Galeocerdo

As many regular readers may have surmised, my son Tim is homeschooled, which is how he’s able to spend so much time in the field with me. As part of his science education he’s required to do various projects (some of which are on his blog), and as his most recent on is on paleontology I decided to post some of his results here.

There’s some interesting potential for studies on Carmel Church sharks, because there are large numbers of teeth that all come from a single stratigraphic horizon and small geographic area, so they may represent a true living population much more closely than do most shark tooth samples.

Tim looked at the two species of tiger sharks found at Carmel Church, Galeocerdo contortus and Galeocerdo sp. Shark tooth fans may know G. sp. as G. aduncas, but Purdy et al. (2001) make the case that G. aduncas is a nomen dubium. In the images at the top G. contortus is on the left and G. sp. is on the right. There is an easy identification feature that separates these species – the shape of the anterior cutting margin is distinctly wavy only in G. contortus (on the left below):

Actually Purdy et al. (2001) found G. contortus to be so different from other Galeocerdo species that they speculated that it may represent a different genus, although they did not assign it to one.

One interesting point with these two species is that (according to Purdy et al.) they are always found together, and that (at least at Lee Creek Mine) they occur in a 2:1 ratio with G. contortus being the more common of the two. Tim’s Carmel Church sample confirms this ratio.

Tim also confirmed what I had long suspected, that G. contortus tends to be larger (or at least taller) than G. sp. (at least at Carmel Church):

More surprising is that there’s a good correlation between tooth height and tooth thickness in G. contortus, but not in G. sp.:

I’m not sure what this indicates, other than that these teeth are quite different from each other (maybe they should be different genera!). Tim speculates that maybe the prey preferences of G. sp. didn’t change as the shark grew (so no change in tooth thickness), while perhaps older, larger G. contortus fed on different, larger prey than juvenile G. contortus.

Tim’s complete report is available on his blog; I encourage you to check it out.

Reference

Purdy, R. W., V. P. Schneiver, S. P. Applegate, J. H. McLellan, R. L. Meyer, and B. H. Slaughter, 2001. The Neogene sharks, rays, and bony fishes from Lee Creek Mine, Aurora, North Carolina. In C.E.Ray and D.J. Bohaska, eds., Geology and Paleontology of the Lee Creek Mine, North Carolina, III. Smithsonian Contributions to No.90, P.71-202.
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4 Responses to Galeocerdo

  1. Brian Beatty says:

    The plots look like they have some overlap, are they statistically significantly different?
    Maybe more importantly, have you been able to eliminate the possibility that the teeth being compared just come from different portions of the toothrow? Shark teeth have a notorious history of having new species described from isolated teeth, with subsequent species being named from teeth from different portions of the toothrow.
    I know I spout on about the need for more intraspecific variation studies in mammals, but I suppose it is even more necessary in sharks. It would be an awesome project to study the qualitative (like a population aggregation analysis) and morphometric variation in some modern species of sharks, eh? Then, once a better understanding of it can be recognized in modern species, fossil taxa can be more readily discerned….

  2. Alton Dooley says:

    There have been suggestions that G. contortus might be the lower teeth from G. sp., but I think that’s quite unlikely. The 2:1 ratio would be very odd, and modern Galeocerdo don’t have such a distinct difference in uppers and lowers (G. sp. is very similar to modern Galeocerdo).

    The wavy cutting edge on G. contortus seems to be a consistent character across different tooth positions (in other words, it occurs in both anterior and posterior teeth), so it seems to be a good distinguishing character independent of tooth position.

    No statistical analysis yet–Tim’s math skills are not yet that far along! However, there is potentially much larger sample size; all Tim’s sample was collected in a single 2-week excavation in May 2006, which is just a tiny portion of what’s already been collected at Carmel Church.

  3. Boesse says:

    I remember reading the upper/lower hypothesis for G. contortus/G. sp.

    I have also seen G. contortus referred instead to the genus Physogaleus; I couldn’t remember why, but a quick check on elasmo.com states that Ward and Bonavia (2001) referred the species contortus to Physogaleus.

    I’ve heard that a character distinguishing G/P. contortus from G.sp. is a twisted crown, which may be the same as the wavy cutting edge.

    Thought I’d mention that when Kenshu Shimada looked at articulated dentitions of Cretoxyrhina mantelli from the Niobrara Chalk, it was possible to synonymize well over 100 taxa with C. mantelli.

  4. Alton Dooley says:

    Yep, the twisted crown and the wavy cutting edge are the same character.

    Purdy et al. mentioned the referral of G. contortus to the new genus Physogaleus by Cappetta, but didn’t like that referral. Apparently, Physogaleus was a new name for a species originally named by Winkler in 1874, for which the types have been lost. Based on Winkler’s descriptions of the types, they thought Winkler’s species might be an orectilobiform shark (carpet, nurse, and whale sharks) rather than a carcharhinoform.

    I think their question was not whether or not Cappetta’s Paleogene teeth belonged in the same genus as contortus, but rather whether Cappetta was correct in assigning those teeth to P. secundus (Winkler). Basically, when Cappetta made Winkler’s species secundus the genotype for Physogaleus, he permanently linked that name to Winkler’s lost (possibly orectilobiform) teeth. Given the uncertain status of Winker’s specimens, it would be incorrect to refer a carcharhinoform (whether contortus or Cappetta’s Paleogene specimens) to Physogaleus.

    I don’t keep completely up-to-date on shark tooth literature, so it’s possible more has been resolved on this since 2001.

    Shimada’s finding doesn’t surprise me; I think in general shark teeth are wildly oversplit taxonomically, and suspect that in many lineages the teeth are simply not diagnostic for identifying species (I think Carcharhinus and Carcharius may be good examples of this).

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