Episode 348 is all about Orodromeus, a small fast ornithopod known from a growth series at Egg Mountain.
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In this episode, we discuss:
News:
- A redescription of Scutellosaurus shows that it was bipedal source
- “Megaripples” found a mile beneath Louisiana were caused by a tsunami after the impact that wiped out the dinosaurs source
- The Kuji Amber Museum in Japan has a display of recently discovered fossilized teeth from three theropod species source
- There’s a time-lapse video of all 292 bones of Dippy getting assembled at Norwich Cathedral source
- Warren Elsmore also in Norwich has a Jurassic Lego exhibition until Aug 30 including 500,000 Lego blocks and elements source
- Whipsnade Zoo in Bedfordshire, England has a new Zooasic Park until Sept 5 source
- A “Guide to vaccination for dinosaurs” was posted in Malaysia after someone got there COVID vaccine in an inflatable T. rex costume source
- Google Chrome’s Dinosaur game has been updated to include Olympic events source
The dinosaur of the day: Orodromeus
- Ornithopod that lived in the Late Cretaceous in what is now Montana, US (Two Medicine Formation)
- Looks like other ornithopods, with a small head, long body and tail, and long legs
- Estimated to be 8.2 ft (2.5 m) long
- Bipedal and small, but fast
- Extremely gracile and a fast runner, based on hindlimb proportions
- Found on the Egg Mountain site (same as Maiasaura)
- Described in 1988 by Jack Horner and David Weishampel
- Jack and Horner and David Weishampel described embryonic dinosaur remains in 1988
- “We report here the first well-documented association of dinosaur eggs containing embryos, juveniles and adults from the fossil record”
- Type species is Orodromeus makelai
- Genus name means “Mountain Runner”
- Genus name alludes to “Egg Mountain” as well as the state of Montana, and to the animals, presumed cursorial habits
- Species name is in honor of “the late Robert Makela for his many dinosaur discoveries including the holotype”
- Holotype is a nearly complete juvenile skeleton without hands and tail
- Referred material includes the clutch of 19 eggs with embryos, hind legs, a couple skeletons, another skull, and a braincase (6 individuals plus the clutch)
- One individual is about 60% complete and smaller, and probably a near-hatchling
- Another is more mature, and has a crushed skull
- Another is also mature, fairly complete disarticulated skeleton
- Found a clutch of 19 unhatched eggs with embryonic skeletons, found with eggshell fragments from other nests containing young Maiasaura
- Said Orodromeus was precocial after hatching (pretty active right after hatching), after comparing nesting patterns and bone histology with Maiasaura
- Teeth from the Judith River Formation, originally referred to Thescelosaurus, was referred to Orodromeus by Galton in 1995
- One specimen and eggs from Egg Mountain are now considered to be a troodontid, possibly Stenonychosaurus
- Horner and Weishampel in 1996 thought they were Troodon (published a correction in a letter to Nature, and said “further preparation of the embryo has revealed the identity to be that of the theropod dinosaur Troodon formosus” (based on the teeth), then in 1997 Varricchio and others described a partial adult Troodon skeleton with a clutch of at least five eggs, probably in a brooding position
- Van der Reest and Currie said it was possible that troodontid was Stenonychosaurus
- Rodney Scheetz wrote a thesis (unpublished?) on the osteology of Orodromeus in 1999
- According to Scheetz, an alternate reason for so many young Orodromeus carcasses in Troodon nesting area may be because Troodon adults brought them in as prey for their young (would think there were more hind-limb elements, “as the hind-quarters of the animal would constitute the largest meat mass” (Scheetz’s assessment found that to be true)
- Still lots of Orodromeus found. Scheetz’s thesis said he described material of 99 other specimens
- Second species is Orodromeus minimus (possibly Laosaurus minimus)
- Charles Gilmore described Laosaurus minimus in 1909 based on a partial left hindlimb and pieces of vertebra found in Alberta, Canada (Allison Formation)
- In 1949 Loris Russell found it to be most like Hypsilophodon, based on the locality where the fossils were discovered having been found in the early 1930s to be the Belly River Group instead of the Blairmore Group
- In the 1980s, Galton suggested it was Troodon, based on unpublished evidence
- In 1990, Sues and Norman found Laosaurus minimus to possibly be Orodromeus minimus, but there weren’t enough fossils to be sure
- Horner and Weishampel said, “Tooth wear indicates reversion to (or retention of) the primitive high-angle, fabrosaur-like style of chewing”
- Family Fabrosauridae now considered to be basal Ornithischia
- Based on a 2013 study by Jordan Mallon and others, a small ornithopod like Orodromeus probably ate food that was at most about 3 ft (1 m) high
- Scheetz said the triangular teeth with the high angle of how the teeth fit together may show a shift to eating insects
- Possible the teeth were that way in infants, and then the known Orodromeus specimens were young, or the adults kept that “neonate condition” even as they grew
- Had boss-like growths on its cheek bones
- Zephyrosaurus may be closely related to Orodromeus, based on both having bosses on their cheeks (talked about in episode 306)
- May have burrowed, like Oryctodromeus, based on how the fossils were found packed together in places that they usually would have been found scattered (but no clear evidence)
- Jack Horner and others in 2009 compared histology and growth of Orodromeus, Dryosaurus, and Tenontosaurus
- Looked at perinate (embryos or recently hatched), juveniles, sub-adults, and adults
- Typical perinate had a femur length of (0.6 in) 16 mm
- Had smaller and larger juveniles
- Sub-adult femur poorly preserved and destroyed by bacterial-mycelial actions
- Adult tibia also invaded by bacteria
- Adult femurs were 7-7.8 in (180-200 mm) or greater in length
- Did reach a growth plateau
- Found first LAG on the late juveniles, which is similar to Maiasaura (but Maiasaura is much bigger)
- Couldn’t count the exact number of LAGs on the adult (too poorly preserved), but counted at least 5 in the better preserved individuals
- Estimated the late juveniles to be 3 to 4 years old, and that Orodromeus reached adult size 1 to 2 years after that
- Orodromeus was relatively small, but still had considerable growth trajectory from embryo to adult
- Grew quickly after hatching, then slowed down as a later juvenile
- Probably grew faster than a crocodile but not as fast as a bird
Fun Fact: Ankylosaurs (& other dinosaurs) could have floated for weeks before finally sinking and fossilizing.
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