Episode 568: Theropods! It Wasn’t Just T. rex with Tiny Arms. From the smallest ones (birds) to the largest (tyrannosaurs) and in between. The evolution of theropods, an extra large tyrannosaur, more evidence that Nanotyrannus is valid, two new small theropods (enantiornithine birds), and much more

News:

• A new Early Jurassic theropod skeleton gives more insight into how theropods evolved source
• There’s a new enantiornithine, Plumadraco bankoorum, and it had really long tail feathers source
• There is a new enantiornithine, Gorgonavis alcyone, and it had a long beak source
• Paleontologists found an extra large tyrannosaur shinbone in New Mexico, which may show Tyrannosaurus evolved in North America source
• A study of the tiny hyoid bone (in the mouth) in Nanotyrannus supports that it is a valid dinosaur source
• How and what theropods ate changed over time as their skulls evolved—a new study breaks down the theropod group changes source
• Five different carnivorous non-avian theropod groups evolved to have shorter arms and bigger heads (not just T. rex) source

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The dinosaur of the day: Avimimus

• Oviraptorosaurian theropod that lived in the Late Cretaceous in what is now Mongolia
• Oviraptorosaurs, were the “egg thief lizards” known for their short beaks and parrot-like skulls (that probably brooded and took care of their eggs)
• Small and bird-like
• About 5 ft (1.5 m) long
• Walked on two legs, had a tail, a short head, body covered in feathers
• Holotype had no tail, so originally thought to have no tail, but more fossils found later included tail bones (caudal vertebrae)
• Had broad hips
• Based on the broad hips, may have had a long tail (don’t know too many tail bones)
• Had long, slender legs, with long shins compared to its thighs, and was probably a fast runner
• Had three toes on each foot with claws
• Had relatively short arms
• Hand bones were fused together
• Had feathers, but probably couldn’t fly
• Had a ridge on the lower arm bone (ulna) which may have been an attachment point for feathers
• Had quill knobs on the ulna
• Had a small skull, but large eyes and a big brain
• Skull bones gave extra support and protection to the brain
• Had a beak and no teeth, but the holotype had tooth-like projections along the tip of the front of the jaw (premaxilla)
• Later specimens found had small premaxillary teeth
• May have been an herbivore, or an omnivore, or eaten insects (insectivore)
• May have used beak to crop plants
• Need to find gut contents to know what it ate
• Back bones did not have openings for air sacs (oviraptorids and caenagnathids have them) so Avimimus may have been more basal
• Had a long, slender neck, made of vertebrae that were longer compared to other oviraptorosaurs
• Type species is Avimimus portentosus
• Genus name means “bird mimic”
• Another dinosaur with a name that means “bird mimic” is Ornithomimus (in Greek); Avimimus means “bird mimic” in Latin
• First fossils found in 1973
• Named in 1981 by Dr. Sergei Kurzanov
• In 1996, a second, nearly complete skeleton was found (described in 2000)
• Second species named in 2018: Avimimus nemegtensis. Named based on subtle differences, such as bones with different openings, wider spots, etc.
• More than 500 dinosaur individuals found in a bonebed in the Nemegt Basin in Mongolia (included avimimids, caenagnathids, and oviraptorids—all of them are closely related oviraptorosaurs) and realized there was a second species
• Both species are from the Nemegt Formation
• A. nemegtensis is older than A. portentosus
• Bonebed found in 2006
• Bonebed had thousands of bone shards (fossil poachers had found the bonebed first)
• First oviraptorosaur bonebed described
• Bonebed included at least 10 Avimimus individuals, adults and subadults
• No juveniles
• Most of the bonebed is Avimimus (160 or over 90% of the 177 documented specimens). The rest of the fossils are from oviraptorids, hadrosaurs (embryonic bones), birds, lizards, mammals, invertebrates, and wood
• Based on how the fossils were preserved, they were buried quickly, then water flowed over the bones and moved them, and they got buried again a short distance away
• Unclear why the Avimimus skeletons were together in the bonebed
• Possible they were found together because they were gregarious, or social. Could be they were flocking or lekking (in an age-segregated group, with only mature Avimimus)
• For lekking, individuals come together to show off to potential mates
• Likely died from a catastrophic mass death, although it’s unclear what killed them
• High proportion of lower leg bones, which is unusual because there aren’t that many hip bones. May mean they were trapped in mud, but there’s no evidence of miring (mud) because there was a second flow event, meaning water flowed over the bones and moved them
• First evidence of gregariousness in oviraptorosaurs
• Possible more dinosaurs than we previously thought were social, and there was a trend to being more gregarious towards the end of the Mesozoic
• Authors said that in the Late Cretaceous, some groups, including ankylosaurs, ceratopsians, hadrosaurs, ornithomimids, and tyrannosaurs, show “strong tendencies to form monodominant assemblages” regardless of how they were preserved or their environment preferences
• Birds are gregarious, so it’s interesting to see bird-like dinosaurs together, to help understand the changes dinosaurs went through to become birds
• Also in 2016, another bonebed described found in northern China, which included at least six individuals of avimimids
• Second bonebed was found in 1959, which used bulldozers to excavate the site. More expeditions to the site in 1987 and 1988, though wasn’t described until 2016
• Some of them may have been juveniles, based on some of the bones beginning to fuse together, although according to the authors of the study, they were “likely functionally analogous to adults”
• They grew quickly (based on histology/cutting in to the bones)
• Possible juveniles reached full size within a year
• Lived in a semi-arid climate, with wet summers and dry winters
• Other dinosaurs that lived around the same time and place include hadrosaurs, pachycephalosaurs like Prenocephale, sauropods, alvarezsaurs like Mononykus, dromaeosaurs like Adasaurus, ornithomimosaurs like Anserimimus and Deinocheirus, caenagnathids like Elmisaurus, oviraptorids like Oksoko and Rinchenia, therizinosaurs like Therizinosaurus, troodontids like Zanabazar, and tyrannosaurs like Tarbosaurus

Fun Fact:

Tyrannosaurus tip-toed instead of stomped. source

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