|
Before I tackle Caudipteryx, I thought I'd
enlighten everyone as to the facts behind one of the most mysterious
dinosaurs around- "Beelemodon".
"Beelemodon" Bakker 1997
Kimmeridgian-Tothonian, Late Jurassic
Morrison Formation, Wyoming, USA
Material- (TATE 546) (~1.5-4 m) tooth (7.1 mm
long, FABL 5.4 mm)
(TATE coll.) tooth (~9 mm)
Diagnosis-
Currently indeterminate pending more detailed
comparison to several theropod taxa.
Description-
This taxon is still a nomen nudum, as it is not yet
diagnosed, nor does it have a species name. Bakker describes it as an
"omnivorouscarnivorous dinosaur of uncertain relations" and an "enigmatic
dinosaur". It is supposedly "coyote-to-wolf size". Although using
tooth size to determine total length is extremely risky, comparison
to various theropods indicates a length of 1.5-4 meters is probable,
depending on body form. It is unclear whether postcranial remains can be
referred to the taxon, as only teeth are described and illustrated. A
single tooth is illustrated in side view and cross section. Another tooth
is plotted in the "denticle-width vs. crown height" graph, indicating a slightly
larger specimen is known as well.
The illustrated tooth is slightly recurved,
laterally compressed (50% as wide as anteroposteriorly long) and missing its
distal tip. Fluting is present on the illustrated side. The root is
constricted, the anterior carina lacks serrations and the posterior carina has
serrations extending to the base. The serrations are small (4.3
per mm, ~35 on the whole crown), pointed and project slightly distally.
The cross section indicates it was fairly symmetrical labiolingually, narrowing
anteriorly and exhibiting a slight anterior expansion labially(?) and a slight
posterior expansion lingually(?).
Relationships-
At first glance, these specimens look very similar
to ornithischian premaxillary teeth. The posterior two premaxillary teeth
of Lesothosaurus have anterior serrations, but lack them posteriorly except at
the tip. This is the reverse of the case in "Beelemodon". The
serrations are comparatively larger (~15 per tooth if they extended as
proximally as in "Beelemodon") and do not extend to the base of the crown.
Drinker has a very similar tooth morphology, with serrations present only on the
posterior carina. These serrations are slightly larger (25-30 per tooth)
and have longer interdenticle slits. The tooth itself is not
recurved, but is otherwise similar in shape. Galtonia also has similarily
shaped teeth, but with larger serrations and anterior serrations present
distally. "Beelemodon" is obviously based on theropod maxillary or dentary
teeth however, as the premaxillary teeth of most theropods have serrations
displaced so that the distance between them is much longer labially than
lingually. Troodontids, tyrannosaurids and ornithischians have
premaxillary teeth that not only have the latter character, but are also much
wider labiolingually than "Beelemodon". The cross section of "Beelemodon"
is very similar to theropod maxillary and dentary teeth.
While "Beelemodon" is obviously theropod, placing
it within that clade is a more difficult task. The constricted root is
known in Compsognathus, Pelecanimimus, segnosaurs, Protarchaeopteryx,
Caudipteryx, mononykines, Archaeornithoides, troodontids, Microraptor,
Archaeopteryx and toothed pygostylians. Therefore, chances are pretty good
this is a coelurosaur. Compsognathus has some teeth that have unserrated
anterior carinae and serrated posterior carinae. These have larger
serrations relative to crown height (20-25 per tooth). They are shaped
similarily and have similar serration morphology. Pelecanimimus has yet to
be described in detail, but has both anterior and posterior carinae
unserrated. Segnosaurs differ in having crowns that are less
recurved, more elongate and labiolingually wider, with much larger posterior
serrations (8-10 per tooth) and equally sized anterior serrations.
Protarchaeopteryx is described briefly, but differs in having anterior
serrations that are slightly larger compared to crown height (20-30 per
tooth). The highly elongate, needle-like teeth of Caudipteryx lack
serrations altogether, so are very dissimilar. Mononykus has unserrated
carinae, more elongate and less recurved crowns. The teeth of
Archaeornithoides differ in lacking both serrations and carinae, as well as
being nearly conical. Archaeopteryx has teeth that are completely
unserrated, lack posterior carinae and are much wider labiolingually. They
are similarily proportioned and have unserrated anterior carinae. Some
troodontid teeth lack anterior serrations, but have them posteriorly, and
are laterally compressed. The teeth of "Beelemodon" differ from
troodontids in being less recurved, lacking hooked serrations and having
comparatively smaller serrations (compared to 15-20 per tooth).
Microraptor is similar in having crowns with unserrated anterior carinae and
posterior carinae with distally projecting serrations. It differs in
having larger serrations in comparison to crown height (20-25 per tooth), longer
blood grooves and a wider crown. Pygostylians have unserrated crowns
without carinae that are very wide, so are similar to Archaeornithoides, but
dissimilar to "Beelemodon". "Velociraptorines" also sometimes lack
anterior serrations, are laterally compressed and have similar amounts of
serrations (15-35), but are more recurved and lack basal
constriction.
Therefore, the greatest resemblence is to
Compsognathus, although the few facts known about Protarchaeopteryx are also in
agreement and a deinonychosaur might also be expected to evolve a similar tooth,
judging by comparisons with Microraptor, Saurornitholestes and Morrison
"velociraptorine" teeth (Britt 1991). Are there any theropods already
known from the Morrison Formation that could have "Beelemodon" teeth?
Although Morrison compsognathids are not known, both Coelurus and Ornitholestes
are close phylogenetically and have poorly described or unknown teeth.
Protarchaeopteryx comes out as a segnosaur-oviraptorosaur in my phylogeny, so
perhaps these teeth belong to the Morrison segnosaur-oviraptorosaur, known from
two cervical vertebrae. Finally, although reported Morrison
"velociraptorine" teeth lack constricted roots (Britt 1991), it is not
inconceivable "Paleopteryx" had a mix of avian and dromaeosaur characters in
it's teeth, like Microraptor. There are therefore several taxa to which
"Beelemodon" could be reasonably referred. However, as it is currently
impossible to chose one over another, they should be left separate. Given
the amount of variation in serration number in a single theropod genus
(Allosaurus- 20-35; Saurornitholestes- 15-35), there is no way to separate
"Beelemodon" from Compsognathus or Protarchaeopteryx at this point.
Because of this, it must remain indeterminate. The fluting
or serration morphology may eventually prove diagnostic, but this cannot be
determined from the available literature. I recommend classifying
"Beelemodon" as a provisionally indeterminate coelurosaurian nomen nudum until
further research is done.
reference- Raptor family values: Allosaur parents
brought great carcasses into their lair to feed their young. Bakker, R. In
“Dinofest International”, Proceedings of a Symposium, Academy of Natural
Sciences, eds Wolberg, Sump and Rosenberg, 51 - 63 (1997).
Anyone who wants a scan of the tooth, ask me
offlist.
Mickey Mortimer
|