Old Earth Ministries Online Dinosaur Curriculum

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Lesson 37 - Troodon

Troodon (or Tro÷don in older sources) is a genus of relatively small, bird-like dinosaur from the Late Cretaceous period (75-65 mya). Discovered in 1855, it was among the first dinosaurs found in North America. Its species ranged widely, with fossil remains recovered from as far north as Alaska and as far south as Wyoming and even possibly Texas and New Mexico.

Its name is Greek for "wounding tooth", referring to the dinosaur's teeth, which are different from those of most other theropods. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet. A partial Troodon skeleton has been discovered with preserved puncture marks.

Troodon

Quick Facts

 

Length:  8 Ft

Weight:   110 lbs

Date Range:   75-65 Ma, Campanian - Maastrichtian Age, Cretaceous Period

 

Troodon

Troodon (Image embedded from National Geographic)

Description

Troodon was a small dinosaur, around 2.4 metres (7.9 ft) in length, and weighing on the
Troodon Scale
Troodon Scale image (Picture Source)
 order of 50 kilograms (110 lb). Troodon is the largest known troodontid. It had very long, slender hind limbs, suggesting that the animal was able to run quickly. It had large, retractable sickle-shaped claws on its second toes, which were raised off the ground when running.

Its eyes were large (perhaps suggesting a partially nocturnal lifestyle), and slightly forward facing, giving Troodon some degree of depth perception. Their light skulls contained a capsule similar to those found in ostrich dinosaurs.

Brain

Troodon had one of the largest known brains of any dinosaur, relative to its body mass (comparable to modern birds). Troodon's cerebrum-to-brain-volume ratio was 31.5% to 63% of the way from a nonavian reptile proportion to a truly avian one.

History and classification
Troodon
Head and neck of Dale Russell's Stenonychosaurus sculpture, Natural History Museum, London  (Picture Source

Troodon was originally spelled Tro÷don (with a diaeresis) by Joseph Leidy in 1856, which was officially amended to its current status by Sauvage in 1876. The type specimen of Troodon has caused problems with classification, as the entire genus is based only on a single tooth from the Judith River Formation.

The first specimen currently assigned to Troodon that was not a tooth was named Stenonychosaurus by Sternberg in 1932, based on a foot, fragments of a hand, and some tail vertebrae from Alberta. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of Deinonychosauria.

A more complete skeleton of Stenonychosaurus was described by Dale Russell in 1969 from the Dinosaur Park Formation, which eventually formed the scientific foundation for a famous life-sized sculpture of Stenonychosaurus. Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with Saurornithoides, it formed the family Saurornithoididae. Based on differences in tooth structure, and the extremely fragmentary nature of the original Troodon specimens, Saurornithoidids were thought to be close relatives while Troodon was considered a dubious possible relative of the family. Phil Currie, reviewing the pertinant specimens in 1987, showed that supposed differences in tooth and jaw structure among troodontids and saurornithoidids were based on age and position of the tooth in the jaw, rather than a difference in species.  All of the specimens once called Stenonychosaurus are now referred to as Troodon in the recent scientific literature.

Distribution

Troodon is known from the Judith River Formation and the upper Two Medicine Formation of Montana, the Judith River Group of Alberta, the Horseshoe Canyon Formation of Alberta, the Prince Creek Formation of Alaska, and in the famous Lance and Hell Creek Formations of the USA. There is some evidence that Troodon favored cooler climates, as it seems to have been particularly abundant in northern and even arctic areas and during cooler intervals, such as the early Maastrichtian. Possible Troodon teeth have been found in the lower Javelina Formation of Texas and the Naashoibito Member of the Kirtland Formation in New Mexico.

It seems unlikely that all of these fossils, which come from localities hundreds or thousands of miles apart, separated by millions of years of time, represent a single species of Troodon. However, further study and more fossils are needed to determine how many species of Troodon existed.

Paleobiology

Troodon is thought to have been a predator like other theropods. This view is supported by its Maniraptoran features (sickle claw on foot) and apparently good binocular vision.

Troodon teeth, however, are different from most other theropods. One comparative study
Troodon Teeth
Above: Troodon teeth (Picture Source)
Below: The holotype tooth
Troodon holotype tooth
 of the feeding apparatus suggests that Troodon could have been an omnivore. The jaws met in a broad, U-shaped symphysis similar to that of an iguana, a lizard species adapted to a plant-eating lifestyle. Additionally, the teeth of Troodon bore large serrations each of which is called a denticle. There are pits at the intersections of the denticles, and the points of the denticles point towards the tip, or apex, of each tooth. The teeth show wear facets on their sides. Holtz (1998) also noted that characteristics used to support a predatory habit for Troodon - the grasping hands, large brain and stereoscopic vision, are all characteristics shared with the herbivorous/omnivorous primates and omnivorous Procyon (raccoon).

One study was based on the many Troodon teeth that have been collected from Late Cretaceous deposits from northern Alaska. These teeth are much larger than those collected from more southern sites, providing evidence that northern Alaskan populations of Troodon grew to larger average body size. The study suggests that the Alaskan Troodons may have had access to large animals as prey because there were no tyrannosaurids in their habitat to provide competition for those resources. This study also provides an analysis of the proportions and wear patterns of a large sample of Troodon teeth. It proposes that the wear patterns of all Troodon teeth suggest a diet of soft foods - inconsistent with bone chewing, invertebrate exoskeletons, or tough plant items. This study hypothesizes a diet primarily consisting of meat

Age determination studies performed on the fossilized remains of Troodon using growth ring counts suggest that this dinosaur reached its adult size probably in 3ľ5 years.

Reproduction

Dinosaur eggs and nests were discovered by John R. Horner in 1983 in the Two Medicine
Troodon eggs
A clutch of Troodon formosus eggs partly encased in matrix. Museum of the Rockies specimen on loan to the Burke Museum.  (Picture Source)
 Formation of Montana. Varriccho et al. (2002) have described eight of these nests found to date. These are all in the collection of the Museum of the Rockies. Horner (1984) found isolated bones and partial skeletons of the hypsilophodont Orodromeus very near the nests in the same horizon and described the eggs as those of Orodromeus. Horner and Weishampel (1996) reexamined the embryos preserved in the eggs and determined that they were those of Troodon, not Orodromeus. Varricchio et al. (1997) made this determination with even more certainty when they described a partial skeleton of an adult Troodon in contact with a clutch of at least five eggs, probably in a brooding position.

Varricchio et al. (1997) described the exact structure of Troodon nests. They were built from sediments, they were dish shaped, about 100 cm in internal diameter, and with a pronounced raised rim encircling the eggs. The more complete nests had between 16 and 24 eggs. The eggs are shaped like elongated teardrops, with the more tapered ends pointed downwards and imbedded about halfway in the sediment. The eggs are pitched at an angle so that, on average, the upper half is closer to the center of the nest. There is no evidence that plant matter was present in the nest.

Varricchio et al.(1997) were able to extract enough evidence from the nests to infer several characteristics of Troodon reproductive biology. The results are that Troodon appears to have a type of reproduction that is intermediate between crocodiles and birds, as phylogeny would predict. The eggs are statistically grouped in pairs, which suggests that the animal had two functional oviducts, like crocodiles, rather than one, as in birds. Crocodiles lay many eggs that are small proportional to adult body size. Birds lay fewer, larger, eggs. Troodon was intermediate, laying an egg of about 0.5 kg for a 50 kg adult. This is 10 times larger than reptiles of the same mass, but two Troodon eggs are roughly equivalent to the 1.1 kg egg predicted for a 50 kg bird.

Varricchio et al. also found evidence for iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. MOR 363 was found with 22 empty (hatched) eggs, and the embryos found in the eggs of MOR 246 were in very similar states of development, implying that all of the young hatched simultaneously. The embryos had an advanced degree of skeletal development, implying that they were precocial or even superprecocial. The authors estimated 45 to 65 total days of adult nest attendance for laying, brooding, and hatching. The authors found no evidence that the young remained in the nest after hatching and suggested that they dispersed like hatchling crocodiles or megapode birds instead.

Varricchio et al. (2008) examined the bone histology of Troodon specimen MOR 748 and found that it lacked the bone resorption patterns that would indicate it was an egg-laying female. They also measured the ratio of the total volume of eggs in Troodon clutches to the body mass of the adult. They graphed correlations between this ratio and the type of parenting strategies used by extant birds and crocodiles and found that the ratio in Troodon was consistent with that in birds where only the adult male broods the eggs. From this they concluded that Troodon females likely did not brood eggs, that the males did, and this may be a character shared between maniraptoran dinosaurs and basal birds.

If you ordered the Test Pack, it is now time to take Test 5.

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