Re: ceratopsians and theropods cont.

["Thomas R. Holtz, Jr." <th81@umail.umd.edu>]

At 08:45 AM 4/7/98, Dave wrote:

>3)  The reason I lean toward ceratopsians as an important prey of big
>theropods, as opposed to hadrosaurs or something else, is that they seem so
>abundant.  This may be an artifact of preservation.  But if not, it is not
>reasonable to me that big theropods would pass up an abundant potential
>prey just because it has horns.

Actually, hadrosaurid abundance is very comparable to ceratopsian abundance
in most of the late Late Cretaceous dinosaurian assemblages of western North
America (in fact, it may even be higher).

I agree, though, that ceratopsians probably were a major potential food
resource for tyrannosaurids.

HOWEVER, I sincerely doubt that healthy, adult, alerted ceratopsians were a
major food resource: as with modern predators, it is much more likely that
the sick, the young, the old, or the just-plain-surprised were the primary
victims.

>5)  Selection for cursorial adaptations will undoubtedly be of a different
>nature in different species.  The selection pressures on kangaroos are
>undoubtedly different from those on cheetahs.  The list of factors that
>would select for cursorial adapations in big theropods seems rather short.
>It may have been sprinting ability for "chase and bite."  But I remain
>skeptical.

The selection for cursorial ability in tyrannosaurids occured *before* they
were tyrannosaurids!  The elongate limbs, arctometatarsus, and so forth are
inherited from their common ancestor with other groups of theropods
(ornithomimids and troodontids), and were developed among forms much smaller
than Late K tyrannosaurids.

>5)  I realize that large theropods, including Tyrannosaurus rex, occurred
>in a variety of habitats, and I do not exclude the possibility of a
>tyrannosaur-ceratopsian chase even in forest.  But modern practitioners of
>"chase and bite" do not have the weaponry of theropods, and must make
>multiple bites on large prey in order to bring it down.

Not true, actually.  Watch footage of hyaenids and big canids preying on
animals their own size or smaller.  They can often take them down in a
single strike.  It is only going after animals much larger than themselves
that they commonly require multiple bites.

>It is difficult
>for us to envision how truly dangerous these walking razor blades were,
>because big predatory mammals do not have their bladed, serrated teeth.

True, although many extinct predators other than dinosaurs do.

HOWEVER, please, Please, PLEASE get a hold of a T. rex tooth (or cast):
advanced tyrannosaurine teeth are ANYTHING but blade-like.  It doesn't
matter how many popular books (or paleontologists) call them "steak-knives":
the teeth of advanced tyrannosaurines like Tyrannosaurus are half-again as
thick side to side (or more) than they are front to back, and the serrations
are offset (they don't run up the front and the back).

More primitive theropods, like Carcharodontosaurus or Allosaurus, DO have
the teeth you describe.

>Those who suggest that tyrannosaurs were not good slashers should perhaps
>spend more time with living, breathing animals.

I would suggest, in turn, that you actually take a look at a tyrannosaurid
tooth, rather than go by what you have read.

>Perhaps they should watch
>a big monitor lizard slice clean through the skull of a rat (not the neck,
>mind you, the braincase) so fast that the whole process is just a blur.

Yep, and these *have* ziphodont teeth.

>Perhaps they should observe a 6-foot Philippine crocodile, with its poorly
>aligned, poorly bladed, unserrated teeth, nearly cut through a broom with a
>quick grab and a few side-to-side yanks.

Hey, using torsional motion in food processing for predators without
ziphodont teeth: what a concept for a DinoFest and SVP'98 talk... :-)

>I reiterate that Tyrannosaurus
>rex, with its well-aligned, nicely bladed,

I challenge anyone to slice meat with a T. rex tooth...  They are NOT
blades: they are banana-shaped.

>serrated teeth and mandibular
>adductors more powerful than the largest great white shark, was an
>EXCELLENT slasher.

Again, excellent tearer: yes.  Excellent slasher: no.

(Note: just because I find the evidence that tyrannosaurids were not
slashers doesn't mean I don't think that they weren't the best thing before
sliced bread!)

>I challenge anyone who doubts it to reconstruct the
>jaws and teeth of this animal, put a machine behind them that even roughly
>approximates the power of its head, neck, trunk, and legs, set it to work
>on a cow carcass, and observe the result.

Sounds like something people would do in Berkeley... :-)

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661