Yet another entry from the fieldguide (though substantially updated and enlarged)…
What might be one of the strangest Cretaceous birds was described in 2004. I refer of course to Aberratiodontus wui of the Jiufotang Formation of Liaoning Province, China.
Named for a near-complete specimen, Aberratiodontus was regarded by its describers (Gong et al. 2004) as one of the most unusual of the enantiornithines or ‘opposite birds’ (though read on!). In keeping with the tradition of Linnaean taxonomy, they gave it its own eponymous ‘family’ and ‘order’ (these being ‘Aberratiodontuidae’ and ‘Aberratiodontuiformes’, respectively). If you know anything about fossil birds you’ll know that the useless practice of creating redundant higher taxa for distinctive species has been all too common [adjacent skull reconstruction from Gong et al. (2004): read on].
While the Aberratiodontus holotype specimen is reasonably complete [holotype specimen LHV0002 shown below (though LHV0001a and LHV0001b are given as the accession numbers in the body of the text), from Gong et al. (2004)], many of its bones are poorly preserved and have broken surfaces, making confident interpretation rather difficult. In fact, many of the claims made in the descriptive paper are arguable and unlikely to be correct. Gong et al. (2004) identified prefrontals and a robust postorbital, for example, but the excellent photos they provided show that the ‘prefrontals’ may just be parts of the nasals (prefrontals actually seem to be absent in birds anyway) while the ‘postorbital bar’ is not definitely complete as they showed in their reconstruction (that’s rather ironic, as the ‘diapsid status’ of the skull was used in the title, as if it were one of the most significant details of the specimen). A complete postorbital bar is seen elsewhere in Mesozoic birds: confuciusornithids and the enantiornithine Protopteryx possess it; Archaeopteryx seems to have one (Tischlinger 2005), and at least a few others do too.
The relatively short toe claws suggest that Aberratiodontus was not a habitual percher, and the proportions of its hindlimbs and its relatively long neck suggest that it was a terrestrial forager. Its large sternum and wing proportions indicate that it was a capable flier. Perhaps it preyed on large arthropods, small vertebrates and other animals (it was reasonably large for a Cretaceous bird, with a skull about 58 mm long and neck 88 mm long). I make all of these assertions without having done any of the appropriate quantitative comparisons, however…
Its most unusual feature is its dentition: it was regarded by its describers as being heterodont. Gong et al. (2004) claimed that 21 teeth can be seen to line each maxilla, but again this is difficult to be confident about, as only a handful of maxillary teeth are actually visible (somewhere round about 8 or 9). Poor preservation of the premaxillary margins mean that the number there can’t be asserted with confidence either (though 4 teeth in each premaxilla is a good guess). There are about 24 teeth in each dentary. This is unusually high (12 or so is more typical for toothed Mesozoic birds [many had less]) and could be diagnostic. What is obvious is that the teeth at the front of the maxilla are smaller than the premaxillary teeth, and than those in the posterior part of the maxilla. The teeth are reported to differ in terms of possession or absence of carinae, but the terminology used in the paper is highly idiosyncratic and difficult to make sense of [the holotype skull is shown here: you might like to compare it to Gong et al.'s reconstruction, shown at the very top].
The sternum of Aberratiodontus has strongly divergent posterolateral processes that have slightly expanded ends. This is reminiscent of what’s seen in various enantiornithines and other Mesozoic birds, though the posterolateral processes aren’t normally as divergent as they are in Aberratiodontus [speculative life restoration of Aberratiodontus shown below].
At the risk of spending more time commenting on the paper than on the taxon it describes, I must also note that the authors interpreted the style of tooth replacement inferred in Aberratiodontus as being more similar to that of crocodilians than that of non-avian theropods; on the basis of this, the authors implied that birds might not be dinosaurs*. I think they were influenced by the fact that the new teeth of Aberratiodontus form concavities on the lingual sides of the older teeth’s roots during their development. I don’t understand why they think this tells you anything about the affinities of birds (exactly the same developmental pattern is seen in maniraptoran theropods).
* Actually, they wrote “It would be a new question about the dinosaurian hypothesis of bird origins accepted widely that meet a new challenge” (Gong et al. 2004, p. 6).
I note a strong and consistent link between (1) bizarre interpretations and poor descriptions and (2) BAND support (BAND = the Birds Are Not Dinosaurs movement). Sorry if that sounds mean, but it’s true. If I were bolder and prepared to stick my neck out, I think I would say that the BAND movement only remains alive because of bad science, not because there are wise, ingenious authors making novel observations and forming strong arguments.
While it’s true that enantiornithines have proved to be a diverse, speciose clade, the problem with such groups is that people soon come to assume that any and all new species are members of said groups. That may be exactly what’s happened here. Enantiornithines can be identified by such characters as their distally projecting third metacarpal, distinctively large posterior trochanter on the femur, and narrow fourth metatarsal. Yet none of these features are visible, or have been reported, in Aberratiodontus.
It’s… it’s… another specimen of Yanornis?
Have any other opinions been expressed on Aberratiodontus since its description? Zhou et al. (2008) – in a footnote – doubted most of Gong et al.’s conclusions, and argued that Aberratiodontus can be excluded from Enantiornithes on the basis of its large cnemial crest. They went even further, and stated that Aberratiodontus is not a distinct taxon at all, but is actually synonymous with Yanornis [holotype of Yanornis martini shown here, from Zhou & Zhang (2001)]. Cau & Arduini (2008) included Aberratiodontus within a cladistic analysis of Mesozoic birds, and found it to be a close relative of Yanornis (see the comments for more on this).
And Yanornis isn’t an enantiornithine – it’s a member of a small clade of Cretaceous birds that are more closely related to neornithines than enantiornithines are (Clarke et al. 2006). Yixianornis grabaui and Songlingornis linghensis are the other identified members of the group: the existing names Yanornithidae Zhou & Zhang, 2001 or Yanornithiformes Zhou & Zhang, 2001 could be used for the group, but Clarke et al. (2006) chose not to use any name for the clade for some reason. And there’s a lot that could be said about the ‘yanornithids’, but it’ll have to wait to another time. The proposal that Aberratiodontus might be a misidentified member of this group is an interesting one that looks likely to be correct, but more work is needed to determine whether Aberratiodontus is a distinct taxon within this clade – as supported by Cau & Arduini (2008) – or synonymous with Yanornis, as suggested by Zhou et al. (2008).
For previous articles on enantiornithines and other Mesozoic birds see..
- Tet Zoo picture of the day # 24 [on archaeopterygids]
- The new Crato Formation enantiornithine
- A stunning new Mesozoic bird… well, new-ish
- Epidexipteryx: bizarre little strap-feathered maniraptoran
- Long and Schouten’s Feathered Dinosaurs, a review
- Cyril Walker
- The Mesozoic birds with weird, plastic-strip-style tail structures
- Alexornis and other ‘alexornithiforms’
And given that I’ve said negative things about the BAND movement, you might like to see…
- Publishing with a hidden agenda: why birds simply cannot be dinosaurs
- … and this comment on the James & Pourtless paper.
Refs – –
Cau, A. & Arduini, P. 2008. Enantiophoenix electrophyla gen. et sp. nov. (Aves, Enantiornithes) from the Upper Cretaceous (Cenomanian) of Lebanon and its phylogenetic relationships. Atti della Societa Italiana di Scienze Naturali e del Museo Civico di Storia Naturale in Milano 149, 293-324.
Clarke, J. A., Zhou, Z. & Zhang, F. 2006. Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of Yixianornis grabaui. Journal of Anatomy 208, 287-308.
Gong, E., Hou, L. & Wang, L. 2004. Enantiornithine bird with diapsidian skull and its dental development in the Early Cretaceous in Liaoning, China. Acta Geologica Sinica 78, 1-7.
Tischlinger, H. 2005. Neue Information zum Berliner Exemplar von Archaeopteryx lithographica H. v, Meyer 1861. Archaeopteryx 23, 33-50.
Zhou, Z., Clarke, J. & Zhang, F. 2008. Insight into diversity, body size and morphological evolution from the largest Early Cretaceous enantiornithine bird. Journal of Anatomy 212, 565-577.
– . & Zhang, F. 2001. Two new ornithurine birds from the Early Cretaceous of western Liaoning, China. Chinese Science Bulletin 46, 1258-1264.