The Big Dinosaur Podcast

Episode 36 is all about Troodon, one of the smartest dinosaurs, and also a theropod, originally named based on just on tooth.

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In this episode, we discuss:

• The dinosaur of the day: Troodon formosus, whose name means “wounding tooth”
• Lived in the Cretaceous
• Fossils found in Alaska, Wyoming, possibly Texas and New Mexico
• Discovered in 1855, one of the first dinosaurs found in North America
• Troodon fossil found in 1855 by Ferdinand V. Hayden, but named by Joseph Leidy in 1856
• Originally spelled Troödon (named by Joseph Leidy in 1856, but changed by Sauvage in 1876
• Type specimen was based on one tooth found in Judith River Formation, which caused some classification problems
• Originally Troodon tooth classifed as a lizard, then became a megalosaurid (1901–wastbasket taxon), then Gilmore suggested in 1924 that it was a Stegoceras (pachycephalosaur), and the family Pachycephalosauridae was known as Troodontidae for a while. But in 1945, Charles Mortram Sternberg said Troodon’s teeth was too similar to carnivore dinosaurs, and reclassified it as a theropod (making Troodontidae irrelevant, so the family was renamed Pachycephalosauridae)
• In 1932, Sternberg found a foot, parts of hand, and tail vertebrae in the Dinosaur Park Formation in Alberta. Named it Stenonychosaurus, but in 1951 decided it was closely related to Troodon, though no other specimens had been discovered yet to test his theory
• Dale Russell found a more complete Stenonychosaurus in 1969 (also from Dinosaur Park Formation), and it was well-known theropod in the 1980s. It was classified as part of the family Saurornithoididae, along with Saurornithoides, which were considered to be close relatives of Troodon. In 1987 Phil Currie analyzed the specimens and found that the different tooth and jaw structures of troodontids and saurornithoidids were because of age, not difference in species. He reclassified Stenonychosaurus inequalis as a junior of Troodon formosus (along with Polydontosaurus grandis and Pectinodon bakkeri). In 1988 Gregory S. Paul also classified Saurornithoides mongoliensis as Troodon mongoliensis, but not all scientists accepted it.
• But Currie and other paleontologists then questioned whether all the specimens were one Troodon species and noted in 1990 that though the specimens from Judith River were Troodon formosus, other troodontids from Hell Creek Formation and Lance Formation may be different species. George Olshevsky said in 1991 that the Lance formation fossils were Troodon bakkeri (formerly Pectinodon bakkeri, then Troodon formosus), and other scientists call the Dinosaur Park Formation fossils Troodon inequalis (formerly Stenonychosaurus inequalis)
• Zanno and a team reviewed troodontids in 2011 and found that Pectinodon bakkeri was a valid genus and many of the Troodon formosus specimens could be more than one species, but would require further study (the holotype Troodon is just one tooth, so Troodon may be a nomen dubium)
• Troodon species come from different areas and several million years apart
• Only way to properly figure it out is to have a neotype specimen (and not just a tooth) to compare all the specimens to and determine if there are multiple species of Troodon
• A neotype specimen “sets the standard for the species”
• Dale A. Russell, who found the first Troodon skull, suggested in 1982 that had Troodon not gone extinct, it may have evolved to have a brain case similar to a humans (since its EQ was 6x higher than other dinosaurs)
• Russell called it a “Dinosauroid” and said Troodontids would have had large eyes, three fingers on each hand, toothless beaks, and fed young like birds with regurgiated food (and language would sound like bird song)
• Many paleontologists have criticized the Dinosaurid as too anthropomorphic
• Small, 3 feet tall (0.9 m), 7.9 ft long (2.4 m), 110 lbs (50 kg)
• Reached adult size at 3-5 years
• Largest Troodon are similar in size to Deinonychus
• Closely related to dromaeosaurids (these theropod groups are the closest relatives to birds)
• Bipedal, stiff tail
• Long hind limbs, so probably could run quickly; also retractable sickle claw on second toes (didn’t touch the ground when running)
• May have had primitive feathers (could have insulated in Alaska)
• Troodon probably preferred cooler climates
• Lived near streams and deltas of coastal plains on Larimidia, along with Euoplocephalus, Dromaeosaurus and Nanuqsaurus
• Very large brain relative to body mass (similar to modern birds)
• Scientists who reconstructed a Troodon’s brain found it may have begun to fold onto itself (different from other reconstructed dinosaur brains); the folded, wrinkled look is seen in humans and animals with lots of brain power
• Hard to know its actual intelligence, though scientists think it was at least as smart as a modern opossum
• Not sure if it was omnivore or carnivore
• May have been an omnivore (serrations are somewhat similar to herbivorous reptiles)
• Troodon jaws meet in a broad U-shape, similar to an iguana (which eats plants), and teeth have large serrations (called denticles) like plant eaters; but it had grasping hands, was smart (similar to raccoons, for example, omnivores)
• Troodon had an opposable finger; not sure how it used it, but could have helped search for prey in branches or on the ground
• But could have been a predator (sickle claw and binocular vision)
• Troodon had recurved teeth (curved towards the tip points to the back of the mouth to help seize prey)
• T-rex also had a U-shaped snout so it could take larger bites of prey
• Good sense of hearing
• Better sense of smell than modern birds, but not as good as crocodiles and large theropods like T-rex
• Large eyes, so may have been nocturnal, and faced slightly forward, so had some depth perception
• Large eyes helped it hunt at night or during long winters
• Troodon in Alaska (due to the dark and having large eyes to see in the dark) may have evolved to be bigger to help fill the gap left by the larger carnivores (didn’t live there)
• One study of Troodon teeth found in northern sites (Alaska), larger than teeth from southern sites, suggesting northern Troodon grew larger and may have been able to hunt larger animals; also found that wear patterns in teeth showed it ate soft foods (not tough plants, but meat)
• One Troodon bone may have had a bite wound
• Some have speculated Troodon may have hunted in packs (no real evidence)
• May have eaten a mix of small lizards, malls, invertebrates, and plant material
• Jack Horner found the nests in Two Medicine Formation in Montana in 1983 along with David Varrichio
• Varricchio and team described in 1997 Troodon nests (dish shaped, about 100 cm in diameter, with a rim encircling the eggs. Complete nests had between 16 and 24 eggs, which were shaped like long teardrops
• Varricchio and team found that (based on nests), Troodon reproduced in a way in between crocodiles and birds; eggs grouped as pairs (so had two functional ovaries like crocodiles–birds only have one to help them fly) and Troodon laid eggs about 0.5 kg for a 50 kg adult (10 times larger than reptiles the same mass, but similar to what birds do)
• Varricchio and team found evidence of iterative laying (lays a pair of eggs every day or two, then delays brooding until finished laying eggs); embryos all in same stages of development, so probably hatched around the same time; may take 45-65 days for laying, brooding and hatching; no evidence they stayed in nest (may have left, like crocodiles)
• The site had a male adult Troodon
• Team also found that male Troodons probably brooded, and not females, based on bone histology of a Troodon specimen that did not have “bone resorption patterns that would indicate it was an egg-laying female”
• Troodon eggs laid vertically, and would have buried bottoms of the egg in mud (more similar to birds than crocodiles, which completely bury eggs)
• Similar to Egyptian Plover bird that broods eggs that are partially buried in sand
• The team studied the eggs and compared them to the number of pores in bird and crocodile eggshells (crocodile eggs have more pores for respiration, since they’re buried). Could help to figure out how future dinosaur eggs found were incubated
• Troodontidae is a small group of maniraptorans
• Small and very bird-like (one Troodontid, called Sinovenator, is very similar to Archaeopteryx
• Some scientists have suggested Troodontidae were ancestors of birds, but most believe it was dromaeosaurs
• Troodontid genuses include Saurornithoides (Mongolia), Troodon (North America), and Sinornithoids
• Other troodontids include Borogovia (named after Lewis Carroll poem) and Zanabazar (named after Mongolian spiritual figure), and Mei
• They have lots of teeth and closely spaced teeth in lower jaw
• Long legs, large curved claw on second toes that retracts when it runs (similar to dromaeosaurids, but smaller)
• Troodontids have sickle-claws and high EQs (very smart)
• Good hearing
• Ears were asymmetrical (one higher than the other, similar to owls), so may have hunted like owls, using hearing to locate prey
• Some may have been omnivorous though most were probably carnivorous
• Some troodont fossils show they roosted like birds (Mei), and supports theory that they probably had feathers
• Chinese Mei long (soundly sleeping dragon) is a specimen with legs folded and nose tucked under one wing-like arm, looks like it’s sleeping (a troodontid)
• Primitive troodontids like Jinfengopteryx and Anchiornis  have been found fully feathered
• Largest Troodontid is Troodon and smallest is Anchiornis
• In 2004 Mark Norell and team found two partial troodontid skulls in an oviraptorid nest; not sure why they were there (nest parasites or prey for the oviraptorids or its hatchlings)
• Fun fact: Birds and reptiles that live today have one opening in their body for urinating, defecating, and reproducing. It’s called a cloaca, which in Latin means “sewer”, and scientists believe dinosaurs also had a cloaca.