Saurornithoides inequalis Carpenter 1982, new combination
Creators: name: Carpenter, Kenneth
Publisher: name: Zenodo
Year: 1982
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Troodontidae
Saurornithoides
Saurornithoides inequalis

description: Saurornithoides inequalis (Sternberg, 1932), new combination
Fig. 2 a-h
Synonomies: Ornithomimus altus Lambe, 1902 (in part)
Polyodontosaurus grandis Gilmore, 1932
Stenonychosaurus inequalis Sternberg, 1932
Troodon formosus Russell, 1948
Saurornithoididae Sues, 1977
Material: Partial left dentary, UCM 41666 (U CMP-V 571 1); and basioccipital, UCM 43218 (UCMP-V5711).
Discussion: This material probably represents one individual, because: (1) the bones were found in the same sack of matrix; (2) of the similar color of the bones; and (3) of the minute size of the bones. The dentary fragment is ll mm long and has 13 alveoli preserved (Fig. 2 a, c, e).
The dentary fragment is similar to the dentary of Saurornithoides inequalis, NMC 8540, differing primarily in size. Comparison of the dentaries shows the following similarities: (1) on the external surfaces (Fig. 2 a, b) a shallow longitudinal groove connects the dental foramina; and (2) this groove shallows anteriorly. There are vertical ﬂutes anteriorly which extend ventrally from the dental border, and each encloses a dental foramen. Internally (Fig. 2 c, d), the inner dental parapet is lower than the outer dental parapet. A narrow, deeply incised meckelian groove extends along the ventral margin of the dentaries to a single foramen near the symphysis. Each alveolus (Fig. 2 e, f) is separated by an alveolar septum; no interdental plates are present.
The only differences between the two dentaries, other than size, are the weakly developed symphysis and lack of a prominent tuberosity below the symphysis for the geniohyoideus muscle in UCM 41666 (compare c and d, Fig. 2). Both of these features reﬂect the extreme immaturity of the specimen, and probably does not indicate taxonomic difference. In addition, there is a peculiar dorsobuccal twist of the symphysis in UCM 41666 which would cause the first dentary tooth to project obliquely away from the jaw. It is not certain whether this was to enable the hatchling to slit through the leathery egg shell with the tooth, or whether the condition was pathological. If pathological, this may be the first reported example of a birth defect in a dinosaur.
The basioccipital (Fig. 2 g, h) is similar to that figured by Russell (1969, Fig. 4), differing only in that some of the structures are underdeveloped; the dorsal neural groove is not as deep as that figured, nor are the basal tubera as prominent or as rugose. The occipital condyle, however, is well developed and separated from the main body of the basioccipital by a constricted neck. Sutural scars for the exoccipitals are present on each side of the dorsal neural groove and indicate that the exoccipitals contribute to only a small part of the occipital condyle, as in most theropods.
The minute size and underdeveloped condition of the dentary and basiooccipital suggest that these bones probably are from a hatchling. The great size disparity between hatchling and adult Saurornithoides is illustrated by comparing tooth row lengths: the first 13 alveoli in the hatchling dentary occupies 8.5 mm, while the first 13 alveoli in ROM 1445 occupies 26.6 mm, 28 mm in NMC 8540, and 30 mm in PMAA P 67.14.39. ROM 1445, NMC 8540, and PMAA P67.14.39 are almost the same size as the type Saurornithoides mongolíensis (AMNH 6516), and are believed to be from adults. It is not possible to measure the length of the first 13 alveoli in AMNH 6515 as the lower jaws are in occlusion (see Osborn, 1924, Fig. 3).
All the hatchling cranial (material is similar to specimens referred to Stenonychosaurus inequalis by Russell (1969). However, as will be shown in detail later (Carpenter and Paul, in preparation), Stenonychosaurus is a junior synonym of Saurornithoides. This was originally suggested by Russell (1969) and again by Barsbold (1974), although neither made the synonymy official. Only one species of Saurornithoides (S. inequalis) is recognized from the Late Cretaceous of Mongolia, and one is known from the Late Cretaceous of Mongolia (S. mongoliensis, Barsbold, 1974, notwithstanding).
The presence of Saurornithoides in North America is not surprising, as similarities between the dinosaur and mammal faunas of North America and Mongolia have been the subject of comment before (e. g., Kielan-Jarowowska, 1975; Fox, 1978).
description: Published as part of Carpenter, Kenneth, 1982, Baby dinosaurs from the Late Cretaceous Lance and Hell Creek formations and a description of a new species of theropod, pp. 123-134 in Contributions to Geology, University of Wyoming 20 on pages 127-129, DOI: 10.5281/zenodo.3238510