Yesterday we broached the subject of pliosaur nasal disappearance, replaced by frontals in their place.This is a standard for pliosaur workers, but it may be a tradition/paradigm that needs to be reexamined. At best the nasals and frontals could be fused, with each bone keeping to its typical position, as shown here (Fig. 1), but I haven’t seen good evidence of that yet. I have seen good evidence of the frontal fused to the parietal (Fig. 2). And I have seen evidence for the nasal fusing to the prefrontal. Both solutions or a third solution could be relevant depending on the taxon or clade.
What you will see here is a variety of interpretations, mine among them.
Figure 1. Rhomaleosaurus skull from Smith and Dyke 2008. Bones and orbits colorized here. Nasals in magenta. Note the premaxillae (yellow) tentatively contact the parietals (brown), probably overlying the frontals (white) as in germanodactylid pterosaurs. Cheek bones (squamosals) are missing here, but color shows where they would be in vivo. Smith and Dyke indicate no suture between the nasals and frontals, but do not acknowledge the fusion, if present. Here the nasals might be fused to the prefrontals, but Smith and Dyke show nasals fused to frontals.
Smith and Dyke (2008) label the frontal the frontal, but they also do not label the nasal anywhere. Sutures appear to be present separating them. Either the nasals and frontals are fused or the prefrontals are fused to the nasals. Closer examination or higher resolution would help here. In either case, the frontals have not migrated.
McHenry (2009) writing about Kronosauurs, reported, “The nasal bone is often considered to be absent in plesiosaurians (Brown 1981, Druckenmiller and Russell 2008, Storrs 1993), but Andrews (1913: p42) mentions that “there is some indication that the posterior and outer borders are formed by a small distinct element, which, if actually present, must be regarded as a nasal”.
Andrews also suggested the nasal bone might be fused to the prefrontal (Fig. 2).
Figure 2. Liopleurodon skull from Andrews 1913. Color added. Here either the parietal is broken posterior to the pineal opening or the frontals include the pineal opening or the frontals are fused to the parietals. In either case the nasals are present and identified (magenta), distinct from the fronals (white). Prefrontal palpebrals striped amber. Frontal palpebrals in white. Brown dashed line indicates typical extent of parietal, indicating fusion of the frontals and parietals, but not the frontals to each other.
Andrews (1913) described the skull of Liopleurodon including the nasals. Here the pineal opening is in front of a break marking the main body of the parietals, but there is no hint of a suture anterior to the pineal opening. This indicates a likely fusion of the frontals to the parietals, but not the frontals to each other. As mentioned yesterday, both frontals are still present regardless of fusion, no matter what the fused bones are now called. The bones could be called a frontoparietal, but nobody does that. If the frontal were really absent a transitional taxon would show it reducing to a vestige or sliver first. That’s the way it works when a bone actually disappears.
We have seen bones migrate, as in the central wrist bones migrating to the medial rim in pterosaurs and certain mammals. But we haven’t seen the frontal migrate yet.
Then a few problem drawings: Simolestes, case in point (Fig. 3). What are those long bones medial to the naris and orbit (in pink)? Very odd.
Figure 3. Simolestes with bones colorized, including where bones are missing. Here the nasal appears to rim the upper orbit, a very strange organization. Compare this image to Fig. 2 and the nasals look like they really are the prefrontals tipped by frontal palpebrals. But then, I’m interpreting a drawing, which is always fraught with danger. Send a jpeg of this fossil if you have a decent one and we’ll solve this problem together. Also see Fig. 4, where the bones rimming the orbit are indeed the prefrontals.
Perhaps the actual fossil skull of Simolestes can clear the air (Fig. 4). It appears to have all the skull roofing bones in their standard positions. But, then, this is a lateral view, not a dorsal view as above (Fig. 3).
Figure 4. Simolestes fossil skull with a few more roofing bones present. Everything appears to be in the right place. Perhaps some sort of fusion is present between the nasal the prefrontal.
Then there’s Dolichorhynchops (Fig. 5). O’Keefe does not identify the nasal, but fuses the frontal and nasal and his prefrontal is smaller than in sister taxa. He gives the palpebral process to the frontal. In my interpretation all bones are in their standard positions and sizes, including the previous unidentified postfrontal.
Figure 5. Dolichorhynchops skull (above), interpreted here (color) and interpreted by O’Keefe (drawing). O’Keefe does not identify the nasal but fuses the frontal and nasal and his prefrontal is smaller than in sister taxa. In my interpretation all bones are in their standard positions and sizes.
Carpenter’s take on the Dolichorhynchops skull is similar in most respects to that of other workers, but different in a few details (Fig. 6).
Figure 6. Dolichorhynchops UCM 35059 by Ken Carpenter (1996), colorized to show standard placement of bones based on sister taxa. Here the premaxilla does indeed tentatively contact the parietal, a trait very few tetrapods, other than germanodactylid pterosaurs share. Carpenter’s SPO is his supraorbital, otherwise referred to as a postfrontal in previous figures. Carpenter gives the palpebral process partly to the prefrontal (PF), but labels the portion anterior to the orbit the lacrimal, different than all other workers do and did. No nasal is identified (colored pink here).
So, there’s some work to do here.
References
Andrews CW 1913. A descriptive catalogue of the Marine Reptiles of the Oxford Clay, Part II. BM(NH), London.
Benson RBJ, Evans M, Smith AS, Sassoon J, Moore-Faye S, Ketchum HF and Forrest RF 2013. A Giant Pliosaurid Skull from the Late Jurassic of England. PLOSOne online here.
Carpenter K. 1996. A Review of short-necked plesiosaurs from the Cretaceous of the western interior, North America”. Neues Jahrbuch für Geologie und Paläeontologie Abhandlungen (Stuttgart) 201 (2): 259–287.
McHenry CR 2009. ‘Devourer of Gods’ – The palaeoecology of the Cretaceous pliosaur Kronosaurus queenslandicus. Ph.D. dissertation, University of Newcastle, 616 pp. online and download here.
O’Keefe FR 2004. Cranial anatomy and taxonomy of Dolichorhynchops bonneri new combination, a polycotylid (Sauropterygia: Plesiosauria) from the Pierre Shale of Wyoming and South Dakota. Marshall University. Marshall Digital Scholar 1-1-2008, 26 pp. online here.
Smith AS and Dyke GJ 2008. The skull of the giant predatory pliosaur Rhomaleosaurus cramptoni: implications for plesiosaur phylogenetics. Naturwissentschaften 95:975-980.
Williston SW 1903. North American Plesiosaurs (Part 1). Field Columbian Museum, Publ. 73, Geological Series 2(1):1-79.
The Pterosaur Heresies 