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Re: Tyrannosaurs gorged on the young



Larry Dunn wrote:
>So, what conclusions about tyrannosaur behavior can
>the lay-dude safely draw from this data?  
        The data support the conclusion that tyrannosaurs died.

>Was preying on sub-adults the tyrannosaur's likely MO
>(like the MO of many extant predators)?  Are there
>modern predators that take exclusively juvenile prey? 
>(I am always a little loath to read too much into
>evidence.  Help me out here.)
        Certainly less loath than the presenter. Let's look at the evidence,
as I remember it (please, anyone with notes, feel free to correct/add to this):
        1) A disarticulated partial tyrannosaurid skeleton was found. It was
said to be probably an in _situ burial_.
        2) Some altered caudal vertebrae of at least one juvenile hadrosaur
were found. These verts show a spongey texture different from the normal
high quality of preservation in the area, and this is interpreted as being
the result of the action of digestive acids.
        3) The depositional environment was interepeted as being low energy
and the bones were not considered likely to have been transported in.
        4) I'm not sure, but I think the presenter implied that these bones
were from the belly area of the animal.

        From this, and the suggested abundance of juvenile hadrosaurs in the
environment (which assertion seems well-founded), it was suggested that
tyrannosaurs liked to eat the juvie duckbills.

        However:
        1) Although claims were made regarding the degree to which the
tyrannosaur skeleton had not been disturbed, it was (IMHO) impossible to
detect any pattern in the locations of the bones indicating the form of the
body. On the other hand, a friend of mine noted that the bones did appear to
be preferrentially oriented, and certainly no evidence (e.g., a rose
diagram) was presented to demonstrate otherwise. Also, it did not appear to
be a complete specimen.
        2) We find bones with this texture all the time in Big Bend. From
their occurrances in bone beds and other situations it is usually suggested
that they are just extremely weathered bones. Otherwise, we would have to
assume an environment replete with digested bone. Weathering can affect
bones differently even at the same site, and bones which have remained at
the surface for a long time will be more strongly affected. It may be true
that this type of weathering is less common in the north, or that may be
more of a collection bias (given the large number of better quality
specimens up there). In any case, without some sort of evidence (e.g., SEM
photos comparing weathered and stomach acid-etched bones), I would regard
this interpretation as speculative.
        3) In Big Bend we find disarticulated skeletons mixed up in
mudstones all the time. In many cases, these skeletons have undergone some
degree of hydraulic sorting, despite the low-energy setting. I hardly think
that my closely associated hadrosaur ate the nasty, "styrofoam"-textured
hadrosaur bones I found with it. In fact, I think those bones are actually
part of its own skeleton.
        4) See #1, above.

        I was originally perplexed by the presence of well-weathered
hadrosaur vertebrae next to a less weathered tyrannosaur femur, since
vertebrae are usually considered to be easily transported relative to
heavier long bones. However, my hadrosaur site has a femur and portions of
both tibia, along with several vertebrae and other indications of being a
transported assemblege.

        And, of course, as Richard pointed out, one instance does not a
lifestyle make.

        Wagner

--------------------------------------------------------------------------------
     Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
  "Why do I sense we've picked up another pathetic lifeform?" - Obi-Wan Kenobi