Episode 392: What makes a dinosaur a dinosaur? How we differentiate dinosaurs from their closest relatives like pterosaurs, crocodilians, and dinosauromorphs.

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The dinosaur of the day: Erlikosaurus

• Therizinosaurid that lived in the Late Cretaceous in what is now Mongolia (Bayan Shireh Formation)
• Appears in Jurassic World: The Game and in Jurassic World: Alive (and has fur like feathers)
• Had a bulky body, with a small head, large claws, and body probably covered in feathers
• Feather assumption based on preserved feather impressions found on therizinosaurs Beipiaosaurus and Jianchangosaurus
• Estimated to be 11 ft (3.4 m) long and weigh 330 to 550 lb (150 to 250 kg)
• Had a large humerus (about 12 in or 30 cm long)
• Had strong arms with long claws
• Had a backwards directed pelvis
• Had claws on its feet, possibly for self-defense
• Had four toes on each foot
• Had a somewhat long snout
• Had a somewhat long brain
• Had an enlarged forebrain, which may have helped with evading predators and social behavior
• Olfactory tract much larger than the brain
• Had a pretty good sense of smell and good hearing
• Had good balance
• Fossils found in 1972 during a Soviet-Mongolian expedition, including a well preserved skull, nearly complete right foot, and nearly complete left humerus (other fossils found, including fragmentary cervical vertebrae)
• Described in 1980 by Altangerel Perle and Rinchen Barsbold
• Type species is Erlikosaurus andrewsi
• Genus name means “Erlik’s lizard”
• Genus name refers to Erlik, “the lamaist deity, king of the dead”
• Species name is in honor of Roy Chapman Andrews
• Second therizinosaur found in the formation
• Found a very complete skull
• Skull has been described as “remarkably uncrushed”
• Holotype skull is about 10 in (25 cm) long
• In 1981, Perle named and described Erlikosaurus again, as if it hadn’t been done already, but spelled it as Erlicosaurus
• Most scientists consider Erlikosaurus to be valid
• First therizinosaur, then called segnosaurs, where a complete skull was found, which helped paleontologists learn more about this group
• Therizinosaurs, known as segnosaurs in the 1970s, were thought to not be dinosaurs. Then in 1970 Rozhdestvensky suggested they were theropods
• In 1980, therizinosaurs were thought to be slow and semiaquatic (but then more therizinosaurs were found, and they were seen to be more theropod-like)
• James Clark and others in 1994 redescribed the Erlikosaurus skull and found the features to be consistent with the idea that therizinosaurs were theropods
• In 2010, Gregory Paul suggested Erlikosaurus was synonymous with Enigmosaurus (named in 1983). They were found in the same formation and only pelvic fossils were found for Enigmosaurus (no pelvis found for Erlikosaurus). But not everyone agrees (the two species were found in different parts of the formation, the upper and lower boundary, and the hip of Enigmosaurus looked very different from Erlikosaurus’s close relative, Segnosaurus
• Herbivorous
• Had a large gut to process food
• Had a well developed keratinous beak
• Had teeth in its jaws
• Could crop plants and strip leaves off branches
• A study in 2013 by Stephan Lautenschlager and others CT scanned the skull of Erlikosaurus and found the beak helped stabilize the skull while it was eating, which made the skull less likely to bend or get deformed
• Before the study, beaks were thought to replace teeth to make the head lighter, and eventually help with flight
• A 2021 study by Fion Waisum Ma and others digitally modeled and simulated over 40 theropod jaws from five groups (included Tyrannosaurus, Velociraptor, ornithomimosaurs, therizinosaurs, and oviraptorosaurs) and found theropods evolved to have more structurally stronger jaws to eat tougher food (jaws got more stable when biting, and less likely to have bone fractures)
• Jaws became stronger in carnivores and herbivores
• Erlikosaurus had downward-bending jaws that could help relieve stress when cropping plants
• Had a relatively weak bite force compared to other theropods
• In 2012, Stephan Lautenschlager found Erlikosaurus bite force was low compared to other theropods and in relation to its body mass
• However, Komodo dragons tend to have weak bite forces for their size
• Keratinous beak was an advantage because it’s always growing, so it could quickly repair fractures
• A 2015 study by Stephan Lautenschlager and others digitally modeled and then analyzed muscle strain for three theropods when opening their jaws: Erlikosaurus, Allosaurus, and Tyrannosaurus (and compared them to crocodiles and birds)
• Muscles that open and close jaws can only stretch so much before tearing, which plays a role in what an animal can eat
• Allosaurus and Tyrannosaurus could open up to 90 degrees, while Erlikosaurus was only about 45 degrees
• Modern carnivores usually can open their jaws wider than herbivores
• Lived alongside therizinosaur Segnosaurus, but were niche partitioned
• Much smaller than Segnosaurus, which was 20 to 23 ft (6 to 7 m) long
• Segnosaurus had different teeth from Erlikosaurus
• Erlikosaurus had symmetrical teeth, and Segnosaurus had complex features, with serrations and cutting edges and could probably eat tougher vegetation
• Erlikosaurus probably used its beak and muscles in its neck to crop plants, and probably processed its food in its gut
• Segnosaurus probably processed its food more with its jaws and teeth
• Lived in a semi-arid climate, with lakes and rivers
• Other animals that lived around the same time and place included therizinosaurs, tyrannosaurs, ceratopsians, hadrosaurs, ornithomimosaurs, troodontids, pterosaurs, and fish

Fun Fact:

Dinosaurs are the most successful terrestrial vertebrates of all time.

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