Sachicasaurus: the first giant nothosaur, not a pliosaur

Páramo-Fonseca, Benavides-Cabra and Gutiérrez 2018
described Sachicasaurus (Figs. 1-3, MP111209-1, Barremian (Early Cretaceous) Columbia; estimated 10m in length, 2m skull length), a taxon they thought was a giant pliosaur related to Brauchauchenius (Fig. 2).

Unfortunately
the authors did not consider comparing their discovery to Nothosaurus. The short flippers are the first clue that perhaps they should have done so. Misidentifying several bones was a problem. The large reptile tree (LRT, 1420 taxa) tests each new taxon against all prior taxa, thereby largely avoiding the paleo-sin of taxon exclusion.

Figure 1. Sachicasaurus skull from Páramo-Fonseca et al. 2018, colors added.

Figure 1. Sachicasaurus skull from Páramo-Fonseca et al. 2018, colors added. Some skull bones restored in color. Note the differences in the preorbital region of the skull between the original interpretation drawing and the DGS color applied to the skull photo.

Sachicasaurus vitae (Páramo-Fonseca, Benavides-Cabra and Gutiérrez 2018, 10m in length) was originally considered a short-flippered pliosaur related to long-flippered Brachauchenius characterized by two autopomorphic characters: a very short mandibular symphysis ending at the mid length of the fourth mandibular alveoli and reduced number of mandibular teeth (17-18). Here this giant nests with Nothosaurus (above). Originially several bones were misidentified. The ilium is uniquely bifurcated with a dorsal and posterior process. In dorsal view the mandibles are convex while the maxillae are concave, leaving quite a gap between them.

Figure 2. Sachicasaurus was the size of the pliosaurs, Kronosaurus and Brauchenia, but was related to Nothosaurus. This is the first known giant nothosaur.

Figure 2. Sachicasaurus was the size of the pliosaurs, Kronosaurus and Brauchenia, but was related to Nothosaurus. This is the first known giant nothosaur.

Several bones were originally misidentified.
The former left ‘scapula’ is really the clavicle. The former right ‘scapula’ is really the coracoid. The former sock-shaped ‘radius’ is the tiny scapula.

Figure 3. Sachisaurus pectoral girdle and flippers reconstructed with new identities provided here.

Figure 3. Sachisaurus pectoral girdle and flippers reconstructed with new identities provided here. Pectoral elements are digitally duplicate and flipped left to right.

Real plesiosaurs have long flippers
with more than the usual number of phalanges per digit. By contrast, Sachicasaurus does not have long flippers and It has the plesiomorphic number of phalanges. Yes, the skull is huge and the neck is short. In the LRT those pliosaur-like traits are not enough to attract Sachicasaurus toward the pliosaurs. Note the different interpretations of the skull bones presented here (Fig. 1). The nasals, in particular, are nothosaurian, not pliosaurian.

Figure 4. Data for Nothosaurus for comparison with Sachicasaurus. The interclavicle could easily be lost.

Figure 4. Data for Nothosaurus for comparison with Sachicasaurus. The interclavicle could easily be lost. Note the plesiomorphic number of phalanges on both the manus and pes. Compare these to those in figure 3.

From the same Early Cretaceous formation
three real pliosaurs have been discovered. This would have been the fourth one, except it’s a nothosaur with pliosaur size and proportions by convergence. Earlier we looked at a similar convergence between toothed whales and baleen whales.

Figure 5. Subset of the LRT focusing on the Eusauropterygia, including Sachicasaurus.

Figure 5. Subset of the LRT focusing on the Eusauropterygia, including Sachicasaurus.


References
Páramo-Fonseca ME, Benavides-Cabra CD and Gutiérrez IE 2018. A new large Pliosaurid from the Barremian (Lower Cretaceous) of Sáchica, Boyacá, Colombia. Earth Sciences Research Journal 220(4):223-238. eISSN 2339-3459. Print ISSN 1794-6190.

wiki/Sauropterygia
wiki/Sachicasaurus

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