Skeletal Adaptations of Ornitholestes, Struthiomimus, Tyrannosaurus
Author
Osborn, H. F.
text
Bulletin of the American Museum of Natural History
1917
1917-12-31
35
733
771
http://digitallibrary.amnh.org/handle/2246/1334
journal article
10.5281/zenodo.1040385
3bd7517c-522d-4aaa-b097-7bc41006546b
1040385
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:
|
Ornitholestes
|
Struthiomimus
|
Allosaurus
|
Tyrannosaurus
|
| Cervicals |
? 9 |
10 |
9e |
10-9 |
| Thoracic dorsals |
?13 |
13 |
14 |
13-14 |
| Cervico-thoracics, presacrals |
23e |
23 |
23 |
23 |
| Sacrals |
4 |
?5 |
5 |
5 |
| Caudals |
?39-44 |
?40 |
?53 |
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.
1
Osborn, H. F.
“
Ornitholestes hermanni
,
a
New Compsognathoid Dinosaur from the Upper Jurassic.
”
Bull.
Amer
. Mus. Nat. Hist., Vol. XIX,
Art
. xii, July 23, 1903, pp. 459
—
164.
2
In the National Museum series of
Allosaurus
there were found
nine
cervicals, fourteen dorsals, and five sacrals; there were two
breaks
in the series, but evidently the whole number was present.
Letter
of C. W. Gilmore,
January
8, 1916.
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.
Fig
. 2.
Incomplete
manus
of
Ornitholestes
hermanni
; Digit
I
is
missing
in this
specimen
.
Amer.
Mus. No.
5
87.
Palmar
(
A
) and internal (B) views,
one-half natural size. After
Osborn, 1903
.
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
).
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,
C
2, p.
C
3 carpalia and
(?)
p.
tendinous ossicle.
One
third natural size.
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.