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Scottish coelophysoid tibia misinterpreted
While reorganizing my theropod file, I came across some interesting
observations concerning the incomplete tibia described by Benton et al.
(1995) from the Upper Broadford Beds Formation (Sinemurian, Early Jurassic)
of Scotland. This specimen (NMS.G.1994.10.1) was interpreted as a right
tibia missing the distal portion, but I think the authors may have oriented
it upside down and assigned it to the wrong side. For those with the paper,
I interpret figure 2a as a left tibia in anterior view, with the distal end
on top. I support my argument with the following reasons-
The "cnemial crest" is extremely low, never coming close to equaling the
"proximal end" in height, but rising gently towards the "proximomedial"
corner. It projects only slightly anteriorly and projects furthest at its
proximodistal midpoint. Those of coelophysoids and other theropods are
subequal or greater than the proximal end in height and have a
proximodistally long and vertical lateral edge to help enclose the incisura
tibialis. They are also much more pronounced and project furthest
anteriorly at their proximal tip. However, the recessed area above the
"cnemial crest" does strongly resemble facet for the astragalar ascending
process in coelophysoids, becoming taller laterally. The "cnemial crest"
would then be the "well developed step" described proximal to coelophysoid
ascending process facets (Carpenter, 1997). The supposed "fibular crest"
would be much too proximally located, and is instead the posteroventral
process. The tibia is compressed anteroposteriorly, like distal tibiae, but
unlike their proximal ends. The "proximal" view is completely unlike any
proximal tibia. The "cnemial crest" is much too short anteroposteriorly,
with the "lateral condyle" projecting far too laterally, with an odd
triangular shape. There is also no indication of a posterior groove between
the lateral and medial condyles. Reinterpreted, anterior is to the left in
figure 2e. The homologies are the same as above, with the "cnemial crest"
being the overhang of the ascending process' facet and the "fibular crest"
the posteroventral process.
How does this reinterpretation affect the specimen's phylogenetic
relationships? The prominent cnemial crest, two proximal tibial condyles
and fibular crest can no longer be used to place it as a theropod. It is
more derived than herrerasaurids because the distal tibia has a prominent
posteroventral process (also in the basal theropod Guaibasaurus), an
ascending process facet visible distally and a laterally rising ascending
process facet (not in Guaibasaurus). Neotheropods have anteroposteriorly
narrower distal tibiae (Rauhut, 2000). Thus, the only comparable taxa are
coelophysoids and Dilophosaurus. Dilophosaurus differs in having a more
expanded posteroventral process, as well as a higher ascending process
facet. These seem to be shared with neotheropods as well, so may suggest
the Scottish specimen is a coelophysoid. Other differences are the concave
anterior surface and sharp anteromedial corner in Dilophosaurus, both in
distal view. Among coelophysoids, I only have good tibial figures for
Liliensternus and Gojirasaurus. Compared to Gojirasaurus, the ascending
process facet doesn't extend as far distally, but is exposed more laterally.
The posteroventral process is stouter and there is a small anterolateral
process in distal view (as in Dilophosaurus). It is most similar to
Liliensternus, differing in the sharper laterodistal corner, anterolateral
process and face of the ascending process facet being angled posteromedally.
It thus seems distinct from the three taxa I can compare it to, and is
provisionally assigned to the Coelophysoidea.
I will send a scan of the figure from the paper to those who ask, as I would
like some opinion on my interpretation. I can also send a scan of the
tibiae of Gojirasaurus and Liliensternus for comparison.