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In the August issue of Discover, there is a nice
photo of the undescribed basal coelurosaur from the Moreno Hil Formation.
Although it would be inappropriate to write a details segment this early, I
thought I'd share some observations. Look forward to a real details
segment once it is described.
unnamed coelurosaur
Turonian, Late Cretaceous
Moreno Hill Formation, New Mexico
Remains- (~2 m, ~10 kg) two partial skeletons
including premaxilla, maxilla, partial lacrimal, jugal fragment, anterior
dentary, teeth, twenty-six vertebrae, manual elements, incomplete pubes, femora,
tibiae, proximal fibulae, incomplete metatarsus
Description-
This coelurosaur is nicknamed Fred, though it has
yet to be described or named. Two skeletons are reported, though I'm not
sure how much of the material shown belongs to each. A herbivorous lizard
was found associated, which is perhaps the theropods last meal. Note the
distal femora are placed on the opposite proximal femora.
The premaxilla was fairly short, with a sixty
degree or so anterior angle. It is toothed. The maxilla is rather
robust, with a maxillary foramen and anteriorly pinted antorbital
fenestra. The antorbital fossa was not well defined. There were at
least ten teeth, probably no more than thirteen in total. The teeth are
large and recurved. The posterior border of the antorbital fenestra is
oddly angled anterodorsally, perhaps due to deformation. The jugal was not
particularily slender. The dentary has a straight ventral edge, curving
upward anteriorly.
Several vertebrae are positioned as cervicals
and dorsals, but no details are available. A possible sacral vertebra has
a low, kidney-shaped centrum and perhaps expanded transverse processes to attach
to the ilium. Caudal vertebrae lack pleurocoels, have constricted ventral
margins and elongate distally to three times central height.
Manual elements are preserved, though I cannot
identify some precisely. Some are articulated and I think they are a
mid portion of phalanx I-1, distal metacarpal II, distal metacarpal III and
phalanx III-1. In that case, the metacarpals are slender and expanded
distally, and III is only slightly more slender than II. III-1 is
surprisingly elongate. The first digit consists of a broken portion of
phalanx I-1, starting about midway through mcII and ending past it. A
second possibility is that I have metacarpals I and III switched. This
would make metacarpal I longer, but phalanx I-1 shorter than expected.
Metacarpal III would be intermediate between I and II and phalanx III-1 would be
more appropriately sized. Four disarticulated elements are shown.
One looks like a short metacarpal I with a laterally expanded base and
asymmetrical distal condyles. This is compatable with the first
interpretation above. Another may be the base of metacarpal II, as it is
quite robust. Of course, it could also be a radial or ulnar end. Two
other elements are elongate, so represent I-1, mcII, II-1, II-2 or mcIII.
Manual unguals are reported.
The pubes are shown in anterior view and are
incomplete proximally and distally. They are joined for about half their
length and the lateral edges taper slightly distally, but are straight
sided.
The femora are missing middle portions. They
are slender with declined heads separated from the shafts by distinct
necks. They are bowed in anterior view and have larger medial than
lateral condyles. The tibiae are straight and missing only small sections
of the shafts. They are about 38% longer than the femora. The
cnemial crest is prominent, a fibular crest seems to be present and
the medial condyle is projected caudally and well separated from the
shaft. The distal end is better developed laterally, and seemingly
still attached to the astragalus and perhaps calcaneum. Proximal fibulae
narrow abruptly distally. Their proximal ends are divided into two
sections, a pointed anterior corner and a larger rounded posterior corner.
Three metatarsals missing most of their ends are preserved. If we can
trust these are placed correctly as II, III and IV, III is slightly more slender
than II and IV, which are curved outward a bit. It was not
arctometatarsalian.
Relationships-
Hard to tell at this point. I trust Tom that
it's a coelurosaur, as would be suggested by the slender manus, declined femoral
head and narrow fibulae. Various combinations of primitive characters
(skull with large recurved teeth, elongate distal caudals, elongate pubic
symphysis, third manual digit with phalanges, slender manual digit I,
non-arctometatarsalian metatarsus) exclude it from the Tyrannosauridae,
Ornithomimosauria, Segnosauria, Oviraptorosauria, Avimimus, Alvarezsauridae,
Troodontidae and Aves. The only options left are basal coelurosaurs, basal
tyrannosauroids, Bagaraatan and non-avian eumaniraptorans.
Compared to Coelurus, the pubes are not
proximolaterally convex and much narrower proximally. Metacarpal I is
short and more expanded medially. Manual phalanx III-1 is more
elongate.
Compared to compsognathids, the skull is more
robust, with a thicker jugal, less pointed snout and larger broader teeth.
The pubes are similar in general shape, but have a narrower and distally pointed
pubic canal. The femur has a mediolaterally shorter head and concave
proximal margin, while the tibia has a more separated medial condyle and more
extensive incisura tibialis. The proximoposterior portion of the fibula is
better developed and the metatarsus less elongate.
Compared to Nedcolbertia, the pubis is not concave
laterally. The femur is more slender. The tibia has a more
proximally developed cnemial crest and more separated medial condyle. The
fibula is more abruptly expanded proximally and has a greater proximoposterior
expansion.
Compared to Nqwebasaurus, metacarpal I is much
shorter and phalanx III-1 is more elongate. Metacarpal III is
shorter. The medial condyle is separated from the tibial shaft
posteriorly, the fibula narrows more proximally and the proximoposterior portion
is more prominent. The metatarsals are more robust.
Compared to Ornitholestes, the antorbital fenestra
is lower, maxilla and jugal more robust and dentary not decurved. The
tibia is much longer compared to the femur. The fibula is better developed
proximoposteriorly.
Compared to Proceratosaurus, the naris is smaller
and snout shorter.
Compared to Scipionyx, the antorbital fenestra is
longer, maxilla deeper, maxillary fenestra placed closer to the antorbital
fenestra and lacrimal narrower. Metacarpal III is shorter, phalanx III-1
is longer. The femur is more gracile, medial tibial condyle much better
developed and cnemial crest stronger. The fibula is more expanded
proximoposteriorly. It should be noted Scipionyx is a hatchling, so any
differences could be ontogenetic.
Compared to Dryptosaurus, the elements are
more slender and the femoral head not elevated. The incisura tibialis
does not project as far distally and the medial tibial condyle is larger, but
not separated from the shaft as much. The fibula narrows more
proximally.
Compared to Eotyrannus, the premaxillary angle is
lower and the antorbital fossa less extensive. Metacarpal I is shorter
and more expanded medially.
Compared to Bagaraatan, the anterior dentary edge
is less angled from the ventral edge. The pubes are joined in a
synphysis. The femoral head is not projected dorsally, the fibular condyle
is not as large and quadrangular and the tibial condyle is not as rounded.
The tibiotarsus is not convex mediodistally. The proximal fibula is not
pointed posteriorly, nor as projecting anteriorly.
Non-avian eumaniraptorans, such as dromaeosaurids,
Sinornithosaurus and Bambiraptor, are also comparable to the new specimen.
The maxilla is shorter, especially anterior to the antorbital fenestra, and has
a more ventrally located maxillary fenestra. The mid-caudal
prezygopophyses are more slender and probably not greatly elongated.
Metacarpal I is a bit shorter and better developed proximomedially.
Metacarpal III is more robust. The tibial condyle of the femur is not as
rounded. The cnemial crest is less extensive distally and the tibiotarsus
is not convex mediodistally. The anterior edge of the fibular shaft is
straighter.
Although the above comparisons suggest the new form
is a valid species, pinning down its relationships is difficult mainly because
of the obvious lack of description and high quality figures, as well as the
uncertain interrelationships of basal coelurosaurs. The short
preantorbital maxilla is similar to non-maniraptorans, and the short metacarpal
I and elongate phalanx III-1 are more primitive than most coelurosaurs, although
dromaeosaurids also have the latter characters. It's not surprising the
specimen was initially identified as a dromaeosaurid, as the appendicular
elements are very similar. However, the probable lack of elongate caudal
prezygopophyses cast doubt on this identification, and more primitive femoral
and dental characters will probably be identified. I recommend placing
this specimen as a basal coelurosaur and look forward with great interest to its
description.
Anyone who wants scans of the skeleton and skull,
ask offlist.
Mickey
Mortimer |