Tetrapod Zoology

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Some weeks ago I wrote a bit about the Matamata Chelus fimbriatus: a weird, flat-headed South American pleurodiran turtle. It’s one of the strangest creatures tetrapods on the planet, and there’s so much to say about it that the previous article ended up being nothing more than the briefest of introductions. Today we start looking at the Matamata in a bit more depth. We start with its affinities and its long neck…

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Chelus is a chelid: that is, a member of the pleurodire turtle clade Chelidae. Despite – or perhaps because of – its bizarre appearance, the Matamata is sometimes regarded as the ‘best known’ chelid, and it’s also the first member of the group to become known to science: it was named Testudo fimbriatus by Johann Gottlob Schneider in 1783, but had been described even earlier (in 1741) by Pierre Barrère. All other chelids – with the exception of the South American flat-headed turtle Platemys platycephala – were named after 1800. This explains why Chelus serves as the type taxon for the clade (in other words, Chelidae is named after Chelus) [adjacent photo, and close-up of head above, kindly provided by Mark Hollowell].

Chelid taxonomic history is a bit confusing, however, once you get back beyond the 1920s: the name Chelydidae (not to be confused with Chelydridae, the snapping turtle clade), used from the late 1880s onwards, corresponds to Chelidae, and Hydraspididae was also used for the group at times. There’s a good discussion of all this in Joyce et al. (2004).

Within Chelidae, the affinities of Chelus have been mildly controversial and several different proposals have been made. One hypothesis is that Chelus is the sister-taxon to a clade that includes Chelodina (from Australia) and Hydromedusa (Gaffney 1977); another is that
Chelus may be the sister-taxon to a clade that includes Platemys, Acanthochelys, Batrachemys and others (Georges et al. 1998). And yet another is that Chelus might be the sister-taxon to Phrynops (Krenz et al. 2005) [in the diagram below - representing a highly simplified version of the cladogram featured in Georges et al. (1998) - Chelodina is outside a clade that has Hydromedusa at its base and Chelus and Phrynops nested within it. Other phylogenies of Chelidae are available! Chelodina, Hydromedusa and Phrynops images from wikipedia].

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One interesting debate that has arisen about the Matamata’s phylogenetic position concerns the evolution of long necks within Chelidae. Are long necks primitive in chelids (in which case, the short necks of some Australasian and South American chelids are derived), or are short necks primitive (in which case, Chelus, Chelodina and Hydromedusa evolved their long necks independently)? Pritchard (1984) argued that long necks had arisen independently. I came up with the idea of showing a few alternative cladograms with the neck skeletons of the different taxa juxtaposed in order for comparison. But, of course, no one has ever figured the neck skeleton of a Matamata (in entirety), nor that of its close relatives. In fact, the Matamata skeleton hasn’t been well described at all – there just seem to be odd comments here and there, scattered about the literature. I tried convincing my Dad a few weeks ago that the anatomy and functional morphology of most animals is not well known at all, and here’s another demonstration vindicating my contention. Matamatas are hardly obscure creatures. In fact, compared to the majority of living things they’re positively familiar. Anyway…

Yet another unusual thing about the Matamata is that it normally holds its head such that its long axis is inclined upwards (at about 35°) relative to the horizontal [normal head-neck posture shown below: picture by your humble author]. This means that these chelid turtles definitely don’t play by the ‘flexed head-neck junction’ rule that is otherwise widespread in tetrapods (more on that in this article on sauropod dinosaur neck posture), but then we already know that the rule doesn’t apply to aquatic tetrapods.

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Next: skull and neck anatomy (if you think it sounds boring, it isn’t), then feeding behaviour.

For the previous Matamata article see…

And for previous Tet Zoo articles on turtles see…

Refs – -

Gaffney, E. S. 1977. The side-necked turtle family Chelidae: a theory of relationships using shared derived characters. American Museum Novitates 2620, 1-28.

GEORGES, A., BIRRELL, J., SAINT, K., McCORD, W., & DONNELLAN, S. (1999). A phylogeny for side-necked turtles (Chelonia: Pleurodira) based on mitochondrial and nuclear gene sequence variation Biological Journal of the Linnean Society, 67 (2), 213-246 DOI: 10.1006/bijl.1998.0300

Joyce, W. G., Parham, J. F. & Gauthier, J. A. 2004. Developing a protocol for the conversion of rank-based taxon names to phylogenetically defined clade names, as exemplified by turtles. Journal of Paleontology 78, 989-1013.

Krenz, J. G., Naylor, G. J. P., Shaffer, H. B. & Janzen, F. J. 2005. Molecular phylogenetics and evolution of turtles. Molecular Phylogenetics and Evolution 37, 178-191.

Pritchard, P. C. H. 1984. Piscivory in turtles, and evolution of the long-necked Chelidae. Symposia of the Zoological Society of London 52, 87-110.

Comments

  1. #1 Dragon
    July 26, 2010

    Thank you!
    I LOVE tortoises/turtles & any other associated species!

    i am so fascinated by their different survival techniques & those that they have in common with each other.

    i have also come to recognize things in common with other species, even with humans.
    } : =8 } )
    TQ!!
    I look forward for MORE!!

  2. #2 Jorge Velez-Juarbe
    July 26, 2010

    Interesting!! I’m very fond of pleurodires, it seems to be a group not well known to the general public, at least not to those in the northern hemisphere. I can’t wait to see the upcoming part on head and neck!!
    By the way, do the Chelidae have a good fossil record?

  3. #3 John Scanlon FCD
    July 26, 2010

    Some authors have lately taken to calling the ‘family’ Cheluidae, which is ugly but arguably correct. Chelus is a latinized form of χελυς, otherwise commonly transcribed as ‘chelys’ and used in this form in many other turtley taxon names. The stem of Chelus, to be used in suffixed forms, is thus not Chel- but either Chely- (which I prefer aesthetically, if incorrect but familiar Chelidae gets the boot) or Chelu-. I think it’s open to interpretation that Chelus, as a transcribed Greek word, should keep its Greek declension rather than being forced to become a Latin u-stem.
    Anyway, cool turtles. All the chelid fossils (and handy comparative specimens) I deal with are short-necks, so I can’t send pics of Chelodina neck vertebrae.

  4. #4 Jaime A. Headden
    July 27, 2010

    While John Scanlon is technically correct on the Greek (the root form of chelys being chely-), it is incorrect to state that the correct name for the taxon Chelidae is Cheluidae, at least insofar as the ICZN is concerned. This is also why all family-level taxa in Ceratopsidae are incorrectly formed when including the S and emmendations are invalid when attempting to “fix” this.

    Nomenclature, even when done incorrectly, is validated despite the author’s misuse of the language.

  5. #5 Darren Naish
    July 27, 2010

    Thanks for comments. Jorge: chelids have a pretty good fossil record, and matamatas in particular are well represented. I’m coming to this – NO SPOILERS PLEASE!

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