Better late than never, I’ve only recently gotten hold of Zhou et al.’s paper on the enantiornithine bird Pengornis houi, published online in Journal of Anatomy back in January but now available in hard-copy. I must say that I really dislike the new trend of publishing things in special, online versions prior to their ‘proper’ publication. Anyway… Pengornis (which is from the Lower Cretaceous Jiufotang Formation of Dapingfang, Liaoning, China) is particularly interesting for several reasons…
It’s very well preserved, with one of the best-preserved skulls of any Mesozoic bird [skull shown below, from Zhou et al. (2008)]. Most of the teeth are small with conical, uncurved crowns, but those toward the back are globular and blunt with wear facets. I’d love to know what Pengornis was doing with these weird teeth (stomach contents from other enantiornithines show that they were feeding on such things as small crustaceans and fishes, so perhaps it was crushing arthropods or little gastropods). Secondly, it’s large for a Lower Cretaceous enantiornithine, and as a basal member of the group (Zhou et al. 2008) it suggests that relatively large size was primitive for the clade: most other basal enantiornithines can be loosely described as finch-sized, whereas (with a skull about 55 mm long and a wing skeleton [humerus + ulna + carpemetacarpus] about 170 mm long) Pengornis was perhaps closer in size to a small corvid (warning: this was a guesstimate and I haven’t done any proper size comparisons).
Pengornis is of further interest because it possesses several characters that were previously thought limited to ornithurines*, including a globular humeral head, a hooked acromion on the scapula and heterocoelous cervical vertebrae. Did these characters therefore evolve earlier in bird phylogeny than hitherto thought, or were they convergently acquired by both enantiornithines and ornithurines? First recognised by Cyril Walker in 1981 from incomplete remains from Upper Cretaceous Argentina (Walker 1981), enantiornithines – sometimes called ‘opposite birds’ – have proved to be the most abundant, diverse and widespread group of Mesozoic birds, and over 35 genera are presently recognised.
* The name Ornithurae has a complex history and it’s been used in different ways by different authors. Here I follow the majority of bird workers in using it for the node-based clade that includes hesperornithines, crown-group birds, and all descendants of their most recent common ancestor [for help, here's a bird phylogeny. From one of Tom Holtz's lectures: full-size version here].
Pengornis is one of several recently described Cretaceous birds which have shown that body size, limb proportions, flight specialisations, and ecology overlapped among the several bird clades that existed during the Early Cretaceous (confuciusornithids, enantiornithines and basal ornithurines like the yanornithids). We can still find characters that allow these groups to be reliably distinguished, but it is becoming increasingly difficult to maintain claims that they were occupying different ecological niches.
Refs – -
Walker, C. 1981. New subclass of birds from the Cretaceous of South America. Nature 292, 51-53.
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.