Ornitholestes hermanni (Osborn, 1903)

Osborn, H. F., 1917, Skeletal Adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus, Bulletin of the American Museum of Natural History 35, pp. 733-771 : 735-738

RESTUDY OF ORNITHOLESTES HERMANNI OSBORN.

Text-figs. 1-3 , Plate XXVI .

The original reconstruction of the skeleton ( Osborn, 1903, fig. 1, p. 460)1 is very inaccurate. It was based partly upon Marsh’s restoration of the skeleton of Anchisaurus . The cervical ribs are incorrectly restored; the presacral vertebrae, restored as twenty-eight, are certainly too numerous; the digits of the manus ( Osborn, 1903, figs. 1, 2, 3) are restored as spreading instead of closely appressed; the skull is incorrectly drawn.

The following characters may be pointed out in the new reconstruction shown in Plate XXVI , which is based upon a restudy of the type specimen ( Amer. Mus. No. 619 ) supplemented by the manus of the second specimen ( Amer. Mus. No. 587 ).

Vertebral formula. The vertebral formula is uncertain; it is estimated in comparison with that of other Theropoda as follows:

It will be observed that there is a constant number of twenty-three pre-sacrals in Struthiomimus, Allosaurus , 2 and Tyrannosaurus , which renders it probable that Ornitholestes also had not more than twenty-three pre-sacrals.

The boundary line between the cervical and thoracic vertebrae in the American Museum skeleton of Ornitholestes is uncertain because the two vertebrae C9 and C10 are missing, but in Struthiomimus , in which the cervical series is complete, the boundary line is clearly indicated by the change in the character of the ribs from the fixed, coalesced condition (C10) to the movable bicipital condition (Th. 1).

Skull. (1) Teeth: premaxillary 4, maxillary 10; crowns not so serrate or prehensile as in typical carnivorous dinosaurs. (2) Cranium with two antorbital openings, prominent premaxillaries, typical theropod arrangement of lachrymals, frontals, postorbitals, parietals, squamosals, quadratojugals, opisthotics. (3) Dentary with twelve small upright or slightly recurved teeth. (4) Jaw with sessile coronoid process.

Vertebrae. Cervicals nine or ten in number, gently opisthocoelous; dorsals amphicoelous; caudals amphicoelous. The supposed 7th and 8th cervicals are moderately elongate, slightly opisthocoelous, with separate attachments for the capitulum of a freely articulated rib on the anterior portion of the centrum and for the tuberculum on the broad diapophysial extension of the neural arch. The neuro-central suture is faintly indicated. It appears that the cervical ribs were slender, free, and two-headed in Ornitholestes while in Struthiomimus they are firmly coalesced with the sides of the cervical centra and arches, a progressive character. For other details of the vertebrae see original description ( Osborn, 1903, p. 460). Four firmly coalesced sacral ribs are suturally distinct. Caudals 39- 44e, anterior caudal ribs coalesced. The very characteristic rod-like elongation of the zygapophyses of the caudal vertebrae begins with the vertebra supposed to be C13; it reaches its maximum between C16 and C24. In these vertebrae the chevrons are depressed, elongate, bifurcated in front, with a posterior keel. In Struthiomimus the rod-like elongation of the prezygapophyses begins with C12 and reaches the maximum in C26-C30, which are so rigidly articulated by the elongate zygapophyses and chevrons as to admit of little or no movement. Pelvic girdle. The pelvic girdle is broadly similar in contour to that of Struthiomimus , but ilium, pubes, and ischia are suturally separate, while in Struthiomimus they are firmly coalesced; pubes with deep symphysial union; ischia more slender, with proximal anterior processes united distally; peduncle of pubes missing in the type.

Hind limb. Femur (.207m.) and tibia (.159m.) estimated to be of unequal length, the femur being the longest bone. In Struthiomimus the tibia is more elongate, a cursorial adaptation. The pes tridactyl; median metatarsal, Mts. III (.117 m.), relatively less elongate than in Struthiomimus : condition of Mts. V unknown. The total length of the hind limb is.483 m., the tibio-femoral ratio is.706, which indicates that the movement of this animal was not so swift as that of Struthiomimus (Ornithomimus) , in which the. tibio-femoral ratio is: 1.11 (T.540-F. 480). It appears from the preserved terminal phalanges that they were more recurved and clawlike than in Struthiomimus (Ornithomimus) .

Fore limb. In the fore limb of Ornitholestes the humerus is somewhat longer (.127 m.) than the ulno-radius, which is also the proportion observed in Struthiomimus . In the figures accompanying the original description of O. hermanni ( Amer. Mus. 587 , see Figs. 7A, 7B , p. 748) the proportions are correctly represented.

We observe a marked anisodactyly and lateral compression of the manus ( Figs. 2 , 3 B ). Mtc. II is enlarged and greatly elongate while Mtc. III is reduced and abbreviated, and Mtc. IV is vestigial; Digit I is partly known in the type as being short and somewhat divergent ( Plate XXVI ); Mtc. I and Ph. I. 2 only are preserved. This manus may, therefore, be described as functionally anisotetradactyl whereas that of Struthiomimus is comparatively isotridactyl, indicating a very marked difference in habit. In other words, the manus of Ornitholestes while subraptorial is more highly reduced than that of Struthiomimus . The central digits are closely appressed to each other (as in Struthiomimus ) while the claws are relatively more powerful and recurved ( Fig. 7B ) than in Struthiomimus . The Ornitholestes manus is losing its fitness as a raptorial grasping type, it is too feeble and too much reduced; it represents a species adaptation to another purpose, recalling in some features the slender and greatly elongated digits of the Aye- Aye ( Cheiromys). In ( Fig. 3 ) Ornitholestes , Struthiomimus , and the manus associated with Ornithomimus ( Fig. 3 a ) metatarsal I is closely appressed to metatarsal II. The phalanges of the pollex are divergent and rotated inwards in the two genera first named, Ornitholestes and Struthiomimus .

The fact that the central digits of Ornitholestes are closely appressed and incapable of spreading laterally harmonizes with the incipient coalescence of Mtc. I and Mtc. II in Struthiomimus . Another harmony is the divergence of the phalanges of Digit I.

Figures

Fig. 1. Reconstructed skull of type of Ornitholestes hermanni, Amer. Mus. No. 619, replacing the original figure (1903). Two-thirds natural size.

Fig. 2. Incomplete manus of Ornitholestes hermanni; Digit I is missing in this specimen. Amer. Mus. No. 587. Palmar (A) and internal (B) views, one-half natural size. After Osborn, 1903.

Fig. 3. Composition of complete manus (B) palmar view of Ornitholestes hermanni. Amer. Mus. 587, partly restored (Digit I) from Amer. Mus. 619, type. One half natural size. Manus of Struthiomimus altus, Amer. Mus. 5339. (A), dorsal aspect; A', palmar aspect, r, radiale, i, intermedium (?) coalesced with cc, centrale. Cl, C2, p. C3 carpalia and (?) p. tendinous ossicle. One third natural size.

Fig. 7. Humerus of Struthiomimus altus and of Choloepus didactylus compared. A Crushed left humerus of S. altus, Amer. Mus. 5339. B Normal left humerus of S. altus, Amer. Mus. 5201. C Right humerus, anterior and posterior views of S. altus, Amer. Mus. 5201. D Right humerus of two-toed sloth, Choloepus didactylus, Amer. Mus. 35513, posterior view. All figures two-ninths natural size.

Plate XXVI. Reconstruction of the skeletons of Struthiomimus altus (left) and Ornitholestes hermanni (right). Struthiomimus 1/10, Ornitholestes 1/6 natural size, The Ornitholestes restoration replaces the original restoration by Osborn in 1903 which is very faulty. The Struthiomimus, Amer. Mus. 5339, mount has the distal end of the tail restored from Amer. Mus. 5355; dotted vertebra from Amer. Mus. 5262, In both restorations the pollex is too closely appressed to the other digits, see Fig. 3.

Fig. 3a. (A1-A3). Genotype specimen of Ornithomimus velox Marsh. Yale Mus. 542. (A1), dorsal aspect of the type left pes, superior aspect of Metatarsals II-IV. (A2) posterior aspect (upper) and exterior aspect (lower) of metatarsals II-IV. (A3) Type tibio-tarsus, anterior, inferior, posterior aspects. (B) Portion of right manus of another individual (Yale. Mus. 548) associated by Marsh with Ornithomimus velox; (upper) metacarpals I-III as they appear in the matrix, (lower) as placed in natural position. (C) Left pes of Struthiomimus altus, genotype specimen, Amer. Mus. 5339. Anterior and exterior aspects of digits II-V. One fourth natural size. After original drawings by the present author. One-fourth natural size.