Last night I actually read Ji et al. 2013 and discovered I was confirming their earlier findings on Sander et al. 2011 — not by matching their tree topology, which matches certain nodes and not others — but in disputing the Sander et al. lumping of several ichthyosaurs under Shastasaurus. Even so, Ji et al. lumped those several ichthyosaurs together in the same clade as Shastasaurus, which the large reptile tree cannot confirm. I also learned that Shang and Li 2009 reassigned Guizhouichthyosaurus tangae (Cao et Luo in Yin et al. 2000) to Shastasaurus, which was an error on their part. Updates have been made.
Sometimes paleontologists like to name new species.
Other times paleontologists consider their latest discoveries variations on old themes. So they lump them together and don’t give them new generic names, perhaps only new species names.
Sander et al. (2011) introduced two short-snouted 33-foot (10 m) ichthyosaurs they suggested were suction feeders possibly tied to a Late Triassic minimum in atmospheric oxygen (fewer fish = more squid). Suction feeding was hypothetically accomplished by rapid retraction of the tongue in a tube-like snout, like a modern-day beaked whale. This news was covered by Brian Switek writing for Smithosonian magazine online here. Previously one of these was described under the name Guanlingsaurus linage (Fig. 1). These two toothless ichthyosaurs were lumped by Sander et al. (2011) with the holotype of Shastasaurus pacificus (Merriam 1895, 1902, 1908; UCMP 9017, Figs. 1, 4) from California. The UCMP specimen did not preserve a rostrum, so whether or not it had teeth or even a short rostrum was considered unknown.
Figure 1. Two Shastasaurus specimens once considered suction feeders. The 3108 specimen nests with the very primitive Mikadocephalus. The 3107 specimen nests with the Cymbospondylus and S. sikkannensis.
The Sander et al. phylogenetic analysis nested the three specimens together (Fig. 2) despite their apparent differences, giving them or retaining their individual species names.
Figure 2. From Sander et al. 2011, phylogenetic relationships of Shastasaurus.
This cladogram represents the strict consensus of 72 most parsimonious trees. Differences in topology among MPTs are mainly found among the outgroup taxa and the basal Merriamosauria. Derived Parvipelvia were part of the analysis but were omitted for clarity.
On a side note,
Motani et al. (2013) formerly dismissed/retracted the suction-feeding hypothesis with Sander on the list of authors. These researchers found that ichthyosaurs did not possess the neccesary rostral features, like elaborate hyoids and a tube-like snout, that allow suction-feeding to work. That story was again covered by Brian Switek, but this time for NatGeo here.
Figure 3. A fifth putative Shastasaurus, S. tangae, IVPP V11853.was erroneously reassigned by Shang and Li 2009. Originally named Guizhouichthyosaurus tangae by Cao et Lu in Yin et al. 2000, it nests close to Ichthyosaurus in the large reptile tree.
A fourth putative Shastasaurus
S. tangae was reassigned by Shang and Li (2009; Fig. 5, originally named Guizhouichthyosaurus tangae by Cao et Lu in Yin et al. 2000,). Distinct from the others it had a long toothy rostrum. More than 60 presacral vertebrae were present and the tail was ventrally bent.
Figure 4. The giant sixth putative Shastasaurus, S. (originally Shonisaurus) sikanniensis (renamed by Sander et al 2011). It nests with the toothless 3107 specimen (Fig. 3).
A fifth putative Shastasaurus,
S. skanniensis (Nicholls and Manabe 2004, originally Shonisaurus, renamed by Sander et al. 2011) was the giant of the group at 21 meters (69 feet; Fig. 6). It did not preserve a rostrum, but no teeth were found with what remained of the giant jaws.
Figure 5. Two Shastasaurus specimens, one of them the holotype, compared to the related and much smaller Hupehsuchus and Eohupehsuchus.
The sixth (but by no means final) Shastasaurus
S. alexandrae (Merriam 1902) is a large ichthyosaur known from a 3D jumbled specimen preserving the material between the nostrils and pectoral girdle only.
Figure 6. Cladogram of ichthyosaurs and kin with five putative Shastasaurus specimens in pink. Bootstrap scores shown. Note: Shastasaurua tangae was erroneously reassigned from Guizhouichthyosaurus tangae (Cao and Luo 2000) by Shang and Li 2009.
Adding these six Shastasaurus specimens
to a selection of basal ichthyosaurs splits them into four clades. The first two specimens, S. alexandrae and S. pacficus surprisingly nested basal to hupehsuchids, drawing these odd-little fellows into the midst of the Ichthyosauria (or not depending on your definition).
The 3108 specimen nests with the basal ichthyosaur Mikadocephalus.
S. tangae nests with the derived Ichthyosaurus and Ophthalmosaurus.
The toothless 3107 specimen and S. sikanniensis nested with the toothy Cymbospondylus.
So toothlessness did not arise only once or twice within the Ichthyosauria, but several more times. The hupehsuchidae may not be as odd and isolated as was once believed. Some of these taxa need new generic names.
A look at the 228th character trait, related to size, indicates that small taxa appeared at the base of each major radiation of ichthyosaurs and proto-ichthyosaurs, as in pterosaurs and other major clades that experienced phylogenetic miniaturization.
I’m not using ichthyosaur-specific character traits here, but relying on the same 228 characters that lumped and split the rest of the 530 taxa now populating the large reptile tree. But the high bootstrap scores are encouraging.
Others who have produced cladograms
of ichthyosaur relationships have not employed mesosaurs and Wumengosaurus as outgroups. I also find it odd that Sander et al. did not recover a closer relationship between S. pacificus and Hupehsuchus, despite their many similarities. Perhaps it was their resistance to employing Thaisaurus, a basal ichthyosaur.
Ji C, Jiang, DY, Motani R, Hao W-C, Sun ZY, and Cai T 2013. A new juvenile specimen of Guanlingsaurus (Ichthyosauria, Shastasauridae) from the Upper Triassic of southwestern China. Journal of Vertebrate Paleontology 33 (2): 340.
McGowan C 1996. A new and typically Jurassic ichthyosaur from the Upper Triassic of British Columbia. Canadian Journal of Earth Sciences 33: 24–32.
Merriam JC 1895. On some reptilian remains from the Triassic of northern California. Am J Sci, 50(3): 55-57.
Merriam JC 1902. Triassic Ichthyopterygia from California and Nevada. Univ Calif Publ, Bull Dept Geol, 3(4): 63-108.
Merriam JC 1908. Triassic Ichthyosauria, with special reference to the American forms. Mem Univ Calif, 1: 1-196.
Motani R, Ji C, Tomita T, Kelley N, Maxwell E., Jiang D., Sander P 2013. Absence of suction feeding ichthyosaurs and its implications for Triassic mesopelagic paleoecology. PLoS ONE. 8, 12: e66075. doi:10.1371/journal.pone.0066075.
Nicholls EL, Manabe M 2004. Giant ichthyosaurs of the Triassic – a new species of Shonisaurus from the Pardonet Formation (Norian: Late Triassic) of British Columbia. Journal of Vertebrate Paleontology 24 (3): 838–849.
Sander PM, Chen X-C, Cheng L and Wang X-F 2011. Short-snouted toothless ichthyosaur from China suggests Late Triassic diversification of suction feeding ichthyosaurs. PlosOne DOI: 10.1371/journal.pone.0019480
Shang Q-H and Li C 2009. On the occurrence of the ichthyosaur Shastasaurus in the Guanling biota (Late Triassic), Guizhou, China. Vertebrata PalAsiatica 2009(7):178-193.
Yin G-Z, Zhou X, Cao Y, Yu Y and Luo Y 2000. A preliminary study on the earlyLate Triassic marine reptiles from Guanling, Guizhou, China. Geology-Geochemisty 28(3):1–23 (Chinese with English abstract).