Tetrapod Zoology

Again, more recycled text from the Mesozoic bird section of the fieldguide…

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Alexornis antecedens from the Upper Cretaceous (?Campanian) La Bocana Roja Formation of Mexico was first described in 1976; its remains were discovered in 1971 by H. J. Garbani and J. Loewe. The bones they found – various elements from the shoulder, wing and leg – were tiny, and evidently from a bird similar in size to a modern finch [adjacent life restoration by Ken Kirkland, from Hilton (2003)]

This discovery pre-dated the 1981 recognition of enantiornithines as a distinct group of Mesozoic birds (see the article on Cyril Walker for more on this subject). Pierce Brodkorb – the fossil’s describer – assumed that Alexornis was a modern-style, neornithine bird (Brodkorb 1976). He even suggested that it might be close to the ancestry of two modern bird groups: the Coraciiformes (rollers and relatives) and the Piciformes (woodpeckers and relatives). The species name – ‘antecedens‘ – reflects this, as it refers to a possible ancestral position for this bird. Larry Martin was later able to show that Alexornis is an enantiornithine (Martin 1983). Like other members of this group, the articular surfaces on its scapula and coracoid are ‘reversed’ relative to those of neornithines*: this had fooled Brodkorb, who had interpreted the scapula as the coracoid and vice versa [Alexornis holotype remains shown below; from Brodkorb (1976)].

* A groove on the ventral surface of the scapula forms the more important part of the triosseal canal, whereas in neornithines it is the dorsal end of the coracoid (specifically, the procoracoid process) that serves this role.

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As with so many Mesozoic birds, Alexornis was initially given its own taxonomic ‘family’ (Alexornithidae), and later authors also gave it its own eponymous ‘order’ (Alexornithiformes). Some authors have recently used these names for more inclusive subgroups of Enantiornithes and one expert, Eugevny Kurochkin, has argued that Alexornis should be united with two very poorly known enantiornithines from Uzbekistan, Sazavis prisca and Kizylkumavis cretacea, in the Alexornithidae (Kurochkin 1996). This is difficult to be confident about, however, and most workers regard all three of these forms as of uncertain affinities within the large enantiornithine clade Euenantiornithes. Alexornis‘s generic name honours the palaeornithologist Alexander Wetmore. What little is known of Alexornis shows that it was small (roughly the same size as a sparrow) and capable of flight. Its skull is unknown.

For previous articles on enantiornithines and other Mesozoic birds see…

Refs – -

Brodkorb, P. 1976. Discovery of a Cretaceous bird, apparently ancestral to the orders Coraciiformes and Piciformes (Aves: Carinatae). Smithsonian Contributions to Paleobiology 27, 67-73.

Hilton, R. P. 2003. Dinosaurs and Other Mesozoic Reptiles of California. University of California Press, Berkeley.

Kurochkin, E. N. 1996. A new enantiornithid of the Mongolian Late Cretaceous, and a general appraisal of the infraclass Enantiornithes (Aves). Russian Academy of Sciences, Special Issue, pp. 60.

Martin, L. 1983. The origin and early radiation of birds. In Perspectives in Ornithology, Essays Presented for the Centennial of the American Ornithologists’ Union, pp. 291-338.

Comments

#1 David Marjanović
April 13, 2010

life restoration by Ken Kirkland

<snarl>

Where’s the alula? And why is the 2nd (and apparently even the 1st) phalanx of the 2nd finger featherless, when that’s where the big remiges attach even in Sinornithosaurus?

I hate artistic traditions that come from what seems logical instead of from looking at the fossils.
#2 Dartian
April 13, 2010

Its skull is unknown.

Did Kirkland have any particular reason for giving Alexornis such a finch-like bill in that reconstruction?
#3 Darren Naish
April 13, 2010

Ken is a regular Tet Zoo reader, he might say.
#4 Andrea Cau
April 13, 2010

Pending Kirkland’s words, the skull seems based on _Gobipteryx_.
#5 Jerzy
April 13, 2010

Cool – I wonder how many forms, and how many independent lineages convergent to particular modern birds remain undiscovered…
#6 Matt Martyniuk
April 13, 2010

@David Marjanović:
Not to mention the seemingly folded, ornithuromorph-style tail feathers. The previous post here discussed the weird strap-tail feathers of enantiornithines. Not sure about the neornithine-like bill, these evolved many times in various lineages, but I think it’s most parsimonoius to assume it was toothed and had a mostly feathery snout like Protopteryx and other basal birds.
#7 Jaime A. Headden
April 14, 2010

Matt, aside from the “strap-like” feathers (also, EPPs!), which occur in some non-birds, there are also enantiornithines without teeth, such as the Gobipterygids, and it is likely that other clades of enants would or could have lost their teeth (especially as it seems more primitive and more derived [relative to the crown clade] around Enantiornithiformes also were losing teeth frequently enough). With enants appearing to have very derived tails (Iberomesornis) and others with primitive and very dinosaur-y looking hands (Cathayornis), enants ran the gamut in morphologies. It’s hard to say they should look like one or another thing.
#8 David Marjanović
April 14, 2010

The head is clearly based on Gobipteryx; in the absence of a phylogenetic analysis, that’s as good a guess as any. I can’t say anything against it. A wide range of alternatives exists, however, all the way to the snout with caniniform teeth seen in Aberratiodontus.
#9 Darren Naish
April 14, 2010

WRT comment 7: enantiornithines are indeed impressively diverse… they might not have been able to do as many things as neornithines have (Kaiser argued that the evolution of locking devices between the dorsal vertebrae and synsacrum allowed neornithines to withstand impact stresses, and hence to use their hindlimbs in predation), but they got pretty close. We now know of enantiornithines that seem to have been waders, swimmers or divers, terrestrial cursors, seed-eaters and sap-eaters, and they ranged in size from finch-like (wingspan less than 20 cm) to hawk-like (wingspan more than 1 m). I was planning to cover Aberratiodontus next, coincidentally.
#10 johannes
April 14, 2010

> and it is likely that other clades of enants would or could have
> lost their teeth (especially as it seems more primitive and more
> derived [relative to the crown clade]

@ Jaime, there can’t be such a thing as an enantiornithine crown group or crown clade, because enantiornithines are extinct, and a crown group is, by definition, the smallest monophyletic group, or “clade”, to contain the last common ancestor of all extant members, and all of that ancestor’s descendants. Enants are, of course, crown group archosaurs, descendants of the last common ancestor of *Passer* und *Crocodylus*.

> I was planning to cover *Aberratiodontus* next, coincidentally.

Happy happy, joy joy
#11 Jaime A. Headden
April 14, 2010

Johannes (@10):

I wrote:

“more primitive and more derived [relative to the crown clade] around Enantiornithiformes”

The crown clade is the only crown clade present, which is the avian crown clade. This is not meant to imply there is some sort of crown involved that includes enants or any other fossil bird (save that it is a member of the crown). My argument was that the presence or loss of teeth was irrelevant to the potential phylogenetic implications of the artist’s rendering, as toothlessness is a derived condition in a bajillion non-crown avian lineages. I hope this clarifies things.
#12 CS Shelton
April 14, 2010

Maybe enantiornithines could adapt to do things living birds can’t, due to the same morphological issue that limited them in other ways. There’s a lot of luck involved in nature and it could be their extinction was more about being in the wrong place at the wrong time than being less fit. Example scenario: 99.99% of dinosaur species go extinct, and the sheltered region that spared a few survivors was populated mainly by ornithurae. Sorry enants! The Universe is utterly indifferent to your evolutionary achievements!
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#13 Nathan Myers
April 14, 2010

Can’t we extend “crown group” for the case of an extinct species to mean those species that would have been in its crown group during the time that it lived, had somebody been there to classify them?

I like the etymological derivation of enantiornithines as “mirror birds”. It gives them a Neil Gaiman quality.
#14 Jaime A. Headden
April 14, 2010

Nathan @12:

Currently, the avian crown (as all other crowns) includes only all living birds, their last shared ancestor, and all descendants (living or extinct) of that ancestor.

Current phylogenetics indicates that all living birds share a single ancestor that excludes forms like Hesperornis or Icthyornis from it (may exclude lithornithids), with as their most basal branching group being ratites.

This does not include ichthyornithids, hesperornitheans, enantiornitheans, Shenzhouraptor-like forms, confuciusornithids, etc. They are avialeans, and in some cases may even be avians (of Aves), but they are not crown birds.
#15 David Marjanović
April 15, 2010

> I was planning to cover *Aberratiodontus* next, coincidentally.

Happy happy, joy joy

Seconded

Can’t we extend “crown group” for the case of an extinct species to mean those species that would have been in its crown group during the time that it lived, had somebody been there to classify them?

You’d have to make that usage explicit in your paper.
#16 John Harshman
April 15, 2010

Can’t we extend “crown group” for the case of an extinct species to mean those species that would have been in its crown group during the time that it lived, had somebody been there to classify them?

Isn’t everything in the crown group, in that case? How can you define “crown group” for extinct clades? If a species is extant at some time horizon, it’s a member of the crown group for that time horizon, by definition.
#17 Jaime A. Headden
April 15, 2010

If an ancestor gave rise to three lineages, one of which is now extinct, the other two form a crown. If that extinct lineages diverged before one extant lineage, but after the other, it is encapsulated by a two extant lineages (bracketed, as it were) and is thus part of a crown group.

All dinosaurs, for example, are part of the Archosauria crown clade (crocodilians and birds), but a healthy dose of fossil avians are NOT part of the avian crown, and none of them are part of the crocodilian crown.
#18 kimberly carillo
January 11, 2011

what caused the extinction of the alexornis antecends?
#19 Bradley
January 11, 2011

The last one died.
#20 Patrick Wilson
February 2, 2011

Please pass this on to Ken Kirkland. I had the distinct pleasure to spend a couple of years at the Smith River Inspection Station in his company and he formed, what I called the FLYING “A” TEAM. Marvoulusly talented man. What a pleasure to see his gifts appreciated and still shaking up the boarish intellectual snobs.