Monday, February 5, 2018

Is Eogranivora's pelvis misinterpreted?

We have another new Jehol bird, this one previously misidentified as Hongshanornis by Zheng et al. (2011).  Zheng et al. (2018) describe it as Eogranivora, and while it's one of those split through the slab specimens with almost no external detail, I do wonder about their pelvic interpretation.

The holotype preserves the pelvis in dorsolateral/ventrolateral perspective, with one lower pubic bone pair curving inward to touch at the tips, or nearly do so.  Zheng et al. think these are the pubes, which seems okay at first although they note "A dorsal or dorsomedially oriented bluntly triangular process is present one-third from the proximal end of the pubes."  I can't recall any examples of this in other Mesozoic birds.  Also, there's really nowhere for the ischia to go, with the posterior edge between the postacetabular process and supposed pubis being as well defined as most edges get in this fossil.  Plus the peduncles of the pubis-ilium junction would need to be unusually long.

Pelvis of Eogranivora edentulata holotype counterslab (STM 35-3) in dorsolateral view as drawn by Zheng et al. (2018) (left), interpreted by them (center left) and interpreted by myself (right).  Pelvis of Apsaravis ukhaana (IGM 100/1017) after Clarke and Norell (2002) (center right).  Ilium is blue, pubis is red, ischium is green and mid-dorsal ischial process is yellow [edited with new label for far right].

But if these long bones were ischia instead, that clean posterior edge makes sense.  Also, Mesozoic ornithuromorph ischia almost always have a mid-dorsal process that would match that in Eogranivora.  Finally, the area where the pubis would attach in this case is fragmented and unpreserved ventrally.  I've compared it to Apsaravis above, which has a very subtle mid-dorsal process compared to most.  Apsaravis also lacks pedal digit I as in Eogranivora but no other Mesozoic birds, so I wonder if that, the short caudal transverse processes, fused dentary symphysis (I'm skeptical of this in Apsaravis though) and the slender and elongate ischia indicate a close relationship.

References- Clarke and Norell, 2002. The morphology and phylogenetic position of Apsaravis ukhaana from the Late Cretaceous of Mongolia. American Museum Novitates. 3387, 1-46.

Zheng, Martin, Zhou, Burnham, Zhang and Miao, 2011. Fossil evidence of avian crops from the Early Cretaceous of China. Proceedings of the National Academy of Sciences of the United States of America. 108, 15904-15907.

Zheng, O'Connor, Wang, Wang and Zhou, 2018. Reinterpretation of a previously described Jehol bird clarifies early trophic evolution in the Ornithuromorpha.  Proceedings of the Royal Society of London B. 285, 20172494.