Wednesday, May 19, 2010

Geosaurus

Type of Geosaurus giganteus (Von Sommerring 1816)(from Young and Andrade 2009)

(sensu Yound and Andrade 2009)
Meaning: "earth lizard"
Species: G. giganteus (type - von Sommerring 1816 as Lacerta giganteus), G. grandis (Wagner 1858), G. lapparenti (Debelmas and Strannoloubsky 1957), G. carpenteri (Wilkinson et al 2008)
Nominal Author: Cuvier 1824
Age: Late Jurassic to Early Cretaceous
Location: Europe (incl. UK and Germany)
Physical Characteristics: a short snouted marine crocodyliform (thalattosuchia)


A Major Revamp of the Genus:
In 2009, Young and Andrade redescribed the type species Geosaurus giganteus and reassessed the genus, butting to an end the "Fraas Misconception" which had resulted in the inclusion of longirostrine species to the genus. Four species, G. vignaudi (Frey at al 2002), G. saltillense (Buchy et all 2006), G. suevicus (Fraas 1901), and G. araucanensis (Gaspirini and Dellape 1976) were renamed as members of the genus Cricocosaurus (bootstrap=67). Rhacheosaurus gracilis Wagner 1852 (renamed Geosaurus gracilis by Fraas 1902) was reestablished. Young and Andrade consider Geosaurus to then be composed of three species sensu stricto (G. giganteus, G. grandis, and G. lapparenti [formerly Dakosaurus lapparenti Steel 1973] bootstrap=50) but also propose G. carpenteri (formerly D. carpenteri Wilkinson et al 2008) and the 'croccodrillo di Portomaggiore' (Leonardi 1956 - still awaiting proper description) as members of the genus (bootstrap=51). Their phylogenetic analysis resulted in a strict consensus of 4104 most parsimonious trees (tree length=578, CI=0.436, RI=0.861, RC=0.376).

Morphology:
Little postcranial material exists for Geosaurus, but based on the available material, it is clear that Geosaurus was adapted to a fully pelagic lifestyle with a streamlined skull and flattened humeri. Proportionally, Geosaurus giganteus has one of the shortest rostra among thalattosuchians and the snouts of other species of Geosaurus are similarly brevirostrine or mesorostrine (Young and Andrade 2009). Its cranial bone as are smooth and lack ornamentation. Geosaurus also has one of the most robust, well developed sclerotic ring (second only to C. schroderi) which encompasses the entire orbit. The teeth of Geosaurus are arranged as opposing blades that are serrated and strongly lateromedially compressed (Young and Andrade 2009). Some general metriorhynchid characteristics include hydrofoil-like forelimbs and a hypocercal tail, convergent with many other Mesozoic marine reptiles. There is also evidence for salt glands as a means of osmoregulation (Fernandez and Gasparini 2008).

Life and Ecology:
Metriorhynchids are the only group of archosaurs to ever fully adapt to life in the marine realm (Steel 1973) and are thought to be stalking predators that fed on fast-moving fish and cephalopods (Hua 1994). Geosaurus is endemic to Europe, with species living from the Late Jurassic (Oxfordian), when they (along with Dakosaurus) replaced medium-sized pliosaurs and Suchodus (Young pres. observ./ Young et al 2010) to the Early Cretaceous (Valangian) when all metriorhynchids went extinct. Geosaurus evolved over time to become more streamlined and hypercarnivorous as it was more able to sustain fast swimming speeds for longer periods and to efficiently slice through fleshy prey (Young et al 2010, Massare 1988). The nature of their sclerotic ring suggests that species of Geosaurus may have even occasionally ventured on deep dives (Young and Andrade 2009).

5 comments:

  1. Metriorhynchids!!!!
    A Major Revamp of the Genus?
    This sound interesting, since I'm currently studying a specimen referred to Geosaurus.

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  2. Very cool. Well, the paper (excellently written, by the way) was in volume 157 of the Zoological Journal of the Linnean Society. I need to update the resources page, but when I do, the full citation will be there. If you would like a copy of the paper tho, let me know and I'll get you the link for Mark Young's CV.

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  3. Thank you very much, Susan: I already have Young et al.'s 2010 paper: Mark himself kindly send me a copy (when it was yet "in press") of his very good paper on metriorhynchoid evolution. I did not realise you were mentioning just that study.
    You have quoted the specimen I'm studying and re-describing: it is the 'crocodillo di Portomagiore' (sic*).

    *the correct Italian name is "coccodrillo di Portomaggiore". :-)

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  4. No problem. I mentioned both the 2009 and 2010 paper by Mark.

    That's very exciting that you're the one who is redescribing the "croccodrillo di Portomaggiore" (I will have to change the spelling).

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  5. Quoting Tom Holtz: wait for the paper! ;-)

    If you're interested, you can read something about that study in progress in the "Crurotarsi"-labelled posts on my blog*:

    http://theropoda.blogspot.com/search/label/Crurotarsi

    *Sorry if the google translator is not always perfect in its work... :-P


    Best,

    ReplyDelete