Thank you, David, for your well-considered thoughts. I have offset David's paras with a single ">" at the start.
>More generally, it hardly tries to quantify anything.
We do have three tables, one of which compares extant v extinct predator/prey ratios. This counts. But by and large the paper is a synthesis of other researchers quantifications.
>That's quite a pity. A population of tetrapods can sustain its numbers if just over two of the eggs a female lies in its lifetime result in hatchlings that survive to reproductive age. Table 3 cites a reference saying that 95 % of ostriches die in their first year (it isn't mentioned if this is counted from hatching or from egg-laying).
We do break it down in the text, however.
>Given such and similar numbers, it should be possible to estimate, with reasonable error margins, how many eggs a *Triceratops* female may have had to lay in a lifetime, given the terminal Cretaceous array of potential predators on eggs and hatchlings, so that two would on average result in reproductive adults. If that number is ridiculously high (say, 30,000 eggs in 15 years), well, then we're looking at a likely cause of extinction! But such a test, or any hint of one, is absent from the paper.
I just don't see it...we have a tough time counting living species let alone extinct. And we are still very far from even knowing how many eggs are produced in a life time.
>== The appearance of mammalian nest predators ==
>I wondered about the gobiconodontids. John, are you still here? Over the years, you've discussed your hypothesis several times, and I remember bringing up the gobiconodontids several times; I don't remember getting an explanation for why they didn't singlehandedly wipe out most dinosaurs from Laurasia in the Aptian or so.
The paper notes dramatic biotic changes approaching the K/Pg. Yes there had been changes earlier and later. We argue that this period was quantitatively different. Stemming from the KTR (as argued by Wilson) and new niches opening up with the development of an entirely new floral biota...culminating in a dramatic convergence of new potential offspring predators just before the K/Pg. And it's not just me, now. Mayr and Tanaka et al. also postulate offspring predation as a prime culprit in bird selection at this time.
In my local breakfast joint I hear the old timers scoffing at carbon accumulation: "Man's carbon is nothing compared to the amount put in the atmosphere from natural causes." They are missing the _additive_ effect. So, I would argue that while dinosaurs coped with significant stresses from predators of all kinds earlier, they had to deal with additional stress prior to their extinction. This is undoubtedly true in the physical realm (volcanoes, sea regression, and bolides, thank you)...we argue species also had an effect.
>The argument...is that large egg-layers can't adapt to sudden increases in the amount of egg/hatchling predators, because they're large and therefore have too long generation times. So far, so good! However, the application of this argument to the K-Pg mass extinction rests on two unspoken assumptions: that the supposed rise of large eutherians, metatherians, multituberculates and gondwanatheres in the Maastrichtian was sudden by this measure
Wait...this is not "unspoken"...it is one of the main points. And the rise of these animals is not "supposed" it is now well documented and quantified.
>and that the appearance and diversification of the gobiconodontids in the early Aptian was not sudden. Not only aren't these assumptions mentioned, they aren't defended either (explicitly or implicitly, let alone with any numbers).
You seem to be arguing that because gobiconodontids did not wipe out the dinosaurs, then nothing else could (except a bolide, of course). You might be right. But we present evidence (standing on the shoulders of others whose hard-won data we use) that regardless of causation, dramatic changes were underway pre-bolide. We argue these developments were more stressful than what had occurred earlier...and that the mammals were an important part of that predatory guild.
>Wilson et al. (2016)...provides a handy overview in its fig. 3 which shows that *Didelphodon* and its fellow stagodontid *Eodelphis* go back deep into the Campanian. There's nothing sudden about the rise of large stagodontid metatherians! We're talking about more than 10 million years before the end of the K here. Further, table 1 features *Nanocuris*; that's a deltatheroidan, and the large deltatheroidan *Deltatheridium* goes back at least as far.
This is the old-timers argument again..."because there was carbon in the atmosphere way back when, it could not cause any problem now." If the hypothesis has any value, it rests upon accumulation of offspring predators.
>The same table misspells *Yubaatar*; and the reference for *Altacreodus*, Fox (2015), is missing from the references list. It is:
Richard C. Fox (2015): A revision of the Late Cretaceous–Paleocene eutherian mammal Cimolestes Marsh, 1889. Canadian Journal of Earth Sciences 52: 1137–1149. DOI: 10.1139/cjes-2015-0113
Thank you for that.
>== The appearance of avian nest predators ==
>The diversification of "ornithurine" (euornithean) birds in the Late Cretaceous is mentioned in a confused three-paragraph section that also mentions the extinction of nonavian dinosaurs and the extinction of non-"ornithurine" dinosaurs, but not the extinction of non-neornithean euornitheans. No attempt is made to show that any Cretaceous euornitheans were potential nest predators, or that any enantiornitheans were not potential nest predators; no attempt is made to show that any Cretaceous euornitheans nested in trees or that any enantiornitheans did not. Nothing is quantified.
I quote from the paper:
"Longrich et al.’s (2011) study of three formations from the final
1.5 million years of the Cretaceous, only 3 of the 17 species are
enantiornithines." That looks like a quantity to me. It might not be a quantity you _like_...because it speaks to a biotic replacement rather than an abiotic cataclysm. I could cop to oversimplification but the bird section is _not_ confusing. It simply says: bird assemblages were changing and had members big enough to prey on dinosaur hatchlings...and, again, Mayr (2016) and Tanaka et al. (2015) suggest offspring predation may have affected these assemblage changes. We simply say, if these new birds had an effect on bird populations, they may well have had an effect on dino hatchlings as well...and that this is parsimonious given the intense predation on ratite chicks by birds today.
>== The appearance of ophidian nest predators ==
>What we can say for sure is that many dinosaur eggs were too small for *Sanajeh*...
??? They could not have been that fussy, surely...or am I misunderstanding you?
>and that whole continents (North America in particular) appear to have remained madtsoiid-free.
Of course, I would relish a NA madtsoiid. I would note, however, on this continent we have well-documented potential offspring predators among Aves and Mammalia, that, in a stressed environment, could have targeted non-avian dinosaurs.
>== Large ground-nesting birds in the Cenozoic ==
>According to the paper's main hypothesis, there shouldn't be any large ground-nesting birds in the presence of Cenozoic levels of nest predation. Therefore the paper hypothesizes that all large Cenozoic ground-nesting birds have lived in environments that provided protection against nest predation – although no attempt is made to quantify how much protection that is.
Ostrich nests in a 2 km sq territory, but cannot be seen by a standing man further away than 10 meters. This is quantification surely.
>Ostriches, emus and phorusrhacids are explained away as being tied to grasslands. Fair enough, if we grant that eggs are exceptionally hard to find in grasslands (see below).
You can't have the hypothesis "explained away" and being "fair enough" in the same para.
>Rheas and dromornithids are also explained away as being tied to grasslands. Problem is, the Paleocene rhea *Diogenornis* lived at a time when grasslands simply didn't exist yet, and the same holds in Australia for all Eocene and Oligocene dromornithids. Brontornithids are not mentioned.
We made an arbitrary cut-off at emu size (30 kg) as the size that nesting adults needed additional concealment...I mean, below some size it is easier to hide. Diogenornis would be well below that. Re Brontornithids and others: our main idea is that dependence on defense emerges with increasing size...and that most non-avian dinosaurs were above this size. I would love to see studies on the value of grass, heterogeneity of cover, etc. relative to the size of an incubating parent. On the hypothesis of predator emergence there is a lot out there. The topic of predator avoidance is fairly well covered for extant species...but there is (understandably) not much on extinct organisms. We put an hypothesis out there...it seems there is some correspondence between grassland evolution and the radiation of big birds. If the correspondence is not perfect, then how did they avoid unsustainable predation? I mean, what are the options: predator avoidance via remote location, concealment, predator swamping, nest defense? This _is_ a valuable conversation and if nothing else, we hope our paper will promote that.
>The existence of cassowaries, moas and kiwis is interestingly blamed on supposed lack of predation on the adults, not on the eggs and hatchlings.
Oh, no...we don't specify that! The paper is about _offspring_ predation...I assumed this could be assumed...sorry if this was unclear.
>The aepyornithids are mentioned in the same paragraph, while a later one blames their existence on wetlands.
I think this is incorrect...I can't find any aepyornithids mentioned with cassowaries...and I am aware of the fossa. Not sure about your point here?
>Wetlands are also made responsible for the Paleocene/Eocene wide distribution and subsequent extinction of the gastornithids, because wetlands hinder "access by mammalian predators" (with references that all seem to be about small birds; "waterfowl" and "the purple swamphen" are mentioned in 2 of the 4 titles). I do have to wonder: you can't just lay an elephant-bird egg into a swamp...? And what kind of "wetland" is it that is too muddy to allow herpestids or oxyaenids to simply walk into it, but not muddy enough that a quarter-ton bird would get stuck?
Swamps have islands. Islands are good places to nest.
>...I'm aware that at least one species of aepyornithid was apparently called vorompatra, meaning "swamp bird" (vorona + patra), but that doesn't mean it was able to nest like a duck or a sedge warbler.
Islands. The more wetlands, the more islands.
>Further, gastornithids are known from all three northern continents; were there really enough "wetlands" for them to nest in and nonetheless spread around the Earth?
Stating the obvious: much is not known re these birds. We have Andors connecting them to wetlands...their bones found in coal swamps, savannas, riparian forests, coastal wetlands...Buffetaut et al. seems mystified at their apparent immunity to invasions of large mammals...again, these birds were undoubtedly complex in their response to predatory threats. We suggest wetlands were an important part of this repertoire...this hypothesis is shared by some workers.
>The Paleogene European "ratites" (*Palaeotis*, *Remiornis*, *Eleutherornis*) are not mentioned, and neither is *Eremopezus* from Fayûm. Messel was full of mammals and crocodiles (in fact, I have no idea how the adults of the _small_ flightless bird *Messelornis* managed to survive), and Fayûm was a scary environment with potential nest predators up to and including *Gigantophis*.
Nor have I! One thing I am certain about: they did not sit on their nests in full view of the local predator communities! This would seem a fairly conservative claim...yet this is our hypothesis for Cenozoic birds.
>Finally, from the "Wetlands" section on p. 8, let me quote this gem: "_*Aepyornis*_. The elephant bird (~500 kg) likely owed its success to the existence of wetland habitat for nesting; wetland contraction is implicated in the extinction of *Aepyornis* (Heuvelmans 2014)." That's the whole paragraph. Yes, you may trust your eyes: the only cited source is a posthumous reprint (not marked as such in the references list!) of the famous cryptozoological work from 1955 (English translation 1958). (Heuvelmans died in 2001.) Catastrophic wetland contraction throughout this huge island within the last thousand years, right at the same time when humans settled the place? Some of the paleontology in Heuvelman's book was probably already outdated in 1955, and yet it's uncritically cited in Historical Biology in 2017. (Note also that *Mullerornis* is not even implicitly mentioned; not all Holocene aepyornithids are *Aepyornis*.)
Well let me apologize for this...I was unaware that this was something I should have noted in the reference section...I do see why it is important to do this. I trust you recognize that I was not trying to mislead...and also, why it might not be a red flag to a reviewer w/out specific knowledge of the work in question.
..."Without the concealment property of grasses, this rate might be higher." Indeed it might. By how much? Would it ever reach 100 %, or 95, or even just 90? No attempt is made to answer this question at all.
Without concealment it would reach 100%...ostriches are fully adapted to grasslands. This is a simple enough conclusion. It is so much a part of their existence as an ocean is to a whale. Would you doubt a whale could survive without its ocean.
>The "concealment property of grasses" is supported only by two anecdotes, by the way..."Bertram (1992) noted that he could not detect an ostrich nest to within 10 m, but that it is sited within a territory of >2 km²."
Again, this is more than just an anecdote...Bertram is a respected and often cited scientist...and this is a careful observation. He couldn't find the birds!
>"Rheas of South America, also nest in grassy open habitat (Bruning 1974) and are similarly inconspicuous (Darwin 1839)."
Darwin has also garnered some respect for his observations...in this case he was riding across the pampas and was surprised that his horse had to avoid unseen nests at the last second...invisible from even horse's pov.
>Comparisons to other vegetated landscapes, quantified or anecdotal, are not provided. How easy is it to find a cassowary nest?
Apart from grasslands, forests of NG and NE Australia are the only other place that support extant large theropods. The paper is a large hypothesis...perhaps should have been a book to cover all the stuff it does. We took out a discussion of forests...in part because most >30kg non-avians nested in the open. Forests obviously work for cassowaries...but there is a limit. Mound-nesting forest megapodes can apparently only make it where continental predators are absent...i.e., there is a number of islands in SE Asia...Borneo/Indonesia area...some have continental predators, some do not. Megapodes are known only on those islands without...presumably because they are aware of the danger (ref on request). In other words, predator quality and density is a limiting factor in a forest. In general, forests do support higher predator density. How does all this relate to cassowaries? We note greater diversity in NG than Australia (a quantity of 3:1) and correlate that with fewer predators. And mainland Australian cassowaries deal with less predators than animals on the Northern continents...the quantity of forest dwelling large theropods is 0!
>We aren't told. And, again, why is it that rheas, emus, dromornithids, apparently ostriches (assuming they're related to some or all of the Paleogene European "ratites") and possibly phorusrhacids were already flightless and pretty large before the forests they lived in turned into grasslands?
We do, however, note the increase in size coincident with the advent of grasslands. Much of this discussion gets back to size. Smaller animals can hide in a multitude of places...we contend oviparous species of large size experience emergent problems. Concealment becomes difficult. And among extant species, at least, defense cannot be the prime strategy. So, what then? At least we are _asking_ that question.
>For dinosaurs above a certain unspecified size, hiding the nest is argued to be impossible because the eggs are just too big and/or the brooding adults (if any) are detectable from afar. (No quantification of course.)
What sort of quantification would satisfy you regarding a sauropod hiding its nest.
>"Running" off to an inaccessible location, like a treetop, a cliff or a remote island, requires the ability to fly or at least climb and is thus out of the question for most if not all nonavian dinosaurs.
Is this really a question for you? Cue the tree-top sauropd!
>Swamping is apparently considered impossible, because (p. 5):
"Perhaps intrinsic to the idea of multiple unattended clutches, is the idea of predator swamping. We contend that it is unlikely that a colony of dinosaurs could abandon their nests and survive excessive offspring predation by overwhelming or satiating predators...Reproductive effort by many turtle species benefits from both of these factors and provides a relevant contrast to nonavian dinosaur nesting ecology because, even when discovered, the contents of turtle nests swamp predators (Santos et al. 2016)."
>As far as I can tell, "we contend that it is unlikely" is the entirety of the argument. The concept of predator swamping is mentioned nowhere else in the paper.
Perhaps you fell asleep at this point...the next _two paragraphs_ contain the argument: turtles have more nest site choices and exercise them; turtle hatchlings limit temporal access by finding refuge in the ocean. As far as I know turtles are the only amniotes that allow straight up unprotected access to offspring with no concealment. This is what swamping must mean...if a word can have a meaning. Provide as many offspring to the predators as they can eat but still have enough left over to carry on. Other swampers (with apologies to one of the greatest rhythm sections) are acorns and oysters...both of whom have relatively cheap offspring. Not true for amniotes.
>On remote nesting I have to say that there are degrees of remoteness. Surely such islands as Egg Mountain and Egg Island afforded _some_ protection? *Maiasaura* did survive the presence of *Gobiconodon* after all.
OK...but our argument is a relative one: non-avians could not be as isolated as avians.
>If we accept that no attainable degree of remoteness is enough and that even a titanosaur nesting colony couldn't swamp nest predators...
Also...because their hatchlings exist amid the same predators as were present at the nest (unlike swamping turtles).
>...that leaves nest defense. The authors correctly point out that nest defense can never be perfect (p. 6):
>"If dinosaurs actively defended their nests, we question the effectiveness of this strategy in the face of an emerging guild of small predators for at least two reasons: (i) dinosaurs needed to maintain structural integrity of their nests and lacked the capacity to defend them against burrowing animals. As a modern analogy, the hairy armadillo (*Chaetophractus* sp.) burrows beneath the nest of the much larger rhea (*Rhea americana*) parent (Fernandez & Reboreda 1998). While this source of predation causes many nests to fail, rheas have no defensive response to it."
>And yet, they still haven't died out. How can that be? It can't be the fact that they nest in grassland – if anything, grassland soil should be easier to dig through than the sand under a rainforest with all those roots in it. I guess that counts as swamping.
Ameliorating factors count for something! The armadillos first have to find the nest. "counts as swamping"...what does the term mean. If it just means having enough offspring to maintain populations, then the term is fairly useless. The thing about amniote eggs is that they are a big, expensive, desirable resources, and they need to be incubated somehow. Clutch size is under intense selective pressure...more eggs means either or both more stress on the parent or less provision for the baby. Swamping is not a good way to go for amniotes.
>"(ii) small [sic] nocturnal predators have an advantage over large diurnal oviparous species. Ostriches that effectively defend the nest from black-backed jackals (*Canis mesomelas*) in the daylight, apparently abandon the nest under similar attack at night (Bertram 1992). Some non-avian dinosaurs might have been nocturnally active (Schmitz & Motani 2011); however, this interpretation has not been widely accepted (e.g. Hall et al. 2011)."
>This is followed by evidence for a nocturnal/fossorial origin of mammals and snakes (all with the implication that Mesozoic dinosaurs were nightblind like ostriches). And yet, ostriches aren't extinct either. Is it the grassland this time? But if so, what happens when a fox or dingo or singing-dog or historically a "Tasmanian" "tiger" attacks a cassowary nest at night?
Cassowaries are strongly diurnal (at least two of the three NG species...evidence from cameras). They are very secretive and they live in an area with no placental predators...historically, anyway. It is possible that different mammals have different perceptual abilities and that marsupials and less endowed in that respect. I'm not the first to suggest this.
>Crocodiles can't really hide their nests or nest in remote places either, so they defend them. This is implied to be insufficient on its own in the following paragraph from p. 7:
>"Nest defense is practiced by most species of crocodilians (Somaweera et al. 2013). This represents an exception to our claim that dependence on nest defense cannot be a viable strategy amid extant predators. We argue that crocodilians have experienced a relatively reduced frequency of offspring predation because their hatchlings find refuge in water. Factors such as turbidity and submerged aquatic vegetation probably enhance crocodilian reproductive success in wetlands (Somaweera et al. 2013); and the semi-aquatic habitat is a particularly effective refuge from mammalian predation (Pasitschniak-Arts & Messier 1995)."
>Crocodylian hatchlings are, of course, eaten by all sorts of large actinopterygians, large frogs, birds, varanids (often good swimmers) and adult crocodylians. Unsurprisingly, parents guard and defend their hatchlings in the water well after hatching, too. I conclude that hiding is insufficient on its own in this very case and needs to be supplemented by defense. Whether defense would be sufficient on its own isn't testable, because hiding is cheaper.
Yes, and we note that big birds will "supplement" hiding by defending (depending on the predator). But our major point here is that the medium of water provides some measure of safety for croc hatchlings...and I don't think you are disagreeing?
>== What about those that could in fact hide? ==
>P. 6: "In comparison to almost all extant terrestrial oviparous vertebrates of <30 kg in body mass, the large size of non-avian dinosaurs precluded stealth when laying and incubating their eggs. We recognize that several small carnivorous non-avian dinosaurs remained into the latest Cretaceous (Turner et al. 2007; Benson et al. 2014; Larson et al. 2016), and that concealment was a likely strategy for them. However, we argue that no single extinction hypothesis can consider every extinct species. Indeed, other scenarios invoke the extinction of carnivorous species following the disappearance of their herbivorous prey (e.g. Alvarez et al. 1980), and we predict a similar fate for the smaller members of the clade Deinonychosauria."
>Come on, authors. Don't you agree that the small deinonychosaurs (< 30 kg) were ideally suited to eat the diversifying mammals? Would they really care if the hadrosaurs died out?
I consider it likely that the main diet of these animals was hatchlings. They are easy to find, they are plentiful (more so than mammalian offspring, right?), and they undoubtedly supported a diverse food chain. Now they are gone...what next? I don't see how this is at all far-fetched. Isn't it also the main mechanism for other scenarios (i.e., first the herbivores then the carnivores)...then why jump at us for this?
>I argue that a single hypothesis for the cause of a _mass extinction event_ should – parsimony! –, and indeed can, consider every species that took part in that event. As far as I can tell, the Chicxulub impact has little trouble explaining the extinction of all deinonychosaurs and a whole lot of mammals at the same time as loads and loads of haptophytes and planktonic foraminifera. But I digress.
No. Parsimony in ecology is that the most complexity you can imagine is still too simple. We are already questioning birds turnover as a thing that is not part of the mass extinction. We do not question the severity of the stress...we just don't see it as a catch all...and even if it was, we still want to explain the paucity of the bau plan in modern ecosystems.
>== Conclusion ==
Ugh...a twenty-year long fool's errand. Well, at least I hope we can provoke some discussion of the issues.
>I was surprised to find that the paper had four reviewers. All of them are anonymous, and any responsible editor is not mentioned (some journals do this, some don't).
>BTW, the authors and the reviewers seem to have expected that the manuscript would be copyedited. Of course it wasn't; very few journals do that anymore. Various typos abound, and (as mentioned above) there's at least one missing reference.
"Abound?" I think this is an unfair criticism.
>== So, why aren't there more large flightless birds in the Cenozoic? ==
>Why weren't there more large flightless birds in the Mesozoic, seeing as *Gargantuavis* proves the concept? I suspect the reason is the same: the ecological niches in question were already occupied. In the Cenozoic, mammals mostly happened to get there first; "everything is the way it is because it got that way" (D'A. W. Thompson, 1917: On Growth and Form).
You must mean "habitat", not "niche". The niche of ratites is so specialized and finely adaptively tuned as to have few competitors! What they have is predators limiting the places where they can safely have their babies. We claim this limits their expansion.