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Description

Allosaurus is inarguably one of the quintessential theropod dinosaurs of the Late Jurassic and it is one of the earliest large discovered. Since the animal was first described in 1877 by Marsh during the “Bone Wars” it has gone on to feature heavily in palaeontology artwork and pop culture, often appearing alongside its contemporaries such as Stegosaurus and Ceratosaurus. Allosaurus was an apex predator and held one of the top trophic levels in its environment where it predated on a wide variety of other Late Jurassic dinosaurs.

The size of Allosaurus

Like most reptiles, the adult size of Allosaurus fragilis varied widely between different individuals depending on various epigenetic factors such as food availability, environmental conditions and health. Fortunately, because Allosaurus fragilis is known from a great number partial and nearly completely skeletons the variation of its adult body size is very well understood. On average it would seem the species was around 8.5 meters in length when skeletally mature (Glut, 1993) however, adult size could range from between just 7.4 meters (USNM 4734) to possibly over 10 meters (NMMNH 26083 and AMNH 5767).

The largest specimen positively attributed to Allosaurus fragilis is the partial skeleton AMNH 680, which consists of three dorsal vertebrae, four proximal caudal vertebrae, an ilium, pubis, ischia, femur, tibia, fibulae, calcaneum, metatarsus and several pedal phalanges. The femur of AMNH 680 possesses a total length of 1008 mm according to (Foster and Chure, 2006), which is 14.5 percent larger than the femur of the specimen UUVP 6000 (An average sized individual), which measures 880 mm (Madsen, 1976). Francisco Bruñén’s skeletal reconstruction of UUVP 6000 yielded a total body length of 8.02 meters (However it was missing the tail), so scaling up to the size of AMNH 680 yields a total length estimate of 9.183 meters. This is slightly less than the 9.7-meter estimate provided by (Mortimer, 2003). I’m not sure what method Mortimer used to obtain her estimates, but it appears she scaled up from an 8.5 meters length estimate for UUVP 6000, which does indeed yield a length of 9.73 meters if scaled to the size of AMNH 680. Considering however that AMNH 680 is incomplete any estimate is going to have a slight error bar.

While Allosaurus fragilis is usually considered smaller than its close relative, Saurophaganax maximus, which reached lengths of over 10.29 meters (OMNH 2114 scaled up from UUVP 6000) it seems that there may be some overlap between the two species in size. The specimen NMMNH 26083 includes a femur with a length of 1100 mm, this is 25 percent larger than UUVP 6000. This would suggest a total body length of 10.03 meters. Despite its large size Mickey Mortimer has identified the specimen as belonging to Allosaurus as opposed to Saurophaganax, which would mean Allosaurus was also able of reaching sizes greater than 10 meters. However, the assignment of this specimen to Allosaurus has been questioned by other authors. So for the time being, AMNH 680 is still the largest specimen attributed to Allosaurus fragilis and appears to be close to the maximum size the species was capable of achieving.

In terms of body weight, a rigorous study using 3D models of the subadult specimen MOR 693 or “Big Al” yielded a weight estimate of between 1400 and 2000 kg, with the average mass estimate being 1500.91 kg (Bates et al., 2009). MOR 693 possessed a total body length of 7.57 meters according to (Bates et al., 2012), so scaling up to AMNH 680 produces an estimated mass of 2679.3 kg for a body length of 9.183 meters. Scaling up to NMMNH 26083 would then yield an estimated weight of 3491.16 kg for an individual measuring 10.03 meters. Of course, this assumes perfect allometry between a subadult and adult specimen.

The integument of Allosaurus

According to Mortimer a partial skeleton belonging to juvenile Allosaurus fragilis, discovered in Wyoming preserves a small skin impression from the left side of the body, about 30 cm2. The impression consists of small scales, between 2 and 3 millimetres in diameter. This suggests that Allosaurus fragilis was primarily covered in small scales, however because of the small size of each scale the animal’s hide would have appeared smooth and leathery.

Outside of Allosaurus there is only one other member of clade Allosauroidea that preserves skin impressions and that is the Carcharodontosaurid, Concavenator corcovatus. Therefore the integumentary covering of Allosaurus must be inferred through the use of phylogenetic bracketing. The holotype specimen for Concavenator (MCCM-LH 6666) preserves several patches of skin impressions from the metatarsals and underside of the tail (Ortega, 2010). The scales around the metatarsals are hexagonal in shape, roughly 1 cm in diameter. The anterior or the metatarsals also preserves scuta and scutella, rectangular in shape and placed laterally along the dorsal digit zone, similar for those found on the feet of modern birds. On the underside of the tail the scales are large and rectangular in shape, similar to the scuta of modern crocodilians. The distal right pedal phalanx preserves soft tissue structures similar to the plantar pads and corneous sheaths seen in the feet of some modern birds such as ratites. This evidence suggests that the integumentary covering on the feet of Allosauroids was comparable to that of modern birds.

The ulna of the Concavenator holotype also preserves an odd posterolateral crest with a series of bumps resembling the quill knobs of modern birds. It has been suggested that these structures were anchoring points for large quill like feathers (Ortega, 2010). However other authors such as Darren Naish have speculated that the bumps could be part of an intermuscular line, with the bumps acting as attachment points for tendons (Naish, 2010), this interpretation was supported by Christian Foth in 2014 (Foth, 2014). In 2015, at a meeting for the Society of Vertebrate Paleontology the authors of the original 2010 paper attempted to reconstruct the arm musculature of Concavenator. They identified a row of muscle insertion points and determined the row of bumps could not have been between any of them and was therefore likely not part of an intermuscular line (Fidalgo, 2015). Some authors such as Mortimer argued the bumps were located on the anterolateral surface of the ulna rather than the posterolateral surface and therefore could not have been anchoring points for quill like feathers at all. In 2018 however, Cuesta Fidalgo published her doctorate thesis where she argued the bumps were indeed located on the posterolateral surface of the bone and were most likely quill knobs as was originally proposed (Fidalgo, 2018).

Interestingly in 2015 a study was done describing a vast and complex system of neurovascular canals present in the front of the snout of Neovenator salerii, another Carcharodontosaurid (Barker et al., 2015). The connection of the canals with the external foramina most likely suggests these passages contained nerves. In modern reptiles these structures indicate a complex tactile sensory system. The implication is that Allosauroid theropods had sensitive faces, which would have given them an advantage in hunting and feeding. Similar traits are possessed by modern crocodilians, Spinosaurids (Ibrahim et al., 2014) and in Tyrannosaurids (Carr et al., 2017). However, the authors noted that the presence of these structures in Neovenator does not indicate what kind of overlying soft tissue type was present.

In order to predict what soft tissue types were present on the face of Allosaurus fragilis the surface bone textures of the skull of the subadult specimen, MOR 693 were examined. The relatively low number of foramina present on the facial bones compared to crocodilians indicates that an immobile extra-oral tissue covering was present, similar to what is observed in present day lizards (Morhardt, 2009). The maxilla, premaxilla and dentary display a hummocky rugosity profile with a dense distribution of neurovascular grooves. According to (Hieronymus, 2009) this indicates an epidermal covering of scales. The nasal and lacrimal bones however display a pitting rugosity profile, which would suggest this part of the skull was covered with either a highly keratinised epidermis or with keratin sheaths.

In terms of general Allosauroid integument, the direct fossil evidence from two species would suggest these animals possessed small scales over the majority of their bodies, hexagonal scales on their feet, rectangular scutes on the metatarsals and underside of the tail, corneous plantar pads on the undersides of the feet and perhaps quill like proto-feathers on the posterolateral surface of the ulna. It can therefore be inferred that being an Allosauroid, Allosaurus fragilis would have possessed these types of soft tissue integument. This is of course just an inference because unfortunately there is very little direct integumentary information for Allosaurus fragilis and do the use of phylogenetic bracketing is appropriate to restore what the animal’s life appearance was like.

References:
Glut, Donald F. (1997). "Allosaurus". Dinosaurs: The Encyclopedia. Jefferson, North Carolina: McFarland & Co. pp. 105–117.

Foster, John R.; Chure, Daniel J. (2006). "Hindlimb allometry in the Late Jurassic theropod dinosaur Allosaurus, with comments on its abundance and distribution". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 119–122.

Madsen Jr, J. H. (1976). Allosaurus fragilis: a revised osteology. Utah Geological and Mining Survey Bulletin, 109, 1-163.

Mortimer, Mickey (21 July 2003). "And the largest Theropod is..." The Dinosaur Mailing List.

Bates, Karl T.; Falkingham, Peter L.; Breithaupt, Brent H.; Hodgetts, David; Sellers, William I.; Manning, Phillip L. (2009). "How big was 'Big Al'? Quantifying the effect of soft tissue and osteological unknowns on mass predictions for Allosaurus (Dinosauria:Theropoda)". Palaeontologia Electronica. 12 (3): unpaginated.

Ortega, F., Escaso, F. and Sanz, J. (2010). A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain. Nature, 467(7312), pp.203-206. (Carpenter, 1999) Carpenter, K. (1999). Eggs, nests, and baby dinosaurs. Bloomington: Indiana University Press

Description

Allosaurus is inarguably one of the quintessential theropod dinosaurs of the Late Jurassic and it is one of the earliest large discovered. Since the animal was first described in 1877 by Marsh during the “Bone Wars” it has gone on to feature heavily in palaeontology artwork and pop culture, often appearing alongside its contemporaries such as Stegosaurus and Ceratosaurus. Allosaurus was an apex predator and held one of the top trophic levels in its environment where it predated on a wide variety of other Late Jurassic dinosaurs.

The size of Allosaurus

Like most reptiles, the adult size of Allosaurus fragilis varied widely between different individuals depending on various epigenetic factors such as food availability, environmental conditions and health. Fortunately, because Allosaurus fragilis is known from a great number partial and nearly completely skeletons the variation of its adult body size is very well understood. On average it would seem the species was around 8.5 meters in length when skeletally mature (Glut, 1993) however, adult size could range from between just 7.4 meters (USNM 4734) to possibly over 10 meters (NMMNH 26083 and AMNH 5767).

The largest specimen positively attributed to Allosaurus fragilis is the partial skeleton AMNH 680, which consists of three dorsal vertebrae, four proximal caudal vertebrae, an ilium, pubis, ischia, femur, tibia, fibulae, calcaneum, metatarsus and several pedal phalanges. The femur of AMNH 680 possesses a total length of 1008 mm according to (Foster and Chure, 2006), which is 14.5 percent larger than the femur of the specimen UUVP 6000 (An average sized individual), which measures 880 mm (Madsen, 1976). Francisco Bruñén’s skeletal reconstruction of UUVP 6000 yielded a total body length of 8.02 meters (However it was missing the tail), so scaling up to the size of AMNH 680 yields a total length estimate of 9.183 meters. This is slightly less than the 9.7-meter estimate provided by (Mortimer, 2003). I’m not sure what method Mortimer used to obtain her estimates, but it appears she scaled up from an 8.5 meters length estimate for UUVP 6000, which does indeed yield a length of 9.73 meters if scaled to the size of AMNH 680. Considering however that AMNH 680 is incomplete any estimate is going to have a slight error bar.

While Allosaurus fragilis is usually considered smaller than its close relative, Saurophaganax maximus, which reached lengths of over 10.29 meters (OMNH 2114 scaled up from UUVP 6000) it seems that there may be some overlap between the two species in size. The specimen NMMNH 26083 includes a femur with a length of 1100 mm, this is 25 percent larger than UUVP 6000. This would suggest a total body length of 10.03 meters. Despite its large size Mickey Mortimer has identified the specimen as belonging to Allosaurus as opposed to Saurophaganax, which would mean Allosaurus was also able of reaching sizes greater than 10 meters. However, the assignment of this specimen to Allosaurus has been questioned by other authors. So for the time being, AMNH 680 is still the largest specimen attributed to Allosaurus fragilis and appears to be close to the maximum size the species was capable of achieving.

In terms of body weight, a rigorous study using 3D models of the subadult specimen MOR 693 or “Big Al” yielded a weight estimate of between 1400 and 2000 kg, with the average mass estimate being 1500.91 kg (Bates et al., 2009). MOR 693 possessed a total body length of 7.57 meters according to (Bates et al., 2012), so scaling up to AMNH 680 produces an estimated mass of 2679.3 kg for a body length of 9.183 meters. Scaling up to NMMNH 26083 would then yield an estimated weight of 3491.16 kg for an individual measuring 10.03 meters. Of course, this assumes perfect allometry between a subadult and adult specimen.

The integument of Allosaurus

According to Mortimer a partial skeleton belonging to juvenile Allosaurus fragilis, discovered in Wyoming preserves a small skin impression from the left side of the body, about 30 cm2. The impression consists of small scales, between 2 and 3 millimetres in diameter. This suggests that Allosaurus fragilis was primarily covered in small scales, however because of the small size of each scale the animal’s hide would have appeared smooth and leathery.

Outside of Allosaurus there is only one other member of clade Allosauroidea that preserves skin impressions and that is the Carcharodontosaurid, Concavenator corcovatus. Therefore the integumentary covering of Allosaurus must be inferred through the use of phylogenetic bracketing. The holotype specimen for Concavenator (MCCM-LH 6666) preserves several patches of skin impressions from the metatarsals and underside of the tail (Ortega, 2010). The scales around the metatarsals are hexagonal in shape, roughly 1 cm in diameter. The anterior or the metatarsals also preserves scuta and scutella, rectangular in shape and placed laterally along the dorsal digit zone, similar for those found on the feet of modern birds. On the underside of the tail the scales are large and rectangular in shape, similar to the scuta of modern crocodilians. The distal right pedal phalanx preserves soft tissue structures similar to the plantar pads and corneous sheaths seen in the feet of some modern birds such as ratites. This evidence suggests that the integumentary covering on the feet of Allosauroids was comparable to that of modern birds.

The ulna of the Concavenator holotype also preserves an odd posterolateral crest with a series of bumps resembling the quill knobs of modern birds. It has been suggested that these structures were anchoring points for large quill like feathers (Ortega, 2010). However other authors such as Darren Naish have speculated that the bumps could be part of an intermuscular line, with the bumps acting as attachment points for tendons (Naish, 2010), this interpretation was supported by Christian Foth in 2014 (Foth, 2014). In 2015, at a meeting for the Society of Vertebrate Paleontology the authors of the original 2010 paper attempted to reconstruct the arm musculature of Concavenator. They identified a row of muscle insertion points and determined the row of bumps could not have been between any of them and was therefore likely not part of an intermuscular line (Fidalgo, 2015). Some authors such as Mortimer argued the bumps were located on the anterolateral surface of the ulna rather than the posterolateral surface and therefore could not have been anchoring points for quill like feathers at all. In 2018 however, Cuesta Fidalgo published her doctorate thesis where she argued the bumps were indeed located on the posterolateral surface of the bone and were most likely quill knobs as was originally proposed (Fidalgo, 2018).

Interestingly in 2015 a study was done describing a vast and complex system of neurovascular canals present in the front of the snout of Neovenator salerii, another Carcharodontosaurid (Barker et al., 2015). The connection of the canals with the external foramina most likely suggests these passages contained nerves. In modern reptiles these structures indicate a complex tactile sensory system. The implication is that Allosauroid theropods had sensitive faces, which would have given them an advantage in hunting and feeding. Similar traits are possessed by modern crocodilians, Spinosaurids (Ibrahim et al., 2014) and in Tyrannosaurids (Carr et al., 2017). However, the authors noted that the presence of these structures in Neovenator does not indicate what kind of overlying soft tissue type was present.

In order to predict what soft tissue types were present on the face of Allosaurus fragilis the surface bone textures of the skull of the subadult specimen, MOR 693 were examined. The relatively low number of foramina present on the facial bones compared to crocodilians indicates that an immobile extra-oral tissue covering was present, similar to what is observed in present day lizards (Morhardt, 2009). The maxilla, premaxilla and dentary display a hummocky rugosity profile with a dense distribution of neurovascular grooves. According to (Hieronymus, 2009) this indicates an epidermal covering of scales. The nasal and lacrimal bones however display a pitting rugosity profile, which would suggest this part of the skull was covered with either a highly keratinised epidermis or with keratin sheaths.

In terms of general Allosauroid integument, the direct fossil evidence from two species would suggest these animals possessed small scales over the majority of their bodies, hexagonal scales on their feet, rectangular scutes on the metatarsals and underside of the tail, corneous plantar pads on the undersides of the feet and perhaps quill like proto-feathers on the posterolateral surface of the ulna. It can therefore be inferred that being an Allosauroid, Allosaurus fragilis would have possessed these types of soft tissue integument. This is of course just an inference because unfortunately there is very little direct integumentary information for Allosaurus fragilis and do the use of phylogenetic bracketing is appropriate to restore what the animal’s life appearance was like.

References:
Glut, Donald F. (1997). "Allosaurus". Dinosaurs: The Encyclopedia. Jefferson, North Carolina: McFarland & Co. pp. 105–117.

Foster, John R.; Chure, Daniel J. (2006). "Hindlimb allometry in the Late Jurassic theropod dinosaur Allosaurus, with comments on its abundance and distribution". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 119–122.

Madsen Jr, J. H. (1976). Allosaurus fragilis: a revised osteology. Utah Geological and Mining Survey Bulletin, 109, 1-163.

Mortimer, Mickey (21 July 2003). "And the largest Theropod is..." The Dinosaur Mailing List.

Bates, Karl T.; Falkingham, Peter L.; Breithaupt, Brent H.; Hodgetts, David; Sellers, William I.; Manning, Phillip L. (2009). "How big was 'Big Al'? Quantifying the effect of soft tissue and osteological unknowns on mass predictions for Allosaurus (Dinosauria:Theropoda)". Palaeontologia Electronica. 12 (3): unpaginated.

Ortega, F., Escaso, F. and Sanz, J. (2010). A bizarre, humped Carcharodontosauria (Theropoda) from the Lower Cretaceous of Spain. Nature, 467(7312), pp.203-206. (Carpenter, 1999) Carpenter, K. (1999). Eggs, nests, and baby dinosaurs. Bloomington: Indiana University Press