Ornitholestes hermanni

Ornitholestes hermanni is the only known species of the extinct genus Ornitholestes ("bird hunter& #34;) of a coelurosaurian theropod dinosaur, which lived at the end of the Jurassic period, approximately 157 to 145 million years ago, from the Kimmeridgian to the Tithonian, in what is now North America. All that is known about Ornitholestes stems from a single specimen discovered in 1900 at the Bone Cabin Mine near Medicine Bow, Wyoming, and described by Henry Fairfield Osborn in 1903. An incomplete hand was initially attributed to Ornitholestes, but is now considered to belong to Tanycolagreus. The name of the only known species O. hermanni, honors American Museum of Natural History curator Adam Hermann. Ornitholestes is a coelurosaurian, similar in many ways to Compsognathus, although much larger. It was an agile carnivore that lived in wooded habitats. It has probably fed on lizards and mammals; although it could also have been an opportunistic scavenger.

Description

Ornitholestes was a bipedal carnivore. Its head was proportionally smaller than most other predatory dinosaurs, but the skull was strongly built, with a short snout and stout lower jaw.. The orbits, the eye sockets, were quite large, measuring more than 25 percent of the skull's length. There is no indication of a bony eyering.

Reconstruction of Ornitholestes

Ornitholestes was a coelurosaurian, similar in many ways to Compsognathus, but larger, measuring just over 2 meters long and 0.80 meters tall, with an estimated weight of 13 kilograms. The head of Ornitholestes is relatively small. In his 1903 description, Osborn wrote that the length of Ornitholestes along the "skull and the restored spine" it was 2.22 metres. However, this reconstruction was inaccurate, as it was based in part on Othniel Charles Marsh's restoration of the basal sauropodomorph Anchisaurus and the neck and trunk were too elongated. David Norman in 1985 and John Foster in 2007 estimated Ornitholestes to be approximately 2 meters long. Gregory S. Paul's Predatory Dinosaurs of the World in 1988 listed the length of Ornitholestes as approximately 2.08 metres. Paul in 1988 and Foster in 2007 estimated that Ornitholestes weighed 12.6 kilograms. John A. Long and Peter Schouten in 2008 suggested a slightly higher figure, 15 kilograms.

Comparison of size Ornitholestes.

It had small, jagged teeth and possibly hunted small animals such as lizards and mammals. However, the skull is more robust, compared to other small theropods, such as Compsognathus and Coelophysis, which must have given Ornitholestes a powerful bite. The front teeth of Ornitholestes were somewhat conical, with reduced serrations. The rear teeth were recurved and more serrated, similar to those of other theropod dinosaurs. Henry Fairfield Osborn in 1903 counted four teeth in the premaxilla, of which the front tooth was the largest in the upper jaw. In contrast, Gregory S. Paul in 1988 described the skull with only three remaining premaxillary teeth, much smaller than those illustrated by Osborn. Each maxilla, the main tooth-bearing bone in the upper jaw, contained ten, and each dentary, in the lower jaw contained twelve teeth. The tooth rows of Ornitholestes were short, with the lower dentary row even shorter than the upper maxillary row, despite the fact that the dentary bone was exceptionally large. long at the back, coming to a point below the center of the eye socket. The teeth did not extend as far as the orbits, and no single row of teeth spanned much more than a third of the skull. The teeth were serrated. two of Ornitholestes clearly identify it as a carnivore, but its exact diet is a matter of debate in the paleontological community. In the original 1903 description, Henry Fairfield Osborn suggests that Ornitholestes must have eaten contemporary birds, based on the "quick and powerful grasp of nimble and delicate prey" suggested by the shape of the hand. In 1917, however, Osborn re-evaluated the hand and determined that it was unsuitable for this purpose. More recently, Robert T. Bakker has speculated that Ornitholestes probably hunted small mammals, noting that "como's hairballs were the exact size of the predator's jaws."

Illustration of the holotype skull.

An area of broken bone near the external nose, blowhole, appears to bulge upward, leading Gregory S. Paul to suggest the presence of a small horn on the snout of Ornitholestes, similar to that of Proceratosaurus, "chicken-like in appearance". Both Oliver WM Rauhut in 2003 and Kenneth Carpenter et al. in 2005 rejected that interpretation and indicated that the upward flash of the bone was due to postmortem crushing of the skull. Paul's updated illustration of Ornitholestes in his 2010 Princeton Field Guide to Dinosaurs no longer contains the nasal horn. Like other theropods, Ornitholestes had a long tail

Hand of the holotype, with the first finger restored to the right.

Ornitholestes had a relatively short neck with a slight sigmoid curve, shaped like an S. half the length of the body, presumably to aid balance. Not all vertebrae were preserved, but Osborn in 1917 estimated that Ornitholestes had nine or ten cervical vertebrae, thirteen dorsal vertebrae, four sacral vertebrae and 39 to 44 caudals. Carpenter et al. in 2005 recorded the specimen as containing five sacral vertebrae. Ornitholestes was a short-bodied theropod, and this was reflected in the short front-to-back dimensions of the cervical and thoracic vertebrae.

The forelimbs of Ornitholestes were relatively long, a little less than two-thirds the length of the hindlimbs. The humerus, the upper arm bone, was strongly built and somewhat longer than the radius and ulna, bones of the forearm. Both the humerus and radius were straight-shafted. The claws on digits I and II of the hand were approximately the same size. Although the third ungual of the hand, the claw bone, was not preserved, extrapolation from the closest relatives of Ornitholestes indicates that it was probably shorter than the first two.

Ornitholestes is often depicted as a fast, long-legged theropod, but its lower limb bones were rather short. Osborn in 1917 calculated that the missing tibia was only 70.6% of the femur. metatarsals were closely spaced, but not fused. As is typical of theropods, the feet were tridactyl, with three clawed, weight-bearing toes. John H. Ostrom in 1969 noted that the claw of digit II, the innermost toe, was larger than that of digits III and IV, and suggested that this digit might have had a modified sickle similar to that of Deinonychus. However, as observed by Ostrom in 1969 and Paul in 1988, the poor preservation of digit II makes this hypothesis difficult to confirm.

Discovery and research

Ornitholestes was the first theropod discovered in the 1900s. The holotype skeleton AMNH 619 was excavated in July 1900 at the Bone Cabin Quarry in Wyoming by a American Museum of Natural History expedition by Peter C. Kaisen, Paul Miller, and Frederic Brewster Loomis. Depicts a partial skeleton with skull, including numerous elements of the vertebral column, forelimbs, pelvis, and hindlimbs. Henry Fairfield Osborn named and scientifically described the specimen in 1903. The genus name Ornitholestes, initially suggested by Theodore Gill means "bird thief" and is derived from the Greek ὄρνις, ornis, ornithos, "bird" and λῃστήσ, lestes, "thief". The species name, O. hermanni, honors Adam Hermann, the Museum's principal preparer, who directed the restoration and mounting of the skeleton.

Osborn assigned an incomplete hand, AMNH 587, to Ornitholestes in his 1903 description of the genus. However, as noted by Gregory S. Paul in 1988, poor preservation of corresponding elements in the type specimen made this association "tentative". In 2005, Kenneth Carpenter et al. described a new small theropod, Tanycolagreus, whose skeleton was found in the Bone Shack Quarry just a few hundred yards from AMNH 587. As AMNH 587 was virtually identical to the preserved hand of the type specimen Tanycolagreus it is now considered to belong to that dinosaur and not to Ornitholestes. Following this reassignment, Phil Senter in 2006 noted that "our knowledge of Ornitholestes can now be drawn from the holotype alone". John Foster in 2007 reported that some fragments from the Cabntera Dry Mesa may belong to Ornitholestes, although they have not yet been described. In 1920 Charles Whitney Gilmore concluded that Ornitholestes was identical to Coelurus. In 1934 Oliver Perry Hay recognized only one difference at the species level, naming a Coelurus hermanni, but in 1980 John Ostrom revived the genre.

Classification

The infraorder Coelurosauria, coined in 1914 by Friedrich von Huene, was traditionally a catch-all taxon into which all small theropods were placed. Ornitholestes, due to their small size, were it was generally classified as a coelurosaur. In 1986, Jacques Gauthier redefined this and several other paleontological terms in a more rigorous way, based on cladistic methods. The Tetanurae were defined as modern birds and all theropods more closely related to modern birds than to ceratosaurs, while Coelurosauria now comprised all members of the Tetanurae more closely related to modern birds than to Carnosauria. In 1988, Gregory S. Paul suggested that Ornitholestes was very similar in skull structure to Proceratosaurus, a theropod from the Middle Jurassic of England. 5] He put these two genera together in the Ornitholestinae, a new subfamily under Allosauridae, and speculated that they were more closely related to the much larger Allosaurus than to other small theropods. However, the classification of Ornitholestes and Proceratosaurus as relatives of allosaurids proved untenable, the latter proving to be a tyrannosauroid and Paul eventually abandoned it. All analyzes Published cladistic studies have shown that Ornitholestes is a coelurosaur as defined by Gauthier. Some analyzes have shown support for the hypothesis that it is the most primitive member of the Maniraptora group, although more extensive analyzes have suggested that it is more primitive than the Maniraptoriformes, and possibly a close relative of the compsognathid Juravenator starki..

Phylogeny

The following family tree illustrates a synthesis of the relationships of major coelurosaur groups based on several studies conducted in the 2010s.

Paleobiology

Paleobiology In a 2001 study by Bruce Rothschild and other paleontologists, 20 foot bones referred to as Ornitholestes were examined for signs of stress fracture but found none.

Forelimb function

A 2006 biomechanical study by Phil Senter used articulated casts of the right forelimb of the Ornitholestes type specimen to determine its range of motion. Senter found that the forearm could swing freely within a 95° range. When flexed, bent inward, at the elbow joint as far as possible, the humerus, a bone of the upper arm, and the radius, a bone of the lower arm, formed an angle of 53°. The ability of Ornitholestes to bend the forearm at an angle significantly more acute than 90° is characteristic of Maniraptoriformes, but absent in more primitive theropods such as Coelophysis and Allosaurus.

Even when the elbow was fully extended, the forearm did not form a right angle and was short 22°. Pronation, turning to make the palmar side of the hand face down the forearm, was impossible, because the radius and ulna lacked rolling surfaces, meaning that the forearm was in a permanent state of supination. When Ornitholestes Bend your elbows, this would cause your forearms to move in toward your midline. It may have used that ability to grab prey with both hands simultaneously.

Diet

This illustration by Charles R. Knight portrays Ornitholestes hunting Archaeopteryx even though they were not contemporary.

Henry Fairfield Osborn, in his 1903 description of Ornitholestes noted its large, conical front teeth, "quick gripping power" hand in hand with him and the & # 34; power of balance & # 34; of its tail and interpreted them as adaptations to prey on contemporary birds. Osborn later repudiated this hypothesis, suggesting in 1917 that Ornitholestes presented the early stages of a transition from a carnivorous to a more carnivorous lifestyle. herbivore, but not before Charles R. Knight drew an influential and widely published illustration of Ornitholestes pursuing Archaeopteryx. Knight's illustration, and others derived from it, continued appearing in non-specialist books on dinosaurs throughout the 20th century.

David Norman in 1985 admitted that it was "simply possible, though not very probable" that Ornitholestes may have caught and eaten primitive birds. However, more recent authors have suggested a diet of small terrestrial vertebrates. Mammals, lizards, frogs, salamanders, rhynchocephalians, and baby dinosaurs would have been possible prey. Gregory S. Paul in 1988 thought that Ornitholestes might have used its conical front teeth for fishing. Norman in 1985 suggested that the robust skull and jaws might have enabled Ornitholestes to cope with "larger and more active prey" than other small theropods. In this sense, David Lambert in 1993 speculated that Ornitholestes if it were a pack hunter, it might have been capable of facing ornithopods as large as a Camptosaurus half grown ".

In 2007, John Foster, a specialist in the Morrison Formation, suggested the possibility of a niche division between Ornitholestes and its contemporary Coelurus, which was within the same range. of size. He theorized that big-eyed Ornitholestes might have specialized for nocturnal hunting, while Coelurus might have focused on prey species that were active during the day. Foster noted, however, that this hypothesis was largely speculative; the lack of preserved skull material from Coelurus makes it impossible to verify whether its eyes were proportionally smaller than those of Ornitholestes. Paul 1988 estimated that an Ornitholestes endothermic 12 kilograms would have a daily dietary requirement of approximately 700 grams of meat.

Feathers

Ornithologist Percy Lowe hypothesized in 1944 that Ornitholestes might have feathers. However, this interpretation was widely ignored for more than half a century; Most reconstructions of theropod dinosaurs, including Ornitholestes, portrayed them with scaly, reptile-like skin. One of the few exceptions to this was Gregory S. Paul's 1988 Predatory Dinosaurs of the World. Robert T. Bakker in The Dinosaur Heresies in 1986 accurately predicted the presence of feathers in dromaeosaurids and held that all dinosaurs were endothermic, however, they did not incorporate feathers into their illustration of Ornitholestes.

In 1996, the primitive coelurosaurus Sinosauropteryx was discovered in China. The well-preserved fossil showed that this dinosaur wore a coat of rudimentary fur-like feathers. As a result of this and similar discoveries, most paleontologists now consider it likely that all coelurosaurs had insulating feathers of some kind, or were descended from ancestors that did. Based on their phylogenetic position, John Foster in 2007 inferred that the feathers of Ornitholestes would likely have been more primitive than those of birds, probably "would have covered the body except for the legs with a short coat, perhaps with feathers longer than they cover the upper part of the skull or the neck and the posterior edge of the forearms". Assuming they were present, these feathers would have been used to isolate and possibly also hatch eggs.

In popular culture

The animated film Fantasia includes it in the "The Rite of Spring" segment, where a large Ornitholestes appears jumping over a small Archaeopteryx i>. A crested Ornitholestes appears in the second episode of the BBC mini-series Walking with Dinosaurs as a predator of small Diplodocus. Ornitholestes also appears in the special The Ballad of Big Al, where he tries to eat Al's brothers.

Ornitholestes is mentioned in Michael Crichton's novel The Lost World when Dr. Richard Levine is examining the corpse of a mysterious reptile on a Costa Rican beach. Later, he mentions that it may have belonged to Ornitholestes . However from a tissue sample he collected he showed that the creature had been able to change color like a chameleon or cuttlefish, an ability attributed to Carnotaurus in the novel implying that the body might actually have belonged to a Carnotaurus.