Gorgosaurus

Gorgosaurus ( ; ) is a genus of tyrannosaurid theropod dinosaur that lived in western North America during the Late Cretaceous Period (Campanian), between about 76.5 and 75 million years ago. Fossil remains have been found in the Canadian province of Alberta and the U.S. state of Montana. Paleontologists recognize only the type species, G. libratus, although other species have been erroneously referred to the genus.

Like most known tyrannosaurids, Gorgosaurus was a large bipedal predator, measuring in length and in body mass. Dozens of large, sharp teeth lined its jaws, while its two-fingered forelimbs were comparatively small. Gorgosaurus was most closely related to Albertosaurus, and more distantly related to the larger Tyrannosaurus. Gorgosaurus and Albertosaurus are extremely similar, distinguished mainly by subtle differences in the teeth and skull bones. Some experts consider G. libratus to be a species of Albertosaurus; this would make Gorgosaurus a junior synonym of that genus.

Gorgosaurus lived in a lush floodplain environment along the edge of an inland sea. It was an apex predator, preying upon abundant ceratopsids and hadrosaurs. In some areas, Gorgosaurus coexisted with another tyrannosaurid, Daspletosaurus. Although these animals were roughly the same size, there is some evidence of niche differentiation between the two. Gorgosaurus is the best-represented tyrannosaurid in the fossil record, known from dozens of specimens. These plentiful remains have allowed scientists to investigate its ontogeny, life history and other aspects of its biology.

Discovery and naming

thumb|left|[[Type specimen of Gorgosaurus sternbergi (AMNH 5664), now recognized as a juvenile Gorgosaurus libratus]]

Gorgosaurus libratus was first described by Lawrence Lambe in 1914. Its name is derived from the Greek (gorgos – "fierce" or "terrible") and (saurus – "lizard"). The type species is G. libratus; the specific epithet "balanced" is the past participle of the Latin verb librare, meaning "to balance".

thumb|left|Specimen [[National Museum of Natural History|USNM 12814 (formerly AMNH 5428)]]

In 1856, Joseph Leidy described two tyrannosaurid premaxillary teeth from Montana. Although there was no indication of what the animal looked like, the teeth were large and robust, and Leidy gave them the name Deinodon. Matthew and Brown commented in 1922 that these teeth were indistinguishable from those of Gorgosaurus, but in the absence of skeletal remains of Deinodon, opted not to unequivocally synonymize the two genera, provisionally naming a ?Deinodon libratus.

Several tyrannosaurid skeletons from the Two Medicine Formation and Judith River Formation of Montana probably belong to Gorgosaurus, although it remains uncertain whether they belong to G. libratus or a new species.

Formerly assigned species

thumb|Cast of specimen PIN 553–1, holotype of now invalid Gorgosaurus lancinator

Several species were incorrectly assigned to Gorgosaurus in the 20th century. A complete skull of a small tyrannosaurid (CMNH 7541), found in the younger, late Maastrichtian-age Hell Creek Formation of Montana, was named Gorgosaurus lancensis by Charles Whitney Gilmore in 1946. This specimen was renamed Nanotyrannus by Bob Bakker and colleagues in 1988. Currently, many paleontologists regard Nanotyrannus as a juvenile Tyrannosaurus. Kenneth Carpenter renamed the smaller specimen Maleevosaurus novojilovi in 1992, but both are now considered juveniles of Tarbosaurus bataar.

Description

left|thumb|G. libratus adult and subadult with a human for scale

Gorgosaurus was smaller than Tyrannosaurus or Tarbosaurus, close in size to Albertosaurus. Adults reached in length from snout to tail, and weighed in body mass. The largest known skull measures long, just slightly smaller than that of Daspletosaurus.

thumb|[[Life restoration]]Gorgosaurus teeth were typical of all known tyrannosaurids. The eight premaxillary teeth at the front of the snout were smaller than the rest, closely packed and D-shaped in cross section. In Gorgosaurus, the first tooth in the maxilla was also shaped like the premaxillary teeth. The rest of the teeth were oval in cross section, rather than blade-like as in most other theropods.thumb|[[Robert T. Bakker|Bob Bakker and a skeleton with several bone injuries, from the "Dinosaur Mummy: CSI" exhibit at the HMNS]]

Gorgosaurus shared its general body plan with all other tyrannosaurids. Its massive head was perched on the end of an S-shaped neck. In contrast to its large head, its forelimbs were very small. The forelimbs had only two digits, although a third metacarpal is known in some specimens, the vestigial remains of the third digit seen in other theropods. Gorgosaurus had four digits on each hindlimb, including a small first toe (hallux) which did not contact the ground. Tyrannosaurid hindlimbs were long relative to overall body size compared with other theropods. The long, heavy tail served as a counterweight to the head and torso and placed the center of gravity over the hips. Scales of some sort were present in this specimen, but they are reportedly widely dispersed from each other and very small. Other patches of isolated Gorgosaurus skin shows denser, and larger though still relatively fine scales (smaller than hadrosaurid scales and approximately as fine as a Gila monster's). Neither of these specimens was associated with any particular bone or specific body area.

Classification and systematics

Gorgosaurus is classified in the theropod subfamily Albertosaurinae within the family Tyrannosauridae. It is most closely related to the slightly younger Albertosaurus. although American paleontologist Thomas Holtz published a phylogenetic analysis in 2004 which indicated it was an albertosaurine. All other tyrannosaurid genera, including Daspletosaurus, Tarbosaurus and Tyrannosaurus, are classified in the subfamily Tyrannosaurinae. Compared to the tyrannosaurines, albertosaurines had slender builds, with proportionately smaller, lower skulls and longer bones of the lower leg (tibia) and feet (metatarsals and phalanges).

The close similarities between Gorgosaurus libratus and Albertosaurus sarcophagus have led many experts to combine them into one genus over the years. Albertosaurus was named first, so by convention it is given priority over the name Gorgosaurus, which is sometimes considered its junior synonym. William Diller Matthew and Barnum Brown doubted the distinction of the two genera as early as 1922. Gorgosaurus libratus was formally reassigned to Albertosaurus (as Albertosaurus libratus) by Dale Russell in 1970, Combining the two greatly expands the geographical and chronological range of the genus Albertosaurus. Other experts maintain the two genera as separate.

thumb|right|Skeletal mount in Japan

Below is the cladogram of Tyrannosauridae based on the phylogenetic analysis conducted by Loewen et al. in 2013.

Paleobiology

Diet and feeding

thumb|Gorgosaurus juvenile specimen TMP 2009.12.14, featuring stomach contents comprising remains of [[Citipes]]

Just like other tyrannosaurids, bite force of Gorgosaurus and Albertosaurus increases slowly among young individuals, and then it increases exponentially when they reach the late juvenile stage. Other paleontologists have produced significantly lower bite force estimates. In 2021, given that the largest known Gorgosaurus had a similar bite force to the similar-sized Tyrannosaurus, Therrien and colleagues proposed that the maximum bite force that could be produced by adult albertosaurines is around 12,200 to 21,800 newtons. In 2022, Sakamoto estimated that Gorgosaurus had an anterior bite force of 6,418 newtons and a posterior bite force of 13,817 newtons.

In 2023, a juvenile Gorgosaurus (TMP 2009.12.14) with its in situ stomach contents containing two Citipes juveniles about a year old intact was reported from the Dinosaur Park Formation. This juvenile would have been 5-7 years old at the time of death, measuring about long and weighing around . It is much larger than the two Citipes juveniles that weigh about , contrary to the assumption that tyrannosaurids fed on prey of their size once they reached , indicating that juvenile tyrannosaurids still consumed much smaller prey after exceeding a certain size threshold. It's a direct dietary evidence that reinforces the theory of 'ontogenetic dietary shift' for tyrannosaurids, as previously inferred by ecological modeling and anatomical features among different age groups. Only the remains of the hindlimbs and caudal vertebrae of juvenile Citipes were present in the tyrannosaurid's stomach cavity, suggesting that a juvenile Gorgosaurus may have had preferential consumption of the muscular hindlimbs. Thomas R. Holtz Jr., a paleontologist who also previously theorized that tyrannosaurs underwent a big dietary shift with maturation, said that the fossil "looks like it was Thanksgiving," as the juvenile Gorgosaurus was mostly eating the legs of Citipes.

Life history

thumb|left|A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Gorgosaurus is shown in blue.

Gregory Erickson and colleagues have studied the growth and life history of tyrannosaurids using bone histology, which can determine the age of a specimen when it died. A growth curve can be developed when the ages of various individuals are plotted against their sizes on a graph. Tyrannosaurids grew throughout their lives, but underwent tremendous growth spurts for about four years, after an extended juvenile phase. Sexual maturity may have ended this rapid growth phase, after which growth slowed down considerably in adult animals. Examining five Gorgosaurus specimens of various sizes, Erickson calculated a maximum growth rate of about per year during the rapid growth phase, slower than in tyrannosaurines like Daspletosaurus and Tyrannosaurus, but comparable to Albertosaurus.

Gorgosaurus spent as much as half its life in the juvenile phase before ballooning up to near-maximum size in only a few years.

thumb|Restoration of a sub-adult

The discovery of two exceptionally preserved juvenile skulls from Gorgosaurus suggests that Gorgosaurus underwent the morphological shift from gracile juveniles to robust adults at an earlier age than Tyrannosaurus, to which it was compared in a study published by Jared Voris et al., suggests that the ontogenetic changes occurred at roughly 5-7 years of age in Gorgosaurus; much earlier than its larger and later relative. However, both tyrannosaur genera underwent these ontogenetic transformations at a similar percent of skull length relative to the large known adult individuals. The study's results likewise indicate that there is a dissociation between body size and cranial development in tyrannosaurs, while simultaneously allowing better identification of juvenile remains that may have been misidentified in museum fossil collections. It is estimated that an ontogenetic dietary shift of Gorgosaurus and Albertosaurus occurs when the mandibular length reaches , indicating that this is the stage when their bite force increases exponentially and when they begin to pursuit large prey.

Another specimen cataloged as TMP94.12.602 bears multiple pathologies. A longitudinal fracture is present in the middle of the right fibula's shaft. Multiple ribs bear healed fractures and the specimen had a pseudoarthortic gastralium. Lesions from a bite received to the face were present and showed evidence that the wounds were healing before the animal died.

As with many tyrannosaurids, several Gorgosaurus specimens show evidence of intraspecific face biting.

alt=TMP 1994.143.1, the skull of a juvenile tyrannosaur from the Dinosaur Park Formation previously thought to be Daspletosaurus sp. but since 2019 assigned to Gorgosaurus libratus. The arrows point at various tyrannosaur bite marks.|thumb|[[Royal Tyrrell Museum of Palaeontology|TMP 1994.143.1, the skull of a juvenile tyrannosaur from the Dinosaur Park Formation previously thought to be Daspletosaurus sp. but since 2019 assigned to Gorgosaurus libratus. The arrows point at various tyrannosaur bite marks.]]

Specimen TMP 1994.143.1, a juvenile skull from the Dinosaur Park Formation with several tyrannosaur bite marks, was previously believed to be Daspletosaurus sp. but was later assigned to Gorgosaurus libratus.

TMP 2017.012.0002 is a Gorgosaurus right maxilla with five raised scars; healed injuries from face biting by conspecifics.

Paleoenvironment

thumb|left|Restoration of Gorgosaurus chasing [[Corythosaurus and Chasmosaurus]]

Most specimens of Gorgosaurus libratus have been recovered from the Dinosaur Park Formation in Alberta. and Gorgosaurus libratus fossils are known specifically from the lower to middle section of the formation, between 76.6 and 75.1 million years ago. The Two Medicine Formation and Judith River Formation of Montana have also yielded possible Gorgosaurus remains. At this time, the area was a coastal plain along the western edge of the Western Interior Seaway, which divided North America in half. The Laramide Orogeny had begun uplifting the Rocky Mountains to the west, from which flowed great rivers that deposited eroded sediment in vast floodplains along the coast. The climate was subtropical with marked seasonality, and periodic droughts sometimes resulted in massive mortality among the great herds of dinosaurs, as represented in the numerous bonebed deposits preserved in the Dinosaur Park Formation. Conifers formed the forest canopy, while the understory plants consisted of ferns, tree ferns and angiosperms. Around 73 million years ago, the seaway began to expand, transgressing into areas formerly above sea level and drowning the Dinosaur Park ecosystem. This transgression, called the Bearpaw Sea, is recorded by the marine sediments of the massive Bearpaw Shale. A Saurornitholestes dentary has been discovered in the Dinosaur Park Formation that bore tooth marks left by the bite of a young tyrannosaur, possibly Gorgosaurus.

Coexistence with Daspletosaurus

thumb|Dinosaur Park specimen (FMNH PR308) of [[Daspletosaurus, mounted at the Field Museum]]

In the middle stages of the Dinosaur Park Formation, Gorgosaurus lived alongside a rarer species of tyrannosaurid, Daspletosaurus. This is one of the few examples of two tyrannosaur genera coexisting. Similar-sized predators in modern predator guilds are separated into different ecological niches by anatomical, behavioral or geographical differences that limit competition. Niche differentiation between the Dinosaur Park tyrannosaurids is not well understood. In 1970, Dale Russell hypothesized that the more common Gorgosaurus actively hunted fleet-footed hadrosaurs, while the rarer and more troublesome ceratopsians and ankylosaurians (horned and heavily armoured dinosaurs) were left to the more heavy built Daspletosaurus. and another bonebed contains the remains of three Daspletosaurus along with the remains of at least five hadrosaurs. While albertosaurine remains have been found in the Hell Creek Formation, it is most likely these are indeterminate remains belonging to a species of Tyrannosaurus.