complexity, which state that air breathers are more likely to become large. For example;
(a) large animals require lots of oxygen; air breathing is more effective at taking up oxygen than water breathing.
(b) large animals require more food; to obtain larger amounts of food requires higher levels of activity; higher levels of activity require more oxygen; air breathing is more effective at taking up oxygen than water breathing.
(c) large animals require more food; to obtain larger amounts of food requires higher levels of activity; tachy-metabolic animals are better able to sustain higher levels of activity; tachy-metabolism requires higher levels of oxygen; air breathing is more effective at taking up oxygen than water breathing.
In each of these there is the likelihood that cause and effect are confusingly tangled, but you get the jist - the array of physiological processes that can potentially link large body size to air breathing is complex indeed. Something to think about...
Conversely, large animals that do rely upon gills for gas exchange may find themselves in a Catch 22 situation. Consider this; gills become more efficient in (a) colder waters, where the oxygen carrying capacity of water is increased, and (b) at high body speeds, where the gill can work higher volumes of water per unit of time. Higher body speeds are facilitated by larger body sizes, and larger body sizes also permit an animal to feed in colder waters whilst maintaining a given thermoregulatary strategy. Colder waters are also more productive, with higher average levels of food species. High levels of food and large body size permit the development of endothermic physiologies; endothermic physiologies require higher levels of oxygen uptake, but allow the higher levels of activity that increase gill efficiency, and also permit operation in still colder water....It is quite possible for water breathers that are exploiting new ecological opportunities through an increase in body size to become locked in a feedback loop that results in them specialising upon temperate habitats. It may be that great white sharks and bluefin tuna, for example, are end results of that feedback loop. The question then remains, does the degree of specialisation that these water breathers have undergone in becoming large make them more vulnerable, in macro-evolutionary terms, to extinction than the air breathers, in which the efficiency of the lungs is not directly linked to cruising speed or water temperature and are thus not forced into a feedback cycle that results in high degrees of ecological specialisation? Perhaps the prevalence of air breathing large marine animals is, to some degree, a macro-evolutionary pattern resulting from higher extinction rates in large water breathing taxa (as well as being simply the result of a physiological constraint that favours the development of large body size in air breathing species in the first place)?
I realise that my attempt at explaining this relationship between gas exchange and large size may not be all that clear, but that reflects my own uncertainty about the precise lines of cause and effect in this case. What persuades me that gas exchange (i.e. air breathing vs. water breathing) is important is the dominance that air breathers have on large body sizes, both today and in the fossil record.
Detoxification: I think it unlikely that the physiology of the liver in different vertebrate classes constrains them from becoming very large, but the following three points are worth noting;
(a) predators concentrate any toxins present in the food chain within their own tissues. The larger the predator, the stronger this effect will be. This may impose a restriction upon the maximum size of predators in ecosystems that are strongly based upon certain types of toxic diatom or dinoflagellate, or at least require that large predator in these ecosystems develops the means to deal with these toxins. Might
also be important for large planktonivores.
(b) industrial pollutants that enter the marine food chains will be concentrated in the tissues of apex predators - this may make large predators especially vulnerable to human pollution.
(c) people fortunate enough to be required to restrain large crocs as part of their job have notice that large specimens are especially susceptible to lacticacidosis, often leading to the death of the croc. Essentially, the struggling large croc builds up so much lactic acid in its tissues that it dies of lactate poisoning. It seems that this susceptibility is a result of crocs opting for a slower, more anaerobic metabolism in order to obtain the longer dive times required for their 'sit and wait' hunting strategy. Could potentially constrain crocs, or any other anaerobic, long diving animal, to sizes below a certain threshold. Maybe.
Osmoregulation: Apart from possibly explaining why there are no large marine frogs, I don't think that osmoregulation has a profound effect on body size.
Digestion: Again, I can't think of any obvious constraints that digestive physiology might have upon the development of large body sizes. In fact, I would expect the digestive processes to be helped along by the mass homeothermy enjoyed by large animals. There is an interaction, in large white sharks, between maintaining the gut tissues, the energy budget of the shark, the preference for prey with a high blubber content, and feeding in temperate waters, but I don’t think that this has any causal relationship with large body size. (But it is interesting that these sharks apparently ‘switch off’ their digestive tract in between their foraging seasons at pinniped ‘pupping’ beaches – apparently it costs the sharks too much, energetically, to maintain the gut when blubbery mammals are not available to eat. I seem to remember that pythons do a similar thing – switching off the gut, that is, not eating seals!)
To summarise, then; the main physiological processes which may constrain a taxa to remaining small are;
1. Method of gas exchange - water breathers very rarely get over 10 tonnes in mass. In contrast, a large number - both absolutely and proportionately (there have always been many more water breathers than air breathers) - of air breathers have become larger than 10 tonnes.
2. Life history - any animal that needs to leave the water for some reason (usually reproduction) will be constrained from becoming very large. The upper limit for animals with such a life cycle may be around 5 - 10 tonnes. All taxa that have achieved 30 tonnes or more are known to have been able to reproduce in the water.
Therefore the largest animals in the water should be tetrapods (as air breathers) that are able to reproduce in the water - which is what we see in nature. Yes, I realise that there is an element of tautology in this statement ("large swimmers are mostly air breathers, therefore to be a large swimmer you need to be able to breathe air"), but it’ll do for now.
The ecology of large size.
As every ecologist should know, an organism's size is probably the most important factor in determining the ecological opportunities that will be available to it. So many constraints / opportunities rely on body size - it is so obvious that it is sometimes overlooked. Having considered what aspects of a taxon's physiology might constrain it from being able to become large, it is important to consider what ecological advantages might result from being big. After all, there is no point in being large just for the sake of it - there has to be some ecological (and hence evolutionary) advantage in a species becoming a bit bigger in order for large body sizes to result. Just what sort of ecological advantages can large size offer to marine animals?
Maximum swimming speed: As we mentioned above, large animals tend to be able to swim faster. It is probable that the top ten fastest swimmers are all between 100 and 1000 kg in mass, and there many of the >1000 kg animals wouldn't be far off the pace. Note, however, that this speed comes at a cost to agility - smaller animals will, as a rule, be more agile than larger ones.
Extending the pattern observed in living animals to fossil taxa, we might predict that the fastest swimmers in ancient seas would have been those animals between 100 and 1000 kg whose anatomies suggest fast swimming (i.e. tunniform shape). Examples might include the thunnasurian ichthyosaurs (Stenopterygius, Ichthyosaurus), the smaller pliosaurs (Archeonectus, Peloneustes, Leptocleidus), the large ichthyodectimorphs (Xiphanctinus), the smaller mosasurs (Clidastes, Platecarpus), various lamnids (Cretolamna, Isurus, Caracharodon), and the smaller archaeocetes and odontocetes.
Diving performance (for air breathers): Larger animals are better divers. Consider two related species (similar physiologies, behaviours, etc) which differ principally in their body size. Because lung and blood volume scale 1:1 with body mass, the large animal can store as much oxygen per kg body weight as its smaller relative. However larger animals tend to have lower specific metabolic rates than their smaller relatives - thus the large animal's oxygen reserves will last much longer during a dive than the smaller animal's, allowing for longer and/or deeper dives.
The champion divers in modern times are the sperm whale, the elephant seal, the leatherback turtle, and the beaked whales - each the largest members of their respective groups. In ancient seas we might expect that the best divers would have been the larger ichthyosaurs (Cymbospondylus, Shonisaurus, the 'Sikanni ichthyosaur', Temnodontosaurus, Leptopterygius, Ophthalmosaurus, and Platypterygius), the big pliosaurs (Liopleurodon, Pliosaurus, Kronosaurus), the larger mosasaurs (Tylosaurus, Plotosaurus, Mosasaurus), the big turtles (Cretachelone, Archelon) and the large archaeocete (Basilosaurus). Of these, taking into account jaw structure and tooth shape, it's probable that the big ichthyosaurs and turtles (and perhaps some of the large mosasaurs) were specialist deep divers, foraging on deep living squid and jellyfish.
Thermal physiology: The advantages of large size for homeothermic / endothermic physiologies should be so familiar to this list that they should not need too much emphasis. Basically, it's very costly to be 'warm blooded' (whichever physiological strategy you might use to be so) if you're not big. Large animal become homeotherms by default, and huge animals are almost by definition warm blooded. Because of this ‘gigantothermic’ effect of large body size, a tachymetabolic physiology becomes much easier to achieve (i.e. cheaper in terms of energetics) for large animals. As we have noted above (see 'gas exchange'), once a species starts to evolve endothermic physiologies it can further push the species (via demands of more food, higher speeds, or hunting in the more productive colder waters) to become larger still, and thus better endotherms, and so on.
Of the extant groups where 'warm bloodedness' is not an ancestral trait (as it is with marine mammals), we see again that the warm blooded species are all amongst the largest in their respective taxonomic groups - e.g. white sharks, tuna, and leatherback turtles.
If we were to hazard a guess as to which fossil species might have been potential endotherms, we should probably be considering any animal of one tonne or more, and certainly any animal over 10 tonnes. The larger ichthyosaurs, pliosaurs, turtles, mosasaurs, and lamnids are obvious candidates. And what about Carcharodon megalodon? It was the largest species of a family well known for endothermy in its extant representatives. As we shall see below, our understanding of its ecology, evolution, and eventual extinction hinges upon how we reconstruct its thermal ecology.
Trophic level: In modern seas the larger animals are invariably apex predators or planktonivores (or curiously, in the case of tuna and leopard seals, both). There is an obvious advantage for large, raptorial predators in being large, and modern apex predators such as Orcinus, Carcharodon, and Physeter have their ancient equivalents in species such as Dunkleosteus, Rhizodus, Temnodontosaurus, Pliosaurus, Kronosaurus, Mosasaurus, Cretoxyrhina, Xiphanctinus, and Basilosaurus (we're perhaps fortunate that there is no modern equivalent of Carcharodon megalodon!). At any one time, the largest animal in the sea has usually been the top predator.
Unusually, that is not the case now. The largest animals in the sea today are planktonivores - of the eleven modern species that regularly exceed 10 tonnes, ten are planktonivores. The only other time in Earth's history where the largest animal in the sea was not an apex predator was during the Middle Jurassic, when Leedsichthys cruised the seas.
The guild that contains the largest terrestrial animals (consumers of macrophytes) doesn’t have a marine equivalent because of the lack of large marine plants. It thus seems likely that the key factor in explaining why the biggest swimmers of all time are with us now is something to do with planktonivory...
Of course, becoming large isn't an infallible recipe for long life and prosperity - there are some down sides to being big. Earlier, we talked about some of the physiological traits that might prevent taxa from becoming large (e.g. gas exchange, reproductive physiology). To this we can add a few ecological negatives about being big.
Firstly, if you're big you are not going to be able to competitively exploit physically complex environments. You are not going to be an efficient predator on rocky shores or coral reefs, where your smaller prey species have plenty of places to hide that are inaccessible to you because of your size. You are probably not going to be very comfortable in lagoons and estuaries, because of the difficulty in navigating a large body through these environments. In fact, being a predator in any near-shore environment is going to have risks (although these are sometimes spectacularly overcome by white sharks, tiger sharks, and orcas). So it's the least biologically productive part of the sea for you - the open blue water.
This leads into the second problem for large animals. The maximum population density of any animal will depend upon the abundance of its limiting resources – usually food, but sometimes certain types of habitat such as nesting places, etc. To remain viable a population can not exceed the carrying capacity of its environment, and there is a strong density dependent effect here. So very big animals whose food demands upon an ecosystem are high must exist at low population densities.
The ‘trophic wastage’ of ecosystems, which leads to the biomass of each tropic level being a maximum of about 10% of the biomass of the trophic level immediately below it, means that this is potentially a signifcant problem for large apex predators. The energy available to an apex predator is a small fraction (0.001% if it is a fourth level consumer) of the primary productivity of that ecosystem. There is a real danger that, in maintaining population densities low enough to avoid over-exploitation, the predator will run out of room, and there won’t be enough of the ecosystem for it to maintain a viable population. (This argument has been used before to reason that large therapods could not have been tachymetabolic endotherms and still have maintained viable population densities).
The way in which nearly every modern huge animal has got around this problem is to cut out the middle man and by-pass the intermediate trophic levels – this is what large planktonivores do. By directly consuming the first level consumers (e.g. krill) planktonivores theoretically have a much larger proportion of the ecosystems primary production (10%) available for their own consumption. The standard wisdom is that plantonivory is what allows the large baleen whales to reach such huge sizes, and the standard wisdom may well be correct in this case. So, by eating krill, blue whales can reach 150 tonnes, have enough energy to be warm blooded, and can still occur at densities sufficient to maintain viable populations (as long as they aren’t chased by monkeys with exploding harpoons).
These two reasons may then explain why killer whales are the largest apex predators alive today, and why the fossil record strongly suggests that apex predators didn’t get above 10 tonnes in weight. Firstly, most ocean habitats are not productive enough to support big apex predators. Secondly, those habitats that are productive enough to support large predators are coastal, and the nature of the physical environment means that any predator larger than a killer whale is not going to be agile enough to hunt effectively in a coastal environment.
How well does the pattern of modern marine animals support this ‘rule of maximum size in marine predators’? As far as we know, killer whales and white sharks hunt predominantly in coastal regions, so they do seem to be tied to the high productivity of these environments. Apex predators commonly found in the open ocean – marlin, blue sharks, ocean white-tipped sharks, false killer whales, pygmy killer whales, pilot whales, etc – are all smaller than orcas, and in any case many of these also hunt in ‘coastal’ ecosystems as well.
But it is the exceptions to this rule which are, as usual, especially illuminating. Of course, killer whales are not the largest apex predators in the ocean today – sperm whales are. Sperm whales are huge open water predators, reaching 60 tonnes, and (prior to whaling) seem to have been remarkably abundant. The beaked whales, family Ziphiidae, are smaller (the largest species reaches 10 tonnes), but are very diverse (17 species in 5 genera) and are apparently quite abundant, even though very little is known about them. So there is certainly no shortage of large predators in the modern open oceans. If these environments are so unproductive, how can this be so?
Go swimming in the big blue, miles from any land, and you’re in the ocean’s equivalent of a desert (especially if you’re near the tropics). But there is food, and a lot of it – huge shoals of squid, bizarre deep water octopus, giant squid over one tonne, billions of slow growing, slow moving fish, sluggish megamouth and goblin sharks. It’s all there – 1000 metres down. To be a large open water predator you need to be able to dive very deep, and this is exactly what sperm whales, beaked whales, elephant seals, and even leatherbacks can do. The reason that there is an opportunity for these animals to be efficient predators of the deep sea systems may be that, as air-breathing divers, they have access to a rich oxygen source and can thus forage more actively than large fish and cephalopods that spend all their time in the deep, oxygen poor waters.
Those offshore predators that forage mainly in the upper part of the water column are, luckily for our argument, much smaller than sperm whales or even killer whales. Pelagic dolphins, marlin, sailfish, dolphin fish, makos and oceanic white-tipped sharks are all less than one tonne. Interestingly, they all take advantage of certain other large species, such as tuna, blue sharks, mola, and swordfish, that regularly forage above and below the thermocline, thus helping to move energy from the deepwater systems to the surface.
So, based upon the pattern of modern large marine animals, we can offer the following ‘rules’ about ecological constraints on large size;
1. Marine animals will be constrained by the availability of food from becoming large. Most of the ocean is biologically unproductive (compared with terrestrial systems), and thus, as predators, marine animals over one tonne will not be able to maintain viable population densities unless they;
(a) specialise on a highly productive marine ecosystem, such as coastal systems.
(b) manage to exploit the large biomass (resulting from low production but enormous physical volume) that exists in deepwater environments.
(c) become plantonivores.
2. Predators in coastal systems will be limited by the physical structure of the environment from becoming huge – the maximum size for predators dependant on coastal systems is approximately 10 tonnes.
3. There is no maximum limit upon the size of deep water predators or planktonivores resulting from the physical structure of these environments. Indeed, it can be the case these strategies become increasingly efficient at huge body sizes;
(a) Large animals are better divers. Diving animals (i.e. air breathers) will have a strong competitive advantage in exploiting deep water systems because they have access to a rich oxygen source. Thus most deep water predators (including all of the ten largest) are air breathing divers. Water breathing divers are smaller - the largest are the hexanchid sharks( < 2 tonnes), the megamouth shark (750 kg), the goblin shark (300 kg), and the large squids (e.g. Architeuthis, 1,000 kg).
(c) Large animals are more efficient planktonivores. Because of their size, large animals are able to catch more plankton per unit time, and because large animals are more efficient swimmers they expend less energy in obtaining their food.
How well do these ‘rules’ account for the pattern of large animals in the fossil record? Well, if we are saying that any animal over one tonne should not exist unless it has adopted one of the strategies under point 1 above, let’s remind ourselves of the fossil animals in our list that exceeded one tonne.
Dunkleosteus, placoderm, 6 - 7 metres (2,000 - 3,000 kg)
Rhizodus, lobe-finned fish, 5 - 6 metres (1,500 - 2,500 kg)
Cymbospondylus, ichthyosaur, 10 metres, (6,000 - 10,000 kg)
Temnodontosaurus, ichthyosaur, 9 metres, (6,000 - 10,000 kg)
Liopleurodon, plesiosaur, 8 -10 metres, (8,000 10,000 kg)
Pliosaurus, plesiosaur, 8 -10 metres, (8,000 - 10,000 kg)
Megalneusaurus, plesiosaur, 8 - 10 metres (8,000 - 10,000 kg)
Kronosaurus, plesiosaur, 8 - 10 metres (8,000 - 10,000 kg)
Platypterygius, ichthyosaur, 5 - 7 metres, (3,000 - 5,000 kg)
lamniform indet., shark 5 - 7 metres (2,000 - 3,000 kg)
Xiphanctinus, bony fish, 4 metres (500 - 1,000 kg)
Brachauchenius, plesiosaur, 5 - 8 metres, (3,000 - 8,000 kg)
Cretoxyrhina, shark, 5 - 7 metres (2,000 - 3,000 kg)
Mosasaurus, mosasaur, 15 metres (8,000 - 10,000 kg)
This group all fit within the "< 10 tonne coastal predator" guild. Their anatomies are consistent with those of macrophagous apex predators, and their fossils are common in shallow marine facies with associated near shore faunas.
Cymbospondylus, ichthyosaur, 10 metres, (6,000 - 10,000 kg)
Shonisaurus, ichthyosaur, 15 metres, (15,000 - 20,000 kg)
'Sikanni ichthyosaur', 23 metres, (25,000 - 50,000 kg)
Temnodontosaurus, ichthyosaur, 9 metres, (6,000 - 10,000 kg)
Cretachelone, turtle, ?4 - 6 metres, (?3,000 - 5,000 kg)
Platypterygius, ichthyosaur, 5 - 7 metres, (3,000 - 5,000 kg)
Mosasaurus, mosasaur, 15 metres (8,000 - 10,000 kg)
This group constitute the "deep water predator" guild. The largest of these, the Sikanni ichthyosaur and Shonisaurus, are too large to be apex coastal predators, and their anatomy is not consistent with macrophagous apex predators. The turtle, like its modern relative Dermochelys, probably specialised upon large deep water plankton, such as jellyfish. The other four taxa are also listed as possible coastal apex predators, but their anatomy and size also allows for the possibility that they were deep water predators.
Leedsichthys, bony fish, 15 - 30 metres, (15,000 - 50,000 kg)
The only large planktonivore in the fossil record, showing that huge planktonivores are not just a Caenozoic phenonmenum. But where are all the others?
pliosaur indet., plesiosaur, 10 - 15 metres, (10,000 - 20,000 kg)
I personally don’t believe it exists, but if it does then it may have been either a problematic exception to the 10 tonne limit on the ‘coastal apex predators’ guild (an earlier case of Basilosaurus), or a very specialised case of large open water predators (as is Carcharodon megalodon).
Basilosaurus, archaeocete whale, 15 - 20 metres, (10,000 - 20,000 kg)
Carcharodon megalodon, shark, 15 metres (40,000 - 50,000 kg)
The really interesting exceptions. Neither of these animals readily fit into our main three categories above. Basilosaurus seems too big to be a coastal predator – at 20 metres long and 20 tonnes in weight it would hardly have been the most agile animal in the sea. It was an unusual shape – very long and thin (though the extreme eel-like shape that many books show is wrong), with a relatively small head. The relatively small skull was still 1.5 metres long, however, and in all the skulls I’ve seen it is the closest to that of a large pliosaur in terms of being a very decent piece of macro-carnivorous equipment. In fact, if you consider the skull alone, you would say that it is equivalent to a medium-to-large coastal apex predator like Brachauchenius or Liopleurodon. In many ways it is like a pliosaur skull stuck on the long body of a mosasaur. Perhaps the long body fits in with some sort of specialisation as an ambush predator – in swimmers a long body is usually taken to indicate the ability to accelerate very rapidly from a standing start, as with pike – but beyond that I’m guessing. I’m am not aware of any work on the locomotory biomechanics of Basilosaurus, although it would be a fascinating thing to look at. My hunch, then, is that Basilosaurus may be a special case of the coastal apex predator.
This is unlikely to be the case with C. megalodon. At 18 metres and 50 tonnes this shark was simply too big to have been a predator of coastal marine systems. Instead, its appearance seems to have been a response to the opportunity created by the evolution of the large baleen whales in the early Miocene (see below). As such, C. megalodon was a highly specialised apex predator of the open ocean systems, preying directly upon the huge planktonivores that were themselves exploiting the plankton blooms of the temperature waters in the Miocene and Pliocene seas. Presumably it was at least as endothermic as the modern white shark, allowing it to hunt effectively in cold temperate waters.
The extinction of C. megalodon has been attributed to the onset of the Pleistocene glaciation and the subsequent specialisation of baleen whales upon the seasonal plankton blooms of the polar seas (even though it probably warm blooded, it was probably not warm blooded enough to cope with the cold waters) and/or the appearance of the killer whale – see, for example,http://www.ncf.carleton.ca/~bz050/megalodon.html . Under this scenario, the large whales retreated to waters too cold for it, and the killer whales were able to out-compete it for the smaller tropical and temperate whales. Another possibility (and perhaps more likely) is that the baleen whales of the Pliocene suffered their own mass extinction event associated with the onset of the Pleistocene glaciations, and by the time the large species of whales reappeared the shark had become extinct.
If the Oxford Clay ‘megapliosaur’ (Group D) does exist, it may have been a specialist mega-planktonivore eater. After all, Leedsichthys is known only from the Oxford Clay….
Understanding the freaks, and the key to huge marine mammals - the late Cainozoic as a great time for plankton (and planktonivores)?
Ok, to recap, we’ve been on a bit of an adventure (a large red herring?) with this one. We’ve established that big (1- 10 tonnes) marine animals become reasonably common in the fossil record once air breathing tetrapods become aquatic, and that most of these animals are apex predators in coastal ecosystems (i.e. where nutrients are derived from river discharge and wave action in shallow waters. We’ve noted that there are a few taxa that become huge (10 – 100 tonnes), and that these are either deep sea predators or planktonivores. We’ve discussed some of the physiological and ecological constraints upon and advantages to the evolution of large body size in marine animals. But what about the nub of Mike’s question – why is it now, of all times, that we see the really huge marine animals?
Well, if you haven’t worked that out by now you’ve probably fallen asleep at some stage during this. The truly remarkable thing about the late Cainozoic, compared to most other times since the Permian, is that we are currently gripped in the throes of an extensive glaciation. Average surface temperatures in the last 2 million years have been lower than at any time since the end of the Palaeozoic – likewise, temperature gradients along latitude at record high levels.
So why does this result in big marine animals? Because the most effective way of making the oceans very productive is to make them cold (this is perhaps counter-intuitive, but it’s true). The act of warming water sets up thermoclines, which prevent nutrients from being brought up from the bottom and thus severely limits photosynthesis in the primary producers (the phyto-plankton). In cold seas, however the thermoclines do not become established, the nutrients at the bottom are available to the plankton at the surface, and primary production is very high – as we can see each in the polar seas each summer. Conversely, the warm seas of today are much less productive, except where nutrients are washed into the sea from the land (i.e. coastal systems) or up from the bottom by storm activity (temperate seas in autumn) or large bottom currents (western continental shelves) that break down the thermoclines
It is because the seas of today are much colder than has been usual for the last 250 million years, that it is probable that the level of primary productivity is far higher than has commonly been the case. More productivity means more plankton, and more plankton means that planktonivores can get really big. Not just huge, but bigger than any animal has ever been before. And what is more, the resident planktonivores during the Cainozoic just happen to be mammals, a group that comes ready equipped with high level tachymetabolic endothermy, and thus able to forage effectively in the recently frigid polar waters.
The largest ones, the big rorquals and right whales, can all be found gorging themselves on the polar plankton blooms each boreal or austral summer. Some of the smaller species of baleen whales (e.g. minkes and Bryde's whales) seem to forage in temperate and/or tropical waters, although the smallest (the pygmy right whale) forages in sub-polar seas.
What is interesting is that the other types of large planktonivore seem to be doing what large planktonivores may have been doing since Leedsichthys. The whale shark feeds on tropical and warm temperate plankton – the much lower levels of plankton in these seas cannot support populations of blue whales, but can support the whale shark with its sluggish metabolism and very slow foraging speeds. The same can be said for manta rays. The basking shark may have a slightly more active metabolism than these two, but it forages in the slightly more productive cool temperate waters.
Right, that's enough for now. There are many holes in this (particularly with the terrestrial gait and the gas exchange sections), but we'd be here for ever otherwise. Also, just about all of this is Phase 2 science - a few hypotheses cobbled together on the back of some observations. What would be really interesting would be to do the third phase - start testing some of these ideas. One lifetime just isn't enough, is it...?
Tel: +61 2 6344 1009