Ornitholestes (meaning "bird robber") is a smalltheropoddinosaur of the lateJurassic (Brushy Basin Member of theMorrison Formation, middleKimmeridgian age, about 154 million years ago^[1]) of WesternLaurasia (the area that was to becomeNorth America).

Ornitholestes Temporal range:Late Jurassic (Kimmeridgian),154 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skeleton mount,Royal Tyrrell Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	Tyrannoraptora
Clade:	Maniraptoromorpha
Genus:	†Ornitholestes Osborn, 1903
Type species
†Ornitholestes hermanni Osborn, 1903

To date,Ornitholestes is known only from a single partial skeleton with a badly crushed skull found at theBone Cabin Quarry nearMedicine Bow,Wyoming, in 1900. It was described byHenry Fairfield Osborn in 1903. An incomplete hand was later attributed toOrnitholestes, although it now appears to belong toTanycolagreus. Thetype (and only known) species isO. hermanni. Thespecific name honors theAmerican Museum of Natural History preparator Adam Hermann.

Ornitholestes was the first theropod to be discovered in the 1900s.^[2] Theholotype skeleton (AMNH 619) was excavated in July 1900 in theBone Cabin Quarry inWyoming by anAmerican Museum of Natural History expedition by Peter C. Kaisen, Paul Miller andFrederic Brewster Loomis.^[3][4] It represents a partial skeleton with skull, including numerous elements of the vertebral column, the forelimbs, pelvis and hindlimbs. Henry Fairfield Osborn named and scientifically described the specimen in 1903.^[5] Thegenus nameOrnitholestes, initially suggested byTheodore Gill, means "bird robber" and is derived from theGreekὄρνις/ornis,ornithos ("bird") andλῃστήσ/lestes ("robber").^[6][7] Thespecies name (O. hermanni) honors Adam Hermann, the head preparator at the Museum, who directed the restoration and mounting of the skeleton.^[3]

An incomplete hand (AMNH 587) was assigned toOrnitholestes by Osborn in his 1903 description of the genus.^[8] However, as Gregory S. Paul (1988) noted, the poor preservation of the corresponding elements in the type specimen made this association "tentative."^[9] In 2005, Kenneth Carpenteret al. described a new small theropod,Tanycolagreus, whose skeleton was found in Bone Cabin Quarry only a few hundred yards from AMNH 587.^[10] Since AMNH 587 was virtually identical to the preserved hand of theTanycolagreus type specimen, it is now considered to belong to that dinosaur and not toOrnitholestes.^[11] Following this reassignment, Phil Senter (2006) noted that "our knowledge ofOrnitholestes can be drawn now only from the holotype."^[12] John Foster (2007) reported that some fragments fromDry Mesa Quarry may belong toOrnitholestes,^[13] though these have not yet been described.

In 1920Charles Whitney Gilmore concluded thatOrnitholestes was identical toCoelurus;^[14] in 1934Oliver Perry Hay recognised only a difference at the species level, naming aCoelurus hermanni,^[15] but in 1980John Ostrom revived the genus.^[16]

In his 1903 description, Osborn wrote that the length ofOrnitholestes along "the skull and vertebral column as restored" was 2.22 m (7.28 ft).^[3] However, this reconstruction was inaccurate, being based in part onOthniel Charles Marsh's restoration of thebasalsauropodomorphAnchisaurus, and the neck and trunk were both too elongated.^[17] David Norman (1985) and John Foster (2007) both estimated thatOrnitholestes was about 2 m (6.6 ft) long.^[18] Gregory S. Paul's 1988Predatory Dinosaurs of the World listed the length ofOrnitholestes as approximately 2.08 m (6.82 ft).^[19] Paul (1988) and Foster (2007) both estimated thatOrnitholestes weighed 12.6 kg (27.8 lb).^[20]John A. Long and Peter Schouten (2008) suggested a slightly higher figure, 15 kg (33 lb).^[21]

Ornitholestes was a bipedal carnivore.^[22] Its head was proportionally smaller than that of most other predatory dinosaurs, but the skull was heavily built, with a short snout and robust lower jaw.^[23] Theorbits (eye sockets) were quite large, measuring over 25 percent of the skull's length.^[24] There is no indication of abony eye ring.^[25]

The front teeth ofOrnitholestes were somewhat conical, with reduced serrations; the back teeth were recurved and more sharply serrated, similar to those of other theropod dinosaurs.^[26] Henry Fairfield Osborn (1903) counted four teeth in thepremaxilla, of which the front tooth was the largest in the upper jaw.^[27] In contrast,Gregory S. Paul (1988) depicted the skull with only three premaxillary teeth remaining, much smaller than those illustrated by Osborn.^[28] Eachmaxilla (main tooth-bearing bone in the upper jaw) contained ten teeth, and eachdentary (tooth-bearing bone in the lower jaw) contained twelve teeth.^[29] The tooth rows ofOrnitholestes were short, with the dentary (lower) row being even shorter than the maxillary (upper) row,^[30] even though the dentary bone itself was exceptionally long at the back, reaching a point below the middle of the eye socket.^[31] Teeth did not extend as far back as the orbits, and neither tooth row spanned much more than one-third of the skull.^[32]

An area of broken bone near the external naris (nostril) appears to bulge upward, which led Gregory S. Paul to suggest inPredatory Dinosaurs of the World (1988) thatOrnitholestes had a nasal horn "rather like a chicken's comb in looks."^[33] Both Oliver W.M. Rauhut (2003) and Kenneth Carpenteret al. (2005a) rejected that interpretation, and indicated that the upward flare of bone was due to post-mortem crushing of the skull.^[34] Paul's updated illustration ofOrnitholestes in his 2010Princeton Field Guide to Dinosaurs no longer contains the nasal horn.^[35]

Ornitholestes had a relatively short neck with a slight sigmoidal (S-shaped) curve.^[36] The tail was long and whiplike, comprising over half of the body's length.^[37] Not all of the vertebrae were preserved, but Osborn (1917) estimated thatOrnitholestes had nine or ten cervical (neck) vertebrae, thirteen dorsal (back) vertebrae, four sacral (hip) vertebrae, and 39 to 44 caudal (tail) vertebrae.^[38] Carpenteret al. (2005a) recorded that the specimen contained five sacral vertebrae.^[39]Ornitholestes was a short-bodied theropod, and this was reflected in the short front-to-back dimensions of the cervical and dorsal vertebrae.^[40]

The forelimbs ofOrnitholestes were relatively long, slightly under two-thirds the length of the hind legs.^[41] Thehumerus (upper arm bone) was heavily built, and somewhat longer than theradius andulna (forearm bones).^[42] Both the humerus and radius were straight-shafted.^[43] The claws on digits I and II of the hand were about the same size.^[44] Although the hand's third ungual (claw bone) was not preserved, extrapolation from the closest relatives ofOrnitholestes indicates that it was probably shorter than the first two.^[44]

Ornitholestes is often portrayed as a fast, long-legged theropod, but its lower limb bones were fairly short.^[45] Osborn (1917) calculated that the, missing,tibia (shin bone) was only about 70.6% as long as thefemur (thigh bone).^[46] Themetatarsals (foot bones) were spaced closely, but not fused together.^[47] As is typical of theropods, the feet were tridactyl (with three clawed weight-bearing toes).^[48] John H. Ostrom (1969) noted that the claw of digit II (the innermost toe) was larger than those of digits III and IV, and suggested that this digit may have borne a modified sickle claw similar to that ofDeinonychus.^[49] However, as both Ostrom (1969) and Paul (1988) noted, the poor preservation of digit II makes this hypothesis difficult to confirm.^[50]

TheinfraorderCoelurosauria, coined in 1914 byFriedrich von Huene, was traditionally ataxonomic wastebasket into which all small theropods were placed.^[51]Ornitholestes, due to its small size, was therefore generally classified as a coelurosaur.^[52] In 1986,Jacques Gauthier redefined this and several other paleontological terms in a more rigorous fashion, based oncladistic methods.Tetanurae was defined asmodern birds and all theropods more closely related to modern birds than toceratosaurs, while Coelurosauria now comprised all members of Tetanurae more closely related to modern birds than tocarnosaurs.^[53] In 1988,Gregory S. Paul suggested thatOrnitholestes was very similar in skull structure toProceratosaurus, a Middle Jurassic theropod from England.^[25] He placed these two genera together in Ornitholestinae—a new subfamily underAllosauridae—and speculated that they were more closely related to the much largerAllosaurus than to other small theropods.^[54] However, the classification ofOrnitholestes andProceratosaurus as allosaur relatives proved untenable (the latter has since proved to be atyrannosauroid), and Paul eventually abandoned it.^[55] All published cladistic analyses have shownOrnitholestes to be a coelurosaur as defined by Gauthier. Some analysis have shown support for the hypothesis that it is the most primitive member of the groupManiraptora, though more thorough analyses have suggested it is more primitive than theManiraptoriformes, and possibly a close relative of the "compsognathid"Juravenator starkii.^[56]

The following family tree illustrates a synthesis of the relationships of the major coelurosaurian groups based on various studies conducted in the 2010s.^[57]

In anabstract presented at the 2021conference of theSociety of Vertebrate Paleontology, Kimberley Chapelle and colleagues found "robust evidence" thatOrnitholestes is the earliest-divergingoviraptorosaur, based on aCT scan analysis of the skull.^[58]

In a 2001 study conducted by Bruce Rothschild and other paleontologists, 20 foot bones referred toOrnitholestes were examined for signs ofstress fracture, but none were found.^[59]

A biomechanical study conducted by Phil Senter (2006) used articulated casts of theOrnitholestes type specimen's right forelimb to determine its range of motion.^[60] Senter found that the antebrachium (forearm) could swing freely within a 95° range.^[61] Whenflexed (bent inward) at the elbow joint to the maximum possible extent, the humerus (upper arm bone) and radius (a lower arm bone) formed a 53° angle.^[61] The ability ofOrnitholestes to bend the forearm to an angle significantly more acute than 90° is characteristic of Maniraptoriformes, but absent in more primitive theropods such asCoelophysis andAllosaurus.^[62]

Even when fullyextended (straightened) at the elbow, the forearm did not form astraight angle, falling short of this by 22°.^[61]Pronation (twisting to make the palmar side of the hand face downwards) of the forearm was impossible, because the radius and ulna lacked rolling surfaces, meaning the forearm was in a permanent state ofsupination.^[44]

WhenOrnitholestes bent its elbows, this would cause the forearms to move inward, towards its midline.^[63] It may have used that ability to grasp prey with both hands simultaneously.^[64]

Henry Fairfield Osborn, in his 1903 description ofOrnitholestes, noted its large and conical front teeth, the "rapid grasping power" of its hand, and the "balancing power" of its tail; these he interpreted as adaptations to prey on contemporary birds.^[3] Osborn later repudiated this hypothesis, suggesting in 1917 thatOrnitholestes presented the early stages of a transition from a carnivorous to a more herbivorous lifestyle, but not beforeCharles R. Knight had drawn an influential and widely published illustration ofOrnitholestes chasingArchaeopteryx.^[65] Knight's illustration, and others derived from it, continued to appear in dinosaur books aimed at a popular audience throughout the 20th century.^[52]

David Norman (1985) conceded it was "just possible, though not very likely" thatOrnitholestes might have caught and eaten primitive birds.^[66] Most recent authors, however, have instead suggested a diet of small terrestrial vertebrates. Frogs, salamanders, mammals, lizards,rhynchocephalians, and hatchling dinosaurs would all have been potential prey items.^[67] Gregory S. Paul (1988) thought thatOrnitholestes might have used its conical front teeth to catch fish.^[33] Norman (1985) suggested that the robust skull and jaws might have allowedOrnitholestes to tackle "larger and more active prey" than other small theropods.^[68] In this vein, David Lambert (1993) speculated thatOrnitholestes, if it was a pack hunter, might have been able to "take onornithopods as big as a half-grownCamptosaurus."^[69]

In 2007, John Foster, a specialist on the Morrison Formation, suggested the possibility ofniche partitioning betweenOrnitholestes and its contemporaryCoelurus, which was within the same size range.^[70] He theorized that big-eyedOrnitholestes might have been specialized fornocturnal hunting, whileCoelurus may have focused on those prey species that were active during the day.^[70] Foster noted, however, that this hypothesis was largely speculative; a lack of preserved skull material fromCoelurus makes it impossible to verify whether its eyes were proportionally smaller than those ofOrnitholestes.^[71]

Paul (1988) estimated that a 12 kg (26 lb)endothermicOrnitholestes would have a daily dietary requirement of about 700 g (1.54 lb) of flesh.^[72]

OrnithologistPercy Lowe hypothesized in 1944 thatOrnitholestes might have borne feathers.^[73] However, this interpretation was largely disregarded for over half a century; most reconstructions of theropod dinosaurs, includingOrnitholestes, portrayed them with reptile-like scaly skin. One of the few exceptions to this was Gregory S. Paul'sPredatory Dinosaurs of the World (1988).^[74]Robert T. Bakker'sThe Dinosaur Heresies (1986) accurately predicted the presence of feathers on dromaeosaurids and contended that all dinosaurs were endothermic, yet did not incorporate feathers in its illustration ofOrnitholestes.^[75]

In 1996, the primitive coelurosaurSinosauropteryx was discovered in China. The well-preserved fossil showed that this dinosaur bore a coat of rudimentary furlike feathers. As a result of this and other similar discoveries, most paleontologists now consider it likely that all coelurosaurs had insulating feathers of some kind, or were descended from ancestors that did.^[76] Based on its phylogenetic position, John Foster (2007) inferred that the feathers ofOrnitholestes would probably have been more primitive than those of birds; they likely "would have covered the body except the legs in a short coat, perhaps with longer feathers lining the top of the skull or neck and the back edge of the forearms."^[77] Assuming they were present, these feathers would have been used for insulation, and possibly also forbrooding eggs.^[77]

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• Osborn, Henry Fairfield (1917). "Skeletal adaptations ofOrnitholestes,Struthiomimus,Tyrannosaurus".Bulletin of the American Museum of Natural History.35 (43):733–771.hdl:2246/1334.
• Ostrom, John H. (1969). "Osteology ofDeinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana".Peabody Museum of Natural History Bulletin.30:1–165.
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• Paul, Gregory S. (1988a)."Ornitholestians and Allosaurs".Predatory Dinosaurs of the World. Simon & Schuster. pp. 302–318.ISBN 978-0-671-61946-6.
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