A hupehsuchid-mimic mesosaur with a duckbill: YAGM V 1401

Cheng et al. 2019
bring us news of a new armored Early Triassic (250 mya) specimen (YAGM V 1401; Figs. 1,2) they attribute to the armored Early Triassic hupehsuchid, Eretmorhipis carrolldongi (Fig. 5; holotype WGSC V26020; Chen et al. 2015). The holotype specimen lacks a skull. The authors considered the new YAGM specimen, complete with skull, conspecific with the WGSC holotype of Eretmorhipis, noting it had small eyes relative to the body and a duckbill-like rostrum.

Instead
the large reptile tree (LRT, 1389 taxa; Fig. 3) nests the YAGM specimen as a derived mesosaur, 32 steps away from the WGSC holotype of Eretmorhipis in the clade of hupehsuchids. The authors assumed Eremorhipis was a hupehsuchid because it looks like one. It really does. That’s easy to see. They are a close match when you eyeball them. Unfortunately Cheng et al. 2019 did not test that assumption using a phylogenetic analysis that included mesosaurs, which nest basal to hupehsuchids (Fig. 3). Once again, it’s taxon exclusion.

The eyes are actually large relative to the skull,
in the new YAGM specimen (Fig. 2), but the skull is tiny relative to the body. The rostrum is narrow relative to the cranium. Typically that enables binocular vision. The authors did not provide a reconstruction of the skull.

The wide, flat rostrum of the YAGM specimen has an open central area,
like Ornithorhynchus the duckbill platypus (Fig. 4) by convergence. Given that bit of morphology the authors sought to extend the duckbill analog by reporting small eyes relative to the body in the YAGM specimen. That gives them an irrefutable headline, but a little mis-leading given the reconstruction (Fig. 2). The authors suggest Eretmorhipis used mechanoreceptors in the rostrum instead of eyesight. They report, “Apparent similarities include exceptionally small eyes relative to the body, snout ending with crura with a large internasal space, housing a bone reminiscent of os paradoxum, a mysterious bone of platypus, and external grooves along the crura.” That’s pretty awesome! Larry Martin would have enjoyed this list of convergent traits. I have no idea how the ox paradoxum bone fit in the YAGM specimen skull. So it remains a paradox.

Figure 1. Eremorhipis in situ and line drawing from Cheng et al. 2019. Colored here using DGS methods. Some bones are reidentified here. See figure 2 for matching colors.

Figure 1. Eremorhipis in situ and line drawing from Cheng et al. 2019. Colored here using DGS methods. Some bones are reidentified here. See figure 2 for matching colors.

The authors created a chimaera
when they added the hands and feet of the holotype WGSC specimen to the new YAGM specimen in their Nature paper. Since the two specimens are not related, that is going to cause confusion. No matter how sure they were, the authors needed a valid phylogenetic analysis to nest their new specimen, now requiring a new generic and specific name.

Figure 2. Reconstruction of Eretmorhipis skull from figure 1, along with in situ specimen and reconstruction from Cheng et al. 2019. Pectoral and pelvic girdles magnified and colored using DGS methods. The skull appears to provide binocular vision due to the narrow rostrum and wide cranium.

Figure 2. Reconstruction of Eretmorhipis skull from figure 1, along with in situ specimen and reconstruction from Cheng et al. 2019. Pectoral and pelvic girdles magnified and colored using DGS methods. The skull appears to provide binocular vision due to the narrow rostrum and wide cranium.

Traditional paleontologists need to catch up to the LRT
and start including thalattosauriforms and mesosaurs whenever they study basal ichthyopterygians, like hupehsuchids. Basal taxa are all closely related and all three taxa include a wide variety of morphotypes, including some that converge.

Figure 3. Subset of the LRT focusing on Mesosauria, Thalattosauriformes and Ichthyopterygia including two specimens referred to Eretmorhipis nesting here apart from one another.

Figure 3. Subset of the LRT focusing on Mesosauria, Thalattosauriformes and Ichthyopterygia including two specimens referred to Eretmorhipis nesting here apart from one another.

It is worth noting
that many mesosaurs, like the SMF R4710 specimen, lack the long, laterally-oriented, comb-like teeth of Mesosaurus. Most mesosaurs have a typical diapsid skull architecture, distinct from the in-filling of the temporal fenestra that Mesosaurus exhibits. Mesosaurs are common in certain Early Permian strata. That provides plenty of time for the highly derived YAGM specimen to evolve by the Early Triassic.

Figure 4. Ornithorhynchus skull with colors added using DGS methods. Note the large opening in the dorsal view of the rostrum, as in Eretmorhipis.

Figure 4. Ornithorhynchus skull with colors added using DGS methods. Note the large opening in the dorsal view of the rostrum, as in Eretmorhipis, by convergence.

It’s also worth noting
that the YAGM specimen has a cleithrum and a ventrally broad clavicle along with an interclavicle and other traits found in mesosaurs, but lacking in hupehsuchids.

Figure 1. The holotype specimen of Eretmorhipis carrolldongi WGSC V26020 compared to the figure drawn form Cheng et al. 2019.

Figure 5. The holotype specimen of Eretmorhipis carrolldongi WGSC V26020 compared to scale to the figure drawn form Cheng et al. 2019 for specimen YAGM V 1401. Cheng et al. created a chimaera when they added the WGSC specimen hands and feet to the new YAGM specimen without first nesting them together in a cladogram. These two specimens do not nest together in the LRT despite the massive convergence. Don’t try to eyeball taxa. Let the software take the bias out of it.

A word to workers: Don’t try to ‘eyeball’ taxa.
Let the phylogenetic software take the bias out of making a taxonomic determination. We’ve seen professional workers make this mistake before by combining diphyletic turtles, whales, seals, and by miss-nesting Vancleavea, Lagerpeton, Chilesaurus, Daemonosaurus by taxon exclusion. Let’s not forget those who keep insisting that pterosaurs are archosaurs (virtually all traditional workers), again by omitting pertinent taxa.

Figure 1. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen.

Figure 6. Mesosaurus origins recovered by the LRT. The fossil record appears to be topsy turvy here with the basal taxa appearing 30 million years later. Fossils are rare and discovery is rarer. Things like this sometimes happen. The YAGM specimen is large, like Mesosaurus, but later (at 250 mya) than Thadeosaurus.

References
Chen X-H, Motani R, Cheng L, Jiang D-Y and Rieppel O 2015. A new specimen of Carroll’s mystery hupehsuchian from the Lower Triassic of China. PLoS One 10, e0126024, https://doi.org/10.1371/journal.pone.0126024 (2015).
Cheng L, Motani R, Jiang D-Y, Yan C-B, Tintori A and Rieppel O 2019. Early Triassic marine reptile representing the oldest record of unusually small eyes in reptiles indicating non-visual prey detection. Nature Scientific Reports Published online January 24, 2019.

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.