[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Cretaceous cormorants -- really?



I've recently reported on the paper that makes a good argument against Cretaceous loons. From this perspective, it could be interesting that cormorants seem to be highly derived members of whatever fragment of "Pelecaniformes", according to both molecules (Fain & Houde) and morphology (http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/balaeniceps.pdf). The oldest Cenozoic cormorants are from the late Eocene, like the loons and many others (source: Feduccia 1996). Naturally I haven't tested if calibration with Cretaceous cormorants makes their relatives impossibly old (or just the tongue twist). But let's have a look at the Cretaceous material referred to cormorants.

For, perhaps, some reason I always imagined in my infinite wisdom that this material consisted of tarsometatarsi. Only today did I check this in Mesozoic Birds. Behold, although I had read the whole book before, I was wrong! Everything Hope refers to Phalacrocoracidae is a scapula from the Nemegt Fm and a femur from the Lance Fm. Here's what she writes about these two bones (p. 367):

"Limited diagnosis -- Specimens are placed in Pelecaniformes based on the following feature: iliac facet of the femur entirely convex (unique). In addition, the lack of elevation of the trochanteric crest of the femur differentiates Pelecaniformes from many other birds."

So in case the scapula turns out not to be phalacrocoracid, there's no reason to consider it pelecaniform.

"Limited diagnosis -- The specimens are placed in Phalacrocoracidae based on the following derived features:"

Then follows a list of 3 to 6 characters of the scapula, depending on how one counts, and 6 to 14 or so unquantified characters of the femur; afterwards comparisons are made of the scapula to Pelecanidae and of the femur to Anhingidae and the late Eocene to early Miocene Plotopteridae; the differences sound like they were adaptations to foot-propelled diving in cormorants, and those between Phalacrocoracidae and Anhingidae are explicitely ascribed to this lifestyle.

The scapula from the Nemegt Fm and the femur from the Lance Fm (p. 368)

"Remarks -- Kurochkin (1995a, b) determined that this specimen came from a very large cormorant. My comparisons with diverse cormorants and other pelecaniforms are consistent with this determination. [...] The specimen is not yet described."

Therefore it is not illustrated, and this means that there is no indication of how complete the scapula is. Personally I would be interested of what a scapula of, say, *Judinornis* (a hesperornithian known from an isolated vertebra from the Nemegt Fm) looked like... One neornithine synapomorphy of the scapula is mentioned on p. 348, but not on p. 368: "the humeral facet of the scapula faces laterally or craniolaterally".

"Remarks -- In the absence of known derived features of the femur in earliest neornithines, the present specimen is referred to Neornithes based on unique features of Phalacrocoracidae."

Hear, hear.

"The head of the femur is damaged,"

This means that one of the two "pelecaniform" characters cannot be coded. I'm not sure whether the trochanteric crest counts as "elevated" or not (fig. 15.9 A), but then this one character is already not considered unique to "Pelecaniformes", see above, so I don't even need to mention the recent dismantling of this group.

"and the shaft is broken off just proximal to the condyles, with a fragment remaining that indicates the very large size and lateral orientation of the lateral condyle;"

Judging from fig. 15.9 A, I'll buy that. But fig. 15.9 C shows an extant cormorant femur. Its lateral condyle is large -- perhaps smaller than what the Cretaceous fragment indicates --, but only slightly laterally oriented from the long axis of the femur, and slightly _medially_ oriented compared to the sagittal plane of the animal! The latter is the case because the femur is strongly bent in cranial view. The Cretaceous femur, on the other hand, is completely straight in caudal view. I wonder if this counts. Mediolateral curvature is not mentioned in the list of phalacrocoracid characters, though, so perhaps it's an autapomorphy of some smaller clade that includes *Phalacrocorax pelagicus*.

"part of the popliteal fossa remains."

But this part doesn't contain any diagnostic characters of cormorants.

"A small but probably significant difference from extant cormorants is the angular truncation of the trochanteric crest. This feature precludes referral to either of the two major extant lineages of Phalacrocoracidae recognized by Siegal-Causey (1988)."

Well. I for one don't think we should expect a _crown_ cormorant in the Cretaceous anyway. :-}

Fig. 15.9 shows the presence of most or all of the other diagnostic characters. But I wonder how many of them are adaptations to foot-propelled diving, and therefore partially correlated with each other. Only 2 to 4 of the 6 to 14 femoral characters in the list are said to be unique to Phalacrocoracidae, but it isn't mentioned where else they occur. Certainly Hope would have noticed if the femur came from a hesperornithid or *Baptornis*, even though she doesn't mention any comparisons to them; but considering the fact that only two characters of the femur are diagnostic for "Pelecaniformes", the femur in question could easily drop out of Neornithes once it would be removed from Phalacrocoracidae. So what other foot-propelled diving birds were there in the Maastrichtian?

I see three possibilities. One are basal, perhaps flying hesperornitheans, like what *Potamornis* seems to have been. No femora or for that matter practically nothing at all is known of these animals. The other are the apparently-not-loons like *Polarornis* and *Neogaeornis* -- if the femur from the Lance Fm belongs in there, it doesn't need to be removed from Neornithes. The third is *Yungavolucris*, according to some interpretations of this isolated tarsometatarsus.

CONCLUSIONS

To settle this question, someone should:
- Find out where cormorants, and many others, sit on the family tree of Neoaves. This would have to be done both with molecules -- so that the calibration experiment can be repeated for both loons and cormorants -- and with morphology -- so that fossils from the Cretaceous to the Eo- or Oligocene can be safely inserted into that tree. Well, don't we all wish. Fortunately both approaches are being pursued and have yielded promising preliminary results (Fain & Houde; Livezey & Zusi).
- Find more fossils of Cretaceous birds, so we get an idea on how realistic my alternative suggestions are. Well, don't we all wish.
- Find out which parts of Hope's diagnosis of Phalacrocoracidae are adaptations to foot-propelled diving and can therefore be partly expected to occur in other foot-propelled divers. This can be done with today's knowledge of phylogeny, and has probably been done years ago.


While I am at it, one important feature that was said to tie *Polarornis* to the loons was its cnemial crest: in hesperorniths it's formed by the patella, in grebes by both tibia and patella, and in loons and *Polarornis* by the tibia alone. But if foot-propelled diving evolved more often than 3 times, then some of these possibilities should have evolved more than once. It already has evolved 4 or more times within Neornithes. Does someone know what the cnemial crest of cormorants and finfoots look like?