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Spinosaurus: Dimetrodon redux?

Ben Creisler

The abstracts for the 2014 GSA Meeting in October are currently posted online:


Unlike the SVP, the abstracts are not embargoed prior to the
presentations and the GSA gives permission to cite content from
abstracts that are posted online.

One of the abstracts raises an interesting issue that may relate to
the semi-aquatic fish-eating Spinosaurus as now envisioned--the Early
Permian synapsid fin-back Dimetrodon apparentlywas doing something
similar. Note too that the related fin-back Secodontosaurus had
elongated crocodile-like jaws with more uniform teeth. Some objections
have been raised to a fin-backed animal being a maneuverable swimming
predator in water.  However, if Dimetrodon (and Secodontosaurus)
preyed on fish and Spinosaurus did likewise, maybe a high fin or sail
served a similar purpose--or was just an amazing coincidence.


ZOEHFELD, K. Weidner.1, MOSSBRUCKER, Matthew T.2, BAKKER, Robert T.1,
and FLIS, Chris J.1, (1) Department of Paleontology, Houston Museum of
Natural Science, 5555 Hermann Park Drive, Houston, TX 77030-1799,
kwzoehfeld@aol.com, (2) Morrison Natural History Museum, 501 Colorado
Highway 8, Post Office Box 564, Morrison, CO 80465
The fin-backed “mammal-like reptiles”, Dimetrodon and its kin, were
the earliest, common apex predators in the terrestrial ecosystem. From
an Early Permian ancestor much like Dimetrodon came the higher
“mammal-like reptiles” which, in turn, included the immediate
ancestors of true Mammalia in the later Triassic. The earliest large
tetrapod herbivores co-occur with Dimetrodon and have been identified
as the principal prey. However, E. C. Olson noted that, in most
samples, the large herbivores are too rare to supply most of the meat
needed by the apex predator. Olson concluded that most Texas
Dimetrodon populations depended upon aquatic prey such as amphibians
and fresh-water sharks.

We have excavated 66 sites in the Craddock Ranch, Baylor County,
Texas, where large Dimetrodons are common and ubiquitous. Large
herbivores are indeed outnumbered by large carnivorous finbacks 8.5 to
1. If Olson’s theory is correct, then we should find shed Dimetrodon
teeth mingled with dismembered fish carcasses. The only common large
fish is a xenacanth shark; the skeleton was highly calcified in life
and preserves as well as do tetrapod bones. Three mudstone lenses are
packed with fragmented shark skeletons and shark head spines,
representing hundreds of individuals. Shed Dimetrodon crowns are
mingled with the shark skeletal elements, confirming that finbacks fed
on the sharks. Lack of shark coprolites indicates little
shark-on-shark feeding.

Olson also noted that the Dimetrodon food web was reciprocal: sharks
should have fed on the finbacks. We excavated pond deposits where
finback skeletons were surrounded by shark coprolites. If the sharks
had preyed upon the finbacks, we should find shark tooth marks on the
bones. Xenacanth teeth had exceptionally sharp apices, far sharper
than those of any large tetrapod. We find exceptionally narrow tooth
marks on finback bones. The long, deeply incised marks show that
carnivores had twisted limb bones around as if employing a body roll
to dismember the Dimetrodon carcass. Thus we ascribe these marks to
sharks and find Olson’s food web theory to be supported.