Dolichorhynchops enters the LRT with some overlooked bones

To no one’s surprise
Late Cretaceous Dolichorhynchops (Fig. 1) nests with the similar and coeval Trinacromerum (Fig. 2) in the large reptile tree (LRT, 1432 taxa, subset Fig. 4). Both nest between plesiosaurs and pliosaurs in the LRT, all derived from a sister to Early Jurassic Rhomaleosaurus.

Figure 1. The FHSM VP 404 specimen of Dolichorhynchops with modification to the pectoral girdle and hypothetical addition of chevrons. Gastalia are not shown, but were probably present based on phylogenetic bracketing.

Figure 1. The FHSM VP 404 specimen of Dolichorhynchops with modification to the pectoral girdle and hypothetical addition of chevrons. Gastalia are not shown, but were probably present based on phylogenetic bracketing.

O’Keefe 2004 reports,
“For most of the twentieth century the polycotylids were classified as true pliosaurs, because they possessed short necks, large heads, and other proportional differences in common with Jurassic pliosauroids such as Peloneustes and Liopleurodon (O’Keefe, 2002; see O’Keefe, 2001 for taxonomic review). In 1997, however, Carpenter questioned the monophyly of the Pliosauroidea as traditionally defined and instead posited a sister-group relationship between the Polycotylidae and the Elasmosauridae, a view also championed by Bakker (1993). O’Keefe (2001) performed a cladistic analysis of the clade Plesiosauria, and found that the traditionally-defined Pliosauroidea were indeed polyphyletic, although a sister-group relationship with the Elasmosauridae was not supported. Instead, O’Keefe found that the Polycotylidae were a derived group of cryptocleidoid plesiosauroids, most closely related to Jurassic taxa such as Tricleidus and Cryptoclidus. The novel phylogenetic position of the Polycotylidae found by O’Keefe (2001) renders the Pliosauroidea polyphyletic as traditionally defined.”

At present
the LRT nests only two polycotylids and they nest together between plesiosaurs and pliosaurs, as superficial appearances suggest. Other poycotylids have not been tested.

From O’Keefe 2004:
POLYCOTYLIDAE Williston 1908 revised definition: A taxon including Polycotylus, Edgarosaurus, DolichorhynchopsTrinacromerum, their most recent common ancestor, and all descendants.

Figure 3. The plesiosaur Trinacromerum has a parietal crest, but it was laterally packed with jaw muscles.

Figure 2. The plesiosaur Trinacromerum has a parietal crest, but it was laterally packed with jaw muscles.

Dolychorhynchops osborni (Williston 1902; Late Cretaceous, 80mya; FHSM VP-404) is a sister to Trinacromerum with a narrow, flat skull. Below, the skull was inappropriately made wider and taller and several bones were overlooked by O’Keefe. Note the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians, overlooked by prior workers. Here hypothetical hemals were added. Gastralia were likely present.

FIgure 2. Skull of Dolichorhynchops along with tracing by Carpenter and reconstruction by O'Keeefe, which seems overblown. Both workers overlooked the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians.

FIgure 3. Skull of Dolichorhynchops along with tracing by Carpenter and reconstruction by O’Keeefe, which seems overblown in width and height, given a constant length. Both workers overlooked the palatal extension of the premaxilla to the internal nares, as in other eusauropterygians. Always best to check the fossils because workers sometimes overlook bones, sutures, etc.

In figure three above,
several bones were overlooked by prior workers. Others have new outlines/sutures. Of particular interest, the premaxilla in palatal view (yellow) extends to the internal nares (red), as in other eusauropterygians.

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

Figure 4. Subset of the LRT focusing on Eusauropterygians (pachypleurosaurs, nothosaurs, plesiosaurs and kin).

Luckily,
someone made a Dolichorhynchops fantasy/documentary, initially designed for 3D IMAX theatres, I think. Here it is on YouTube:

While watching…
ask yourself if the creators/producers/animators actually used the cuter-looking Trinacromerum (Fig. 2), or the inflated O’Keefe restoration (Fig. 3), rather than the flat-headed FHSM VP404 holotype of Dolichorhynchops (Fig.1), which has the cuter name and nickname “Dolly”.


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.
O’Keefe FR 2001. A cladistic analysis and taxonomic revision of the Plesiosauria (Reptilia: Sauropterygia). Acta Zoologica Fennica 213:1-63.
O’Keefe FR 2004. On the cranial anatomy of the polycotylid plesiosaurs, including new material of Polycotylus latipinnis Cope, from Alabama. Journal of Vertebrate Paleontology. 24 (2): 326–340.
Williston SW 1902. Restoration of Dolichorhynchops osborni, a new Cretaceous plesiosaur. Kansas University Science Bulletin 1 (9): 241–244.
Williston SW 1908. North American Plesiosaurs: Trinacromerum. Journal of Geology 16(8): http://www.jstor.org/stable/30068152

wiki/Trinacromerum
wiki/Dolichorhynchops

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