Plesiosaur phylogeny: revisiting O’Keefe 2001

O’Keefe 2001 analyzed members of the Plesiosauria
(Fig. 1) nesting long-necked Rhomaleosaurus (Fig. 3) with short-necked Kronosaurus (Fig. 3) and long-necked Thalassiodracon was the last common ancestor). O’Keefe also nested short-necked Dolichorhynchops with long-necked Styxosaurus and long-necked Plesiosaurus was a last common ancestor. Why the mix up? Let’s hear from the author himself.

O’Keefe reported,
“Characters from the entire skeleton support these relationships, although characters of the skull roof and palate are especially useful.”

Figure 1. Cladogram from O’Keefe 2001, colorized here with purples and blues for pliosaurs, reds and oranges for plesiosaurs.

By contrast
the large reptile tree (LRT, 1869+ taxa, subset Fig. 2) nests Rhomaleosaurus basal to the dichotomy that splits long-necked taxa from short-necked taxa. A clade of long-necked taxa with only two long hind flippers (Thalassiodracon + Yunguisaurus, Figs 3, 6) precedes Rhomaleosaurus (Fig. 3). Surprisingly a clade of four-long-flippered taxa (Hauffiosaurus and kin, Fig. 3) nest independent of other four-flippered taxa. The LRT is able to recover examples of convergence like this.

Figure 2. Subset of the LRT focusing on Sauropterygia. Colors applied based on figure 1. These results more or less randomize the results of O’Keefe 2001, but puts long-neck taxa with long-neck taxa.
Figure 3. Pliosaur origins according to the LRT, beginning with Pistosaurus.

Let’s compare and contrast the two results.
O’Keefe had firsthand examinations of his plesiosaur taxa. I worked only from the literature. On a side note, I remember seeing O’Keefe examining plesiosaurs in Germany while I was examining pterosaurs. He’s an interesting guy.

We both agree
that long-necked, no flipper Pistosaurus (Fig. 3) was basal to most later clades of four-flipper plesiosaurs however they divide thereafter.

O’Keefe splits his Plesiosauroidea and Pliosauroidea
at this points and nests long-necked Thalassiodracon at the base of his short-neck Pliosauroidea (= Kronosaurus + Rhomaleosaurus clade).

The LRT also nests Thalassiodracon basal to Rhomaleosaurus,
but both prior to the long-neck vs. short-neck dichotomy (Fig. 3). Hauffiosaurus nests 11 steps prior to the other four-long-flipper taxa, one node prior to Pistosaurus. So this appears to be an independent and convergent acquisition not recognized nor recovered by O’Keefe 2021.

O’Keefe 2001 did not have
the benefit of the LRT when he reported, “The Sauropterygia is a clade of basal diapsids, more closely related to lepidosaurs than archosaurs but near this basal dichotomy.”

In the LRT, the closer relationship is with marine younginiforms and archosauriforms, not lepidosaurs.

O’Keefe also reported,
“Some recent work has indicated that Testudines is the sister group of Sauropterygia, although this work is controversial.”

This phrasing is typical of workers who have no idea how clades are related to one another and either take the word of a fellow worker or not, depending on the general mood. I would have said the same thing at the time. The LRT (2010-2021) resolves all such issues, but it really is the job of paid professionals to do this. The LRT currently fills this vacuum, hopefully only temporarily.

Back in 2001, O’Keefe was also filling a vacuum
when he reported, “No comprehensive review of the plesiosaur skull has been attempted since the work of Andrews (1910, 1913).”

Setting an outgroup
O’Keefe reports, “The condition of the Permian plesiomorphic diapsid Araeoscelis, described in detail by Vaughn (1955), is an acceptable model from which to derive the sauropterygian skull roof.”

The LRT likewise nests Araeoscelis at the base of the Sauropterygia, but many overlooked transitional taxa (Fig. 4) fill the gap between them, several with a diapsid morphology.

Figure 3. Spinoaequalis and descendant marine younginiformes.
Figure 4. Spinoaequalis and descendant marine younginiformes. These give rise to pachypleurosaurs, plesiosaurs, placodonts, mesosaurs, ichthyosaurs and thalattosuchians. Araeoscelis and other more primitive diapsids are not shown. These are some of the transitional taxa overlooked by O’Keefe twenty years ago, which is forgivable. I made similar mistakes back then.

O’Keefe reports,
“The nasal is lost in all Plesiosauroidea, including Plesiosaurus.” There are lots of Plesiosaurus skulls. This one (Fig. 5), P. dolichodeirus appears to keep the frontal (blue) and nasal (pink) separate.

Figure x. Plesiosaurus skull (BMNH 39490) showing nasals.
Figure 5 Plesiosaurus skull (BMNH 39490) apparently showing nasals.

O’Keefe 2001 introduced Hauffiosaurus
(Fig. 3) to the world of paleontology, He wrote, “The skeleton is approximately 2.5 m long, and displays an interesting mix of plesiomorphic, derived, and apomorphic features.”

According to the LRT, which minimizes taxon exclusion, Hauffiosaurus developed four long flippers independently, and phylogenetically before and convergent with the last common ancestor of all pliosaurs and plesiosaurs… given the present taxon list (Fig 2).

Building the O’Keefe cladogram
O’Keefe did what lots of paleontologists still do. He chose his outgroup taxa. He wrote, “Three taxa were chosen as outgroups for this analysis.”

The LRT chooses valid outgroup taxa for you because it minimizes taxon exclusion.

O’Keefe wrote,
Thirty-one plesiosaur genera were coded for inclusion in the phylogenetic data matrix.”

By contrast,
only 19 sauropterygian taxa are more derived than Pistosaurus (Fig. 3) in the LRT (subset Fig. 2). Twenty are more primitive. Among them is Hauffiosaurus (Fig. 3). Nesting taxa in the outgroup that O’Keefe considered ingroup taxa affects the way the rest of the taxa relate to one another and we’re off to a rocky start.

O’Keefe wrote,
“All known clades are well-represented, however, and the omission of some ingroup taxa from some clades should not influence the results reported here.”

That’s a bold statement falsified by the LRT some twenty years later. Two Hauffiosaurus LRT sisters, Acostasaurus and Anningsaura, known from skulls only, were described after O’Keefe published in 2001, so he can’t be held responsible for those.

The character list
O’Keefe wrote, “The 34 taxa listed above were scored for 166 morphological characters. Of these characters, 107 concerned the skull and 59 were postcranial.”

238 multi-state characters nest taxa in the LRT.

O’Keefe wrote,
“A second analysis was performed with the ‘morphometric’ characters removed, based on the finding that the pliosauromorph body type may have evolved convergently (O’Keefe 2002).”

Note this citation is a year later than O’Keefe 2001. How did he cite a future publication? It was in ‘in press’. Sometimes workers produce more than one paper from a single study.

Figure 3. Albertonectes reconstructed. This 11 m elasmosaur is the longest thusfar recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus.
Figure 6. Albertonectes reconstructed. This 11 m elasmosaur is the longest thus far recorded. This may be the breathing pose, swallowing air, then submerging the neck. When horizontal the air could be passed back to the lungs, as hypothesized for Dinocephalosaurus. This could be a feeding pose, exhaling a bubble net from a long trachea to surround a school of fish overhead.

O’Keefe’s results
“The topology of clade Plesiosauria replicates many of the findings advanced by earlier workers. The basic dichotomy between the Plesiosauroidea and the Pliosauroidea is a well-supported finding.”

Not really. If memory serves, plesiosaurs traditionally had small heads and long necks, while pliosaurs traditionally had larger heads and shorter necks. O’Keefe nests long-necked Thalassiodracon (Fig. 3) at the base of the pliosaurs. The LRT nests it with another long-hind-flipper only taxon, Yunguisaurus, not described until 2006.

Figure 7. Muraenosaurus enters the LRT from these data.
Figure 7. Muraenosaurus data used in the LRT.

Muraenosaurus enters the LRT today alongside Rhomaelosaurus.
O’Keefe reported, “The elasmosaur-like long neck and small head evolved independently in Muraenosaurus.”

This is supported by the LRT. Let your cladogram do what you want it to do… nest taxa without bias or preconception.

O’Keefe FR 2001. A cladistic analysis and taxonomic revision of the Plesiosauria
(Reptilia: Sauropterygia). Acta Zool. Fennica 213: 1–63.
O’Keefe FR 2002. The evolution and functional morphology of plesiosaur and pliosaur morphotypes inthe Plesiosauria (Reptilia: Sauropterygia). — Paleobiology 28(1). [In press at the time in 2001].

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