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Brief Summary

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First discovered in 1897, Euoplocephalus tutus was an ankylosaur dinosaur that lived in the Late Cretaceous, approximately 76.5 to 67 million years ago (1). This large-bodied herbivorous dinosaur was 5-6 m long, 2.4 m wide, and had has been estimated to weigh over 2000 kg. Only Ankylosaurus itself rivals E. tutusin size among ankylosaurids. Like all ankylosaurs,E. tutuswas heavily armored, with a club-like tail that it may have used to defend itself against predators (2). Abundant samples have been found of this species, though questions have been raised on the taxonomic identity of some of these specimens (3).

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General Ecology

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Euoplocephalus tutus lived between 76.5 and 67 million years ago of the Late Cretaceous, though recent reclassification of certain specimens may placeE. tutusin the 76.5 to 75.6 million years ago range. Specimens have been found in deposits throughout Alberta and Montana (1, 2).E. tutus, like other ankylosaurids, was an herbivore (3). Its broad-muzzled appearance implies that it was a non-selective feeder, similar to the feeding strategy of a modern hippopotamus (4). Traditionally, it has been thought that ankylosaurs were limited to simple up and down jaw motion during feeding (5), but by studying tooth wear and jaw articulations it has been found thatE. tutuscould have performed a range of complex jaw movements and was able to move the jaw from side to side during feeding (6). Possible predators ofE. tutusinclude Gorgosaurus, which has been found in similar deposits (7).

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Untitled

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Euoplocephalus tutuswas discovered in 1897 but not described until 1902. Originally Lawrence Lambe named this speciesStereocephalus tutus, but it was renamed in 1910 when it was discovered that the nameStereocephaluswas already in use (1, 2). TheE. tutusholotype consists of the upper part of a skull, and a series of six small plates of armor that protected the neck (1). When another armored dinosaur was found in the Hell Creek Formation,Ankylosaurus, these two species were placed into the family Ankylosauridae. While only three specimens ofAnkylosaurushave been unearthed until this day, over 40 specimens ofE. tutushave been discovered from theUpper Campanian and Lower Maastrichtianof Alberta and Montana, from approximately76.5 to 67 million years ago. This includes 17 skulls of varying completion (3).

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Taxonomy

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Euoplocephalus tutus was the first ankylosaur discovered and as such, the group Ankylosauria was not yet formed. Because of this,E. tutuswas originally classified with the other armored dinosaurs in Stegosauria. It wasn’t until 1923 that this was addressed andE. tutuswas reclassified in the newly defined Ankylosauria (1). Throughout the years, many new members of Ankylosauria have been discovered. One of the recent phylogenetic analyses of this group placesE. tutusas a close relative ofAnkylosaurus and an early member of this clade (2).

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Armor

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Like other ankylosaurids, Euoplocephalus tutus had a highly armored body. Their armor consisted of bony plates called osteoderms, and of smaller round bones (< 5mm). The armor formed nonoverlapping bands on the body, and was not fused into a single unit, which allowedE. tutusto maintain some flexibility (1). The head ofE. tutusarmored on the top and the sides, similar to a football helmet. The armor was formed by numerous small bones which were fused to the underlying bones of the skull. Arcs of bone protect the neck (2). The limbs were only partially armored, with plates present on the upper portion of the arm and only scattered smaller armor and spikes on the lower limbs (1).

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Nasal tract

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Early studies predicted a simple nasal tract in Euoplocephalus tutus, but a reconstruction by Witmer and Ridgely shows that this may not have been the case. This model shows evidence of high degrees of complexity in the nasal cavity ofE. tutus, accompanied by an increase in the number of blood vessels. However, it does not appear that this increased area was used for the sense of smell. Instead, the enhanced nasal cavity has been hypothesized to have functions in temperature or water regulation, or acoustic functions (1,2). This is similar to what is seen in many living birds and mammals (3, 4).

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Tail club

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Ankylosaurid tail clubs have often been hypothesized to function in defense. Hardened bony tendons can be found near the end of the tail in Euoplocephalus tutus, which would have strengthened this area against impact (1). Hardened tendons are not present in the part of the tail closest to the body, which would have allowed for flexibility while maintaining stronger rigidity nearer to the club (2). Paleontologic research indicates thatE. tutuscould not raise its club very high off the ground, but it still could have been very effective used in low swings at attacking predators (3). Further analysis found that although smaller and average-sized clubs might not, larger ankylosaurid clubs would be able to generate enough force to break bones during impact (4).

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