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Two plesiosaurs: Plesiopleurodon and Luskhan, enter the LRT

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This post had its genesis
with a viewing of a YouTube video hosted by paleontologists Valetin Fischer re-describing two plesiosaurs, Luskhan (Fig. 1), and Plesiopleurodon, (Fig. 2), using µCT scanning techniques. He labeled the former a polycotlid-like pliosaurid, and the latter a pliosaur-like polycotylid. Both apt.

My beef today
is with the presentation technique of Fischer et al. 2017, who took a perfectly good dorsal view photograph of a specimen and inked in the sutures they identified. That provided no opportunity to wonder if they got it all right or not. A better solution would be to provide a photograph and a side-by-side diagram, or to colorized a photograph in which the sutures remain visible. The authors’ attempt at improving their presentation had the opposite effect.


Figure 1. The nearly complete skeleton of Luskhan, a less dangerous relative to Kronosaurus in the LRT. Top: the published tracing of the dorsal skull is what NOT to do. The authors took a perfectly good photo and ruined it with pen lines (top), obscuring the sutures from proper view. If you’re going to outline bones with ink, keep the diagram separate from the photo. Or use transparent colors without outlines on the bones. Below the original is a revision with standardized colors applied. The overall diagram is modified here from the original.

Luskhan itilensis
(Fischer et al. 2017, YKM 68344/1 262, Early Cretaceous, 6.5m in length) nests as a sister to Kronosaurus in the LRT, but with a slender snout and short teeth not adapted to large prey.


Figure 2. Plesiopleurodon skull in several views, here colorized.

Plesiopleurodon wellesi
(Carpenter 1996; 71cm skull length; NHMUK R4058; Late Cretaceous) is a traditional enigma, here nesting at the base of the pliosaurids, derived from Plesiosaurus. Note the elongate snout and longer premaxilla. The prefrontals contact the premaxilla, separating the nasals from the frontals.


Figure 3. Subset of the LRT focusing on plesiosaurs. There is little to no strangeness or convergence here. Plesiopleurodon is the first of the plesiosaurs with a longer skull.

Here
in the large reptile tree (LRT, 1889+ taxa, subset Fig. 3) Luskhan nests with Kronosaurus. Pliesiopleurodon nests at the base of the short-neck pliosaurs, derived from long-necked Plesiosaurus.

References
Carpenter K 1996. A review of short-necked plesiosaurs of the Western Interior, North America. Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen 201(2):259-287.
Fischer et al., (8 co-authors) 2017. Plasticity and Convergence in the Evolution of Short-Necked Plesiosaurs, Current Biology, http://dx.doi.org/10.1016/j.cub.2017.04.052

wiki/Luskhan
wiki/Plesiopleurodon



Source: https://pterosaurheresies.wordpress.com/2021/07/23/two-plesiosaurs-plesiopleurodon-and-luskhan-enter-the-lrt/


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