Segisaurus

*Segisaurus halli*

Segisaurus (meaning "Segi canyon lizard") is a genus of small coelophysid theropod dinosaur, that measured approximately 1 metre (3 feet) in length. The only known specimen was discovered in 1933 in early Jurassic strata in Tsegi Canyon, Arizona, for which it was named. Segisaurus is the only dinosaur to have ever been excavated from the area. It was later described in 1936 by the paleontologist Charles Lewis Camp. The fragmentary fossil skeleton consisted of portions of the limbs, pelvis, and vertebrae, but no cranial material was located. Segisaurus appears to have been closely related to the better-known Coelophysis. One distinction, however, was that Segisaurus had solid bones, while those of Coelophysis' were hollow. This even made some scientists consider if Segisaurus was even a theropod at all. Later investigations into the specimen in 2005 showed that Segisaurus did in fact have hollow bones, and lead scientists to believe that Segisaurus was related more closely to Procompsognathus.[1]

## Description

Segisaurus lived about 183 million years ago during the Jurassic period. Segisaurus was roughly the size of a goose and was a primitive bipedal theropod. Segisaurus was roughly 1 metre (3 feet) long, half a metre (1.5 feet) tall and weighed about 4-7 kilograms. It was nimble and insectivorous, although it may have scavenged meat also. It was bird-like in structure, with a flexible, elongated neck and stout body. Segisaurus was three-toed and had powerful legs that were long compared to its body length. Like its legs, Segisaurus had a long tail and long forearms. Its collar bone was not unlike a bird's, thus strengthening scientists' arguments that dinosaurs were related to avians.
Segisaurus is described from the only specimen ever found. The holotype, however, was a sub-adult, so that the full size of Segisaurus as an adult may never be known. Strangely, clavicles were found on the Segisaurus specimen, which were unknown in other dinosaurs from that era. A speculative interpretation by Charles Lewis Camp was that the "splint-like" neck ribs supported a Draco-like patagium along the neck, to improve the animal's ability to move quickly. *Segisaurus* is significant because it demonstrates that the clavicle was primitively present in early theropods.

## Classification

*Segisaurus* was described in 1936 by the paleontologist Charles Lewis Camp, based on specimen UCMP 32101, a fragmentary fossil skeleton which consisted of portions of the limbs, pelvis, and vertebrae. Cranial material was not recovered. *Segisaurus* went relatively ignored for the next half century. When the specimen was examined during this period, all who viewed it commented on the supposed presence of clavicles and the apparently "solid" bones that the dinosaur had. *Segisaurus* appeared to be closely related to the better-known *Coelophysis*, but unlike the hollow bones of *Coelophysis*, *Segisaurus* had solid bones. This caused some scientists question whether *Segisaurus* was a theropod at all. In 2005, a re-examination of the *Segisaurus* holotype revealed that contrary to reports it did in fact have hollow bones and that the clavicles were instead fragmented furculae.[1] In this study, Carano et al. found that although it was very unusual, *Segisaurus* was firmly a coelophysoid, and probably a close relative of *Procompsognathus*.[1]

### Distinguishing anatomical features

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.

According to Rauhut (2003), *Segisaurus* can be distinguished based on the following features:[4]

* the dorsal centra are not very constricted ventrally
* the scapula is slender
* the humeral shaft has stronger torsion (~50 degrees) than does that of Coelophysis
* the presence of a large ischial fenestra (according to Carrano et al., 2005)[1]
* the humeral deltopectoral crest is rectangular

## History of discovery

The first discovery of Segisaurus was by Max Littlesalt, a Navajo Indian who kept livestock inside Tsegi Canyon. After discovering the remains in the Navajo Sandstone formation of Arizona, Little Salt pointed out the fossils to archeologists on an expedition inside the canyon. Other than the first finding of Segisaurus, no other specimens have been discovered. After its initial description by Charles Lewis Camp, the holotype went relatively ignored for the next half century. When the specimen was investigated during this period, all who viewed it commented on the presence of clavicles and the solid bone structures the dinosaur had. According to recent reports, Segisaurus is vital in understanding the evolution of early theropods.

Restoration of the holotype skeleton from 1936
When the specimen of Segisaurus was discovered, Camp likened it to that of a "sitting hen", due to the position the dinosaur's remains were in.[2] Other theropods used this positions to sleep or stay sheltered during sand and ash storms, and indeed the Segisaurus holotype was found in a bed of sand stone, suggesting that the dinosaur had been submerged in a layer of sand and died. This is still only a hypothesis, as no nest or den materials were discovered along the Segisaurus specimen. Another speculative interpretation from Camp has attracted less attention: that the "splint-like" neck ribs supported a Draco-like patagium along the neck, to improve the animal's ability to move quickly.[2]
In September 2005, the Journal of Vertebrate Paleontology published a report which re-examined the remains of the Segisaurus holotype. The authors concluded, that although very unusual, Segisaurus was firmly a coelophysoid. They also noted that Segisaurus was probably a relative of Procompsognathus.

## Taphonomy

The *Segisaurus* holotype was found in a bed of sandstone, suggesting that the dinosaur had been buried in a layer of sand and died. This is still only a hypothesis, as no nest or den materials were discovered with the specimen. Geological features of the Navajo Sandstone Formation suggest that this genus lived in an environment resembling modern sand dunes.

## JPInstitute.com Description

Segisaurus is an Early Jurassic meat-eater that seemed adapted for life in a dry, sandy environment. Though all early dinosaur predators had evolved to be quicker than the animals they replaced, scientists believe Segisaurus has features that made it extra fast.

The North American landscape where Segisaurus lived was very arid and sandy. This little dinosaur seemed to have features that would have made it well adapted to squatting down and sitting out a sandstorm. With only post-cranial remains, the exact look of this dinosaur is unknown.

## Appearance in other media

### *Jurassic Park*

* Segisaurus was planned to be an attraction in Jurassic Park with 48% of its genome created, but it is sadly put on hold due to the Jurassic Park Incident of 1995. It was subject to cruelty in the past yet it is unknown if there are surviving populations of Segisaurus on Isla Nublar.

## Links

https://web.archive.org/web/20030405110346/http://www.jpinstitute.com/dinopedia/dinocards/dc\_segisaurus.html