More Eaters of Ammonites

I posted earlier my initial skull reconstruction of the durophagous mosasaur Globidens dakotensis (here), but it occurs to me that I’ve not done so to the updated version of the skull that now appears on the banner above, and I figured a quickie fix was in order:

Skull of Globidens dakotensis.

For the record, the caudal half of the mandible is largely hypothetical, and reconstructed from “scaling” Prognathodon spp. jaws as well as from Globidens phosphaticus (Bardet & Suberbiola, 2005). Globidensin mosasaurs (“subtribe” Globidensini) are generally confined to the various species referred to Globidens and Igdamanosaurus (blunted, spherical distal teeth; Russell, 1975, Lingham-Soliar, 1991 [see Zdansky, 1935]), Carinodens (narrow but elongated, “plagiaulacoid” distal teeth; Schulp et al., 2009 [see Thurmond, 1969]) and Prognathodon and Plesiotylosaurus (blunted, but conical distal teeth: Camp, 1942, Polcyn et al., 2010, Russell, 1967). The jaws of Globidens, Igdamanosaurus, and Prognathodon are appropriately robust, extremely deep, and had high coronoid bones and concordant processes; those of Carinodens, however, are shallower, narrow, as were their teeth, which evidence a functional dissimilarity (Schulp, 2005).

Broad ridges and lateral sculpturing of the coronoids in the Globidens-like globidensins suggests that the anterior mAME [m. adductor mandibular externalis] group muscles inserted onto the lateral coronoid, and significantly more so than in other mosasauroids relative to both jaw length and jaw depth, while the pterygoideus group muscles are large, but didn’t incorproate as much of the lateral surface of the jaw as did the mAME muscles. These features suggest that the jaws possessed highly developed orthal adductors, but less well-developed protractor-adductors attaching to the palate (but, this doesn’t mean they aren’t strong).

Unlike most mosasauroids, globidensins have highly reduced or even absent palatal dentition (Russell, 1975), which when present are small and conical, but not really recurved as in classic mosasaurs like Tylosaurus or the like, as in Prognathodon. High incidence of durophagy has been inferred, and indeed supported by later samples of stomach contents (Martin & Fox, 2007) of bivalves. However, it is not unlikely that globidensins also pursued free-swimming animals, such as smaller fish, mosasaurs, plesiosaurs, or especially hard-shelled animals such as ammonites or even turtles.

Globidensins have sectorialized dentition; while what above seems to demonstrate only a few types of teeth — such as conical rostral crows, blunted distal crowns — systematists have recognized far more and generally recognize a broad range of morphotypes that may grade along the jaws. However, most of these morpholotypes consider the aspect of the crown in lateral view, degree of incrassation (breadth to length or height), longitudinal sulci or ridges and their degree of extent around the crown circumference, and the development of a basal “cingulum” or distal nubbin or “nipple”-like tip. These features grade into one another along either upper or lower jaw, with the rostral crowns consistently conical, the distal crowns more oblate and incrassate, and intermediate crowns … well, intermediate in morphology. The gradient of subsidiary features has been used to define species (and indeed, Martin, 2007 further redefines these morphologies and distributes them across the species of Globidens to validate a new species, schurmanni, based only on isolated teeth). Despite this, I am wary of the use of “morphotype” distribution because the low degree of recovery of bone specimens with in situ dentition in globidensin mosasaurs makes the use of morphotype segregation problematic. Despite this, it is a functional paradigm that allows categorization, and it can always be changed later. I would think that we may eventually realize that the same changes occur along the jaws in virtually all globidensins, and that most of them develop similar changes along the jaws.

Distinct sectorialization suggest several functional aspects when related to diet, and in this case the degree of extent of different regions of dental morphologies can be useful in discriminating dietary flexibility. For one, more or less rostral conical teeth influence prey seizure and rostral prehension, so that more conical teeth would suggest more active-prey grasping; more blunted distal teeth suggest more of the jaw is used in durophagy, while less suggests a more likely generalized diet. Shearing is indicated by the “plagiaulacoid” teeth in Carinodens spp., but how much suggests the relative flexibility of diet (see Schulp, 2005, for more details). I would further hypothesize that the relative rostral placement of the jaw adductors coincides with blunter distal teeth, and larger regions of attachment for adductors with more blunt teeth than conical, but the extreme brevity of complete jaws makes this difficult to test on globidensin jaws directly. It can, however, be tested by modelling efficiency and mechanics from hypothetical jaw series.

Globidens spp. represent one of the more unusual species to come across my interests, as it would otherwise look “normal,” externally and postcranially, but exhibit high cranial disparity. Qualifying this disparity is of interest to me, as well as the changes of tooth/jaw/skull morphology from “tylosaurin” to “globidensin” mosasaurs.

Mosasaurs, compared

Body outlines with details of the median and distal mandibular teeth of a platecarpine (Platecarpus tympanicus; conical, recurved teeth), and two globidensin mosasaurines (Globidens dakotensis; oblate, spherical & Carinodens belgicus; oblate, “plagiaulacoid”). Body outlines are hypothesized from generous soft-tissue remains.

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Bardet, N., Pereda Suberbiola, X., Iarochène, M., Amalik, M. & Bouya, B. 2005. Durophagous Mosasauridae (Squamata) from the Upper Cretaceous phosphates of Morocco, with description of a new species of Globidens. Netherlands Journal of Geosciences — Geologie en Mijnbouw 84(3):167-175.
Camp, C. L. 1942. California mosasaurs. Memoirs of the University of California 13:1–68.
Gilmore, C. W. 1912. A new mosasauroid reptile from the Cretaceous of Alabama. Proceedings of the United States National Museum 41:479-484.
Dollo, L., 1889. Première note sur les mosasauriens de Mesvin [First note on a mosasaur from Mesvin]. Bulletin de la Société belge de Géologie, de Paléontologie et d’Hydrologie 3:271-304.
Lingham-Soliar, T. 1991. Mosasaurs from the upper Cretaceous of Niger. Palaeontology 34(3):653–670.
Martin, J. E. 2007. A new species of the durophagous mosasaur, Globidens (Squamata: Mosasauridae) from the Late Cretaceous Pierre Shale Group of central South Dakota, USA. Pages 177-198 in Martin, J. E. and Parris D. C. (eds.) The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas. Geological Society of America, Special Paper 427.
Martin, J. E. & Fox, J. E. 2007. Stomach contents of Globidens, a shell-crushing mosasaur (Squamata), from the Late Cretaceous Pierre Shale Group, Big Bend area of the Missouri River, central Soth Dakota. pp.167-176 in Martin & Parris (eds.) The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas. Geological Society of America, Special Paper 247.
Polcyn, M. J., Jacobs, L. L., Schulp, A. S. & Mateus, O. 2010. The north African mosasaur Globidens phosphaticus from the Maastrichtian of Angola. Historical Biology 22(1-3):175-185.
Russell, D. A., 1967. Systematics and morphology of American mosasaurs. Bulletin of the Peabody Museum of Natural History, Yale University 23:1-241.
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Schulp, A. S. 2005. Feeding the mechanical mosasaur: what did Carinodens eat? Netherlands Journal of Geosciences 84(3):345–357.
Schulp, A. S., Polcyn, M. J., Mateus, O., Jacobs, L. L. & Morais, M. L. 2008. A new species of Prognathodon (Squamata, Mosasauridae) from the Maastrichtian of Angola, and the affinities of the mosasaur genus Liodon. pp.1-12 in Everhart (ed.) Proceedings of the Second Mosasaur Meeting. Fort Hays Studies, Special Issue 3:1-12.
Schulp, A. S., Bardet, N. & Bouya, B. 2009. A new species of the durophagous mosasaur Carinodens (Squamata, Mosasauridae) and additional material of Carinodens belgicus from the Maastrichtian phosphates of Morocco. Netherlands Journal of Geosciences — Geologie en Mijnbouw 88(3):161-167.
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3 Responses to More Eaters of Ammonites

  1. Molluscivore marine sauropsids always fascinate me.

    By no chances would these mosasaurs be analogous to placodonts ecologically, would they?

    • Possible, although because placodonts are essentially Triassic, the Cretaceous globidensines would never have seen then, so a direct replacement of one over the other wouldn’t have happened.

      There are some quirky things about placodont biology that suggest they had a different ecology — and it may vary strongly among species — and that despite large crushing teeth, the durophagous mosasaurs were more pelagic than the placodonts were. Placodonts will be covered in a bit more detail soon, but for now I wrote a little on their biology here. Darren Naish covered some placodonts before, such as here, and here, but more generalized placodonts were not covered very much there.

      • Thanks.

        I suspected that these mosasaurs were indeed perhaps too pelagic for a direct comparasion.

        Durophagy has also been suggested for marine sphenodontians and gondwanathere and kollikodontid mammals, so they’re probably closer analogues

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