PLIOSAUR EATEN BY A LARGER PLIOSAUR?

In the Jurassic seas off the coast of present-day UK – did a bus-sized monster make a meal out of its smaller brethren?

Leviathan by Bob Nicholls (c)2009 (used with permission)
Were pliosaurs cannibals? Did pliosaurs eat their own kind? Let’s find out. From my collector’s case: the centrum portion of what was listed as a sub-adult “Pliosaurus macromerus” vertebra. The specimen have many interesting features, including evidence that strongly suggests the pliosaur was eaten by a larger pliosaur. Starting at the top, there are facets to mark what must have been an impressively large neural arch. In terms of overall shape, the centrum is more like a compressed circle or oval, as opposed to round.
Sub-adult Pliosaurus macromerus centrum (anterior view. inverted)
Dorsal view of the anterior portion of a Pliosaurus macromerus centrum showing facets of the large neural arch (absent). Raised mammilla with nutrient foramen visible in the center. Image credit: Max Hawthorne
Both anterior and posterior surfaces are slightly concave, with a heavy ridge/lip around the outer edge/perimeter. Said ridge/lip is distributed fairly evenly on the posterior surface. On the anterior surface it is far more robust in the ventral portion, indicating a possible pathology or arthritic changes brought on by injury. The affected region is ~28% thicker than the centrum’s unaffected portion.
Pliosaurus macromerus cervical centrum (anterior view) showing possible pathology on the ventral portion. Note tooth gouges in the center.

CRIME SCENE?

There are several irregular grooves carved into the anterior portion of the centrum’s articulating surface. These appear to be unhealed tooth scores, indicating either predation or scavenging post mortem. An additional puncture with a possible corresponding fracture appears on the lateral surface of the centrum, between the superior rib facet and the neural arch. Said damage may have also been caused by predation or been inflicted post mortem. Given that this was a relatively young animal (a sub-adult) it is likely that the pliosaur was killed by another, larger pliosaur. The big pliosaur may have eaten the smaller one on the spot, it may have killed it out of territoriality and left it to rot, or it may have fed on it then and there but consumed only part of it. If so, the remaining carcass would have been fed upon by other pliosaurs and/or assorted scavengers.
Pliosaurus macromerus centrum showing tooth gouges (centermost) caused by predation/scavenging
Pliosaurus macromerus centrum, right side/lateral view, showing possible tooth puncture between the neural arch (left) and the right-side superior rib facet (right). Image credit: Max Hawthorne

 

A finely developed boss (mammilla) is present in the center of each of the centrum’s articulating surfaces. It is more robust on the anterior side, vs nearly flush on the posterior. The boss is pierced by the nutritive foramen. On the ventral surface of the centrum, one nutrient foramen is present, with the other being obscured by concretions. Where it remains undamaged, the lateral surfaces of the centrum between both the superior rib facets and the neural arch, as well as on the ventral surface between the inferior facts, appear smooth. Lengthwise, the vertebra is surprisingly short, with a length that is less than one third of either its width or height.
Pliosaurus m. centrum ventral view (image credit Max Hawthorne)
The lateral surfaces of the centrum bear the double facets that are characteristic of the cervical vertebrae of many species of pliosaur. The superior rib facets are both oval in outline, with the uppermost portions being more triangular in shape or somewhat “pinched”. The right inferior facet is almost round, whereas the left appears amorphous in shape. The relative sizes of the rib facets compared to one another is of interest, with the left superior being 50% larger than its inferior, whereas the right superior is easily eight times the size of its inferior.
Pliosaurus centrum lateral/left side view, showing evenly-sized superior and inferior rib facets. Dorsal portion = right side of image.
Pliosaurus centrum right-side/lateral view showing the superior and inferior rib facets. The superior facet is disproportionately larger than the inferior. Dorsal portion of centrum = left side of image.

 

 

What kind of vertebra is it, and how big was the pliosaur?

Although the dealer identified this as a dorsal vert, based on its narrowness, the rib facets emerging from the lower lateral portions of the centrum, and the fact that there are no chevrons present on the ventral to indicate it was a caudal (tail), I believe it to be a cervical (neck). Based on studies of (admittedly uncommon) known pliosaur specimens, I estimate the animal would have been the size of an average bull or large cow Orca: probably ~22 feet in length (7 meters) with a comparable mass of ~4 tons. I did a nice piece on P. macromerus in late 2017 that shows possible upper size limits for this amazing predator; you can check it out here. The image below of Liopleurodon – another, similar member of the pliosauridae – shows the possible location of the centrum, as well as proportions related to the skeleton as a whole. The animal’s neck would’ve been around .75 meters (30 inches) thick.
Liopleurodon skeletal reconstruction. Likely location for the centrum is above the marked area (red dot) (Newman & Tarlo, public domain image)

Where did it come from and how old is it?

Upper Jurassic, Kimmeridgian, 160mya, from Abingdon quarry, Oxfordshire England. Measurements of the cervical vertebra (centrum only sans processes): Width = 4.53” (115 mm) x Height 4.17” (106 mm) x Length 1.33” (34 mm). Pathology-affected portion width: 1.7” (43.3 mm)

 

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