This week's big news was Baminornis (Chen et al., 2025), a bird with a supposed large pygostyle from a well dated Tithonian locality (Nanyuan Formation- in the same layer Fujianvenator is from, which emerges sister to Serikornis in the Lori matrix btw). This supposed pygostyle is at least 74% of ilial length, so is not like the comparatively undeveloped fused structures in Deinocheirus, Beipiaosaurus and some oviraptorosaurs. Instead, it's out of place in the Jurassic and out of place phylogenetically given the authors recover Baminornis as just crownward of Archaeopteryx in both the TWiG and O'Connor matrices, stemward of Jeholornis in both. Chen et al. call this "The observed ‘chaos’ of bird tail evolution" and note the Late Jurassic Solnhofen and Yanliao Biotas lack short-tailed birds, thus concluding "It would be intriguing to explore why no short-tailed avialans have been found from these two localities."
The obvious answer is- that structure in Baminornis isn't a pygostyle. It's pretty obviously a synsacrum. The first thing to note is that despite the clean, rounded end drawn in their Extended Data Fig. 1b (see below), the posterior end is clearly broken in the photo Extended Data Fig. 1a above it, so it may have included six or more vertebrae in life. This isn't an argument for either identification, as Archaeopteryx can have five (most specimens) or six (Thermopolis specimen), while Jeholornis has six. In any case, the arguments for identification as a synsacrum are-
1. Size. As noted above, even with its broken posterior end, the structure in Baminornis is 74% of ilial length despite only including five vertebrae. As Chen et al. say "As in confuciusornithids, the pygostyle is longer than metacarpal II, but the opposite is true in most other early avialans such as Sapeornis and Cratonavis" yet confuciusornithids fuse eight-ten vertebrae to make their longer pygostyle and enentiornithines usually fuse more as well (e.g. Iberomesornis, IVPP V15664) (Rashid et al., 2018). Excluding the very different elongated distal caudals of basal paravians, bird caudals are short, so fusing so few together wouldn't result in such a large structure. Using Wang and O'Connor's (2017) Table 1, the pygostyle/femoral length ratio of Sapeornis with 4 fused caudals is 19-29%, Jehol euornithines' with 3-5 fused caudals is 19-35%, confuciusornithids' is 49-71%, and non-pengornithid Jehol enantiornithines' is 39-80%. Baminornis' ratio is >44%.
2. Curvature. Chen et al. state the bone "curves dorsally—reminiscent of, but to a lesser extent than, the derived condition in ornithuromorphs in which the pygostyle is characteristically plough-shaped. By contrast, the bone is straight in other early avialans such as confuciusornithids and enantiornithines." It actually looks more curved than their figured 'ornithuromorphs' (= euornithines) Bellulornis and 'Abitusavis' (= Yanornis), but in any case I reinterpret the concave side to be ventral, which matches many paravians from Saurornitholestes to Ornithodesmus to Zhyraornis. It's usually not obvious in Lagerstatten birds where the synsacrum is often preserved in dorsoventral view or below the ilium in lateral view. It's of course possible that a basal avialan pygostyle could be curved either dorsally or ventrally, but it definitely matches the listed paravian synsacra more than any pygostyle I can think of.
3. Supposed proximoventral processes. The authors say "The pygostyle has a pair of proximally distributed ventrolateral processes which terminate posterior to the proximal articular facet", but they are asymmetrical with the supposed left one being a small prong while the supposed right one is a more posteriorly placed and ventrally projected, longer blade. Taphonomy maybe, but you know what these perfectly match if the structure is flipped dorsoventrally? A prezygapophysis and a neural spine lamina, respectively. Chen et al. note Sapeornis lacks these processes and obviously Jeholornis doesn't have them, so it would be less parsimonious if Baminornis converged with pygostylians in developing them (or they were lost in Sapeornis) anyway.
4. Anteroposteriorly concave vertebral edges. At least the first, third and fourth vertebrae have concave edges along the concave side. This matches the ventral edges of each centrum in most archosaurian vertebrae, while the dorsal edge of pygostyles is either smooth or convex at each vertebra to reflect the neural spines (see Chen et al.'s euornithine examples, or Fukuipteryx).
5. Taphonomic position. Under my interpretation, you only have to rotate the synsacrum 90 degrees instead of also flipping it upside down, plus the only well-preserved free caudal is next to its posterior end. These aren't in themselves good arguments, since the right pectoral girdle, ischium etc. obviously suffered more displacement than just rotation, but they do match this hypothesis better. Similarly, the synsacrum would be missing otherwise, and as a centrally placed element we would expect it to stay close to the skeleton more than the distal tail.
6. Parsimony, as mentioned above. A pygostyle in an early-diverging avialan like this introduces homoplasy, especially such an elongate one with proximoventral processes like confuciusornithids and most enantiornithines, unlike the shorter and simpler form in Sapeornis and basal euornithines. Whereas a synsacrum is exactly expected.
Once you see the synsacrum in Baminornis, it's pretty hard to un-see. The details all make sense, like the simple and smooth centra and the more complicated topology where the sacral ribs are. Indeed, I'd say if it were a pygostyle my orientation would still make more sense with the centra, prezygapophysis and neural spine lamina, but the vertebrae would still be too big, and why wouldn't it be a synsacrum at that point since it looks just like one and a pygostyle in this taxon is unexpected. Another case of supposed giant Nature falling for sensationalism *cough Oculudentavis cough*.
References- Nessov, 1992. Mesozoic and Paleogene birds of the USSR and their
paleoenvironments. In Campbell (ed.). Papers in Avian Paleontology
Honoring Pierce Brodkorb. Natural History Museum of Los Angeles County
Science Series. 36, 465-478.
Howse and Milner, 1993. Ornithodesmus - a maniraptoran theropod
dinosaur from the Lower Cretaceous of the Isle of Wight, England.
Palaeontology. 36, 425-437.
Wang and O'Connor, 2017. Morphological coevolution of the pygostyle and tail feathers in Early Cretaceous birds. Vertebrata PalAsiatica. 55(4), 289-314.
Rashid, Surya, Chiappe, Carroll, Garrett, Varghese, Bailleul, O’Connor, Chapman and Horner, 2018. Avian tail ontogeny, pygostyle formation, and interpretation of juvenile Mesozoic specimens. Scientific Reports. 8:9014.
Chen, Wang, Dong, Zhou, Xu, Deng, Xu, Zhang, Wang, Du, Lin, Lin and Zhou, 2025. Earliest short-tailed bird from the Late Jurassic of China. Nature. 638(8050), 441-448.
If Baminornis lacks a pygostyle, does it change its phylogenetic position?
ReplyDeleteHad a similar concern, since we had similar issues with Shuilingornis fused vertebrae, with some coauthors believing they are sacrals and others instead considering them fused caudals. Provisionally, in my matrix the pygostyle characters are coded "?" in this taxon.
ReplyDeleteIn the case of Shuilingornis, as I write on the Database "the length matches a synsacrum and is much greater than euornithine pygostyles." Since it's obviously a euornithine, I don't see any reason a pygostyle identity would be favored. Just like if you had a bird with an isolated tiny manual phalanx that anatomically could be III-1 or IV-1, you'd be pretty safe calling it III-1.
DeleteI use to not code an element if the identification is phylogenetically-significant and might impact the topology. I mean, even if I agree that a sacral identity is more likely, could we exclude it being a basal euornithine with a long pygostyle? I'd avoided the risk of a circular reasoning (it's euornithine so it could not have a long pygostyle) and provisionally did not code that element.
DeleteThat said, my preliminary analysis not coding the ?pygostyle places Baminornis as sister taxon of Jeholornithiformes + Pygostylia. So, I am inclined to not consider that element a fused distal tail.
Yet, even removing the pygostyle from the preserved bones, the scapulocoracoid and hand are quite ornithothoracine in overall morphology.
ReplyDeleteA Jurassic pygostylian would be cool. But unfortunately it doesn't seem like this one is real. Still an interesting taxon either way, going by the tree in the paper (and Cau's result he mentioned in the comments) it could be the most derived known Jurassic bird.
ReplyDeleteAny thoughts on Mexidracon or the Bissekty ornithomimosaur getting its own genus?
ReplyDeleteI was having this same conversation with colleagues on campus. I'm not sure how it got through peer-review as a pygostyle.
ReplyDelete