Tracks and Taphonomy (was: sauropod quantity)

[Josh Smith <smithjb@sas.upenn.edu>]

"Jaime A. Headden" wrote:

> Josh Smith wrote:
> <<Where is this coming from, George? First of all, if
> we are talking truly "chicken-sized" and
> "sparrow-sized" in the Triassic and Jurassic, then we
> are talking about the Norian to the, say Toarcian, ja?
> If these are the ages we are discussing, then there
> are not "thousands and thousands" of tracks in this
> size class. Indeed, these tracks are biased towards
> the mid-range (from about 4-8 inches in track-length).
> If we discuss only the Newark Supergroup ichnofauna,
> then I would doubt that there are more than a couple
> hundred such tracks.>>
>
> and Dinogeorge replies:
>
> <Just finished reading the relevant section of
> Lockley's The Eternal Trail, wherein he notes chicken-
> to sparrow-size theropod tracks from Navajo Sandstone
> (prints 1.5 inches long), from Connecticut, from
> Newark, from Argentina, from Brazil (Botucatu Fm.).
> See pp. 121-122. I believe Ellenberger has tracks in
> this size range from South Africa, too. These small
> dinosaurs were apparently distributed worldwide during
> Late Triassic through Late Jurassic times, most
> widespread during Early to Middle Jurassic.

    I am not going to (yet) go into rampant Lockley-bashing, but I don't
think he has a good concept of the Newark Supergroup ichnoassemblage.
There really are not many REALLY small "theropod" tracks from these
units.  It depends on his definition of "widespread and abundant", but
the data for "very small 'theropod' tracks" from the Newark series
certainly don't reflect MY definitions of those words.

>
>
> These are not Coelophysis-size tracks but several
> times smaller. The smallest dinos I can think of off
> the top of my head that are earlier than Compsognathus
> are Segisaurus and Procompsognathus, which might have
> made the >largest< of those small theropod tracks.
> There are no "sparrow-size" nonavian dinos known from
> skeletal remains (maybe some teeny teeth) at all,
> period. The tracks compare in size with, but are
> otherwise different from, those of small mammals found
> in the same localities.>
>
>   It is true that we know of no adult sparrow-sized
> dinosaurs in the Triassic, and even through the EJ,
> but around the Aves-formative years of the MJ and LJ,
> certainly this is a bit anticipatory? No adults, but
> juveniles, certainly, there is an abundance of. The
> smaller the animal, the smaller the baby, generally,
> though on occasion (as in the kiwi) there is a severe
> misproportion of juvenile-to-adult ratio. These tracks
> may represent juveniles of some larger animals.
>
>   And how would they need be theropods? During the
> time frame suggested above (Nor-Toarc) there are
> perhaps a superabundance of non-theropods that walked
> three-toed and would have left slender-toed prints
> without impression of the hallux, including smaller
> "prosauropods" like *Thecodontosaurus*, and
> "hypsilophodontids," especially, and all basal
> ornithschians, generally. These tracks could pertain
> to a wide range of taxa, especially in the Karoo
> though later stages in the South African region, where
> *Heterodontosaurus*, baby prosauropos like
> *Euskelosaurus*, and *Massospondylus* (my fave) would
> have roamed.
>
>

    The data that I and some others have generated (namely Jim Farlow
and Paul Olsen) for small, Newark tridactyl tracks are preliminary but
tend to indicate a stronger theropod signal than a non-theropod.  It is
a question of where you place the phalangeal articulations within the
digits of the footprints, but the data supporting the prosauropod
hypothesis are even weaker than the data supporting the theropod
hypothesis.
    And I know that Lockley runs around saying that these tracks HAVE to
be theropod because they look like theropods would have made them, but
come on, this is the same guy that published a paper saying that a large
"theropod" track from the Maastrichtian was a _T. rex_ footprint because
we don't have another large carnivore known from the record that could
have made such a track.

    Jaime's point is well made, logical, and perhaps probable.  The data
don't really support his hypothesis right now, but I will be the first
person to say loudly that the data really don't tell us that much yet.
Lack of data never seems to stop us from arm-waving in paleo, but if
Tom's legacy is going to be "wait until the damn paper comes out,
please" then I guess I won't have too much of a problem being the "uhm,
ok, but where are the data to support that idea?" guy.


--
Josh Smith
Department of Earth and Environmental Science
University of Pennsylvania
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