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.
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
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.
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.