Tyrannosauroidea
Creators: name: Brusatte, Stephen L.
name: Norell, Mark A.
name: Carr, Thomas D.
name: Erickson, Gregory M.
name: Hutchinson, John R.
name: Balanoff, Amy M.
name: Bever, Gabe S.
name: Choiniere, Jonah N.
name: Makovicky, Peter J.
name: Xu, Xing
Publisher: name: Zenodo
Year: 2010
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Dinosauria

description: Here we assess the current state of tyrannosaur research, with a focus on the phylogenetic relationships and large-scale evolutionary patterns exhibited by the group, the biology of tyrannosaurs as living organisms, and information revealed from the newest discoveries. <b>Phylogenetic Relationships and Evolution of Tyrannosaurs</b> Tyrannosaurs, which formally comprise the clade Tyrannosauroidea, are a relatively derived group of theropod dinosaurs, more closely related to birds than to other large theropods such as allosauroids and spinosaurids (<i>1</i>, <i>17</i>). Approximately 20 tyrannosauroid genera are currently known, 5 of which were described during the past year. To assess their interrelationships, we conducted a phylogenetic analysis, which includes all well-known genera scored for 307 morphological characters (<i>18</i>). The data set is based on personal observation of specimens and includes 123 novel characters (40% of total) based on recently discovered tyrannosaur taxa (<i>4</i>, <i>19 – 21</i>). 1 Division of Paleontologyι American Museum of Natural Historyι central Park West at 79th Streetι New Yorkι NY 10024ι USA. 2 Department of Earth and Environmental Sciencesι columbia Universityι New Yorkι NY 10964ι USA. 3 Department of Biologyι carthage collegeι Kenoshaι WI 53140ι USA. 4 Department of Biological Scienceι Florida State Universityι Tallahasseeι FL 32306ι USA. 5 Department of Veterinary Basic Sciencesι The Royal Veterinary collegeι Hatfieldι Herts AL9 7TAι UK. 6 Department of Geology and Geophysicsι Yale Universityι New Havenι cT 06520ι USA. 7 Department of Biological Sciencesι The George Washington Universityι Washingtonι Dc 20052ι USA. 8 Department of Geologyι The Field Museum of Natural Historyι chicagoι IL 60605ι USA. 9 Institute of Vertebrate Paleontology and Paleoanthropologyι chinese Academy of Sciencesι Beijing 100044ι People’ s Republic of china. *To whom correspondence should be addressed. E-mail: sbrusatte@amnh.org †These authors contributed equally to this work. Tyrannosaurs are a long-lived group that originated by the Middle Jurassic, ~165 million years ago (<i>5</i>) (Fig. 2). The oldest and most basal tyrannosaurs comprise a speciose subclade, Proceratosauridae, which includes mostly small-bodied animals no larger than a human, many of which possessed elaborate cranial crests (<i>22</i>). Progressively more derived tyrannosaurs form a pectinate series on the line toward Tyrannosauridae, the subclade of multi-ton, deep-skulled behemoths from the terminal Cretaceous (Campanian-Maastrichtian), including <i>Tyrannosaurus</i>, <i>Tarbosaurus</i>, <i>Albertosaurus</i>, and close relatives (<i>1</i>). Taxa phylogenetically intermediate between proceratosaurids and tyrannosaurids include a range of genera from the Late Jurassic–early Late Cretaceous of Asia, North America, and Europe, most of which have been recently discovered (<i>4</i>, <i>14</i>, <i>20</i>, <i>21</i>, <i>23</i>, <i>24</i>). These taxa run the gamut from small to medium size (~1.4 to 9.0 m in length), and few were likely apex predators in their ecosystems (<i>21</i>). Until recently, the prevailing notion was that tyrannosaur body size gradually, and progressively, increased over time, in concert with the piecemeal accumulation of signature tyrannosaur skeletal features (<i>14</i>, <i>21</i>). However, new discoveries have led to a reassessment. The Early Cretaceous proceratosaurid <i>Sinotyrannus</i> may have approached 10 m in body length, demonstrating that tyrannosaurs could attain a large size early in their history (<i>25</i>). More striking, the close tyrannosaurid outgroup <i>Raptorex</i> is only 2 to 3 m in length, suggesting that there was great size variability among close tyrannosaurid relatives and perhaps that the immediate ancestors of tyrannosaurids were small animals (<i>4</i>). Truly enormous size, however, is restricted to the latest Cretaceous tyrannosaurids, some of which grew to lengths of 13 m and masses of 5 to 8 tons (<i>8</i>). Therefore, for the first 80 million years of their history tyrannosaurs were mostly small- to mid-sized animals that lived in the shadow of other giant predators (e.g., allosauroids, megalosauroids), and only during the final 20 million years of the Mesozoic did they develop into some of the largest terrestrial carnivores to ever live (<i>26</i>). The dominance of tyrannosaurs as megapredators was purely a latest Cretaceous phenomenon. Tyrannosaur Anatomy The spate of new discoveries has prompted a renewed focus on tyrannosaur anatomy, including external, internal, and soft-tissue morphology (Fig. 3). All tyrannosaurs are bipedal predators and possess several unique features, including a small premaxilla with D-shaped “incisor”-like teeth, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head (<i>6</i>, <i>27</i>, <i>28</i>). A number of derived specializations characterize the giant tyrannosaurids: a large and deep skull with powerful jaw muscles, robust teeth, reinforced sutures between skull bones, and tiny forelimbs (<i>6</i>, <i>29</i>), features often considered adaptations for a hypercarnivore to function at large size (<i>4</i>). Basal tyrannosauroids, in contrast, have smaller skulls and longer arms and generally resemble sleek, bird-like theropods more than their enormous tyrannosaurid cousins (<i>14</i>, <i>22</i>). New discoveries have shown, however, that the hallmark tyrannosaurid body plan (large and deep skull, robust teeth, etc.) does not uniquely or uniformly characterize the tyrannosaurid clade. Most of these features are now known to be present in <i>Raptorex</i>, the man-sized tyrannosaurid outgroup that lived 40 million years before tyrannosaurids originated (<i>4</i>). Furthermore, the gracile and long-snouted <i>Alioramus</i>, a tyrannosaurid that is about half the size of close relatives such as <i>Tarbosaurus</i> and <i>Tyrannosaurus</i>, lacks a deep and muscular skull and thick teeth (<i>19</i>). Thus, characteristic tyrannosaurid features did not evolve as a consequence of large body size, but likely originated in small animals, and not even all derived, Late Cretaceous tyrannosaurids are united by a characteristic morphotype (<i>4</i>, <i>19</i>). Much is also known about the internal anatomy of the tyrannosaur skull, thanks to the discovery of exceptionally preserved fossils and the application of digital techniques such as computerized tomography (CT) scanning (<i>7</i>, <i>30</i>, <i>31</i>). Tyrannosaurs possessed the required neuroanatomy to lead the active, predatory life-style expected of derived theropods (<i>7</i>, <i>30</i>). Their encephalization quotient <i>—</i> an estimate of relative brain size <i>—</i> varies between 2.0 and 2.4, larger than in basal theropods but lower than that of birds and their closest relatives (<i>19</i>). Large olfactory lobes indicate a strong sense of smell (<i>7</i>, <i>31</i>). Elongate cochlear and semicircular canals apparently support elevated sensitivity to low-frequency sound and highly coordinated head and eye movements (<i>7</i>). Several tyrannosaurid specimens have been reported to preserve integumentary structures and other soft tissues, which rarely fossilize in dinosaurs. Although impressions of scaly skin have been described for large tyrannosaurids (<i>32</i>), simple filamentous integument, interpreted as homologous to feathers, is clearly preserved in a specimen of the basal tyrannosauroid <i>Dilong</i> (<i>14</i>). These branched filaments appear to have extensively covered the body, as they are observed near the skull and tail. A recent study suggests that much larger tyrannosauroids were covered with elongate, broader integumentary structures (<i>33</i>), which were likely used for display (<i>34</i>). Several easily degraded soft tissues, such as cells, blood vessels, and collagen,have been reported from a specimen of <i>Tyrannosaurus</i> (<i>15</i>, <i>35</i>). Some of these findings have been met with skepticism (<i>36</i>), and they remain to be validated by other research groups. However, if correct, they promise to give radical new insight into the process of fossilization and may allow for molecular phylogenetic analysis of these extinct taxa (<i>37</i>). Tyrannosaur Growth Arguably we know more about tyrannosaur biology than that of any other dinosaurs (Figs. 3 and 4). Much of this knowledge has been gained over the past 20 years, through the collection of skeletons of both adults and juveniles, bones of their prey with bite marks, coprolites (fossil feces), stomach contents, and pathological specimens (<i>38</i>). Much attention has focused on how tyranno- saurs grew, especially on how giants such as <i>T. rex</i> achieved such massive size and how their skel- etons changed during the transition from embryo to multi-ton adult. Comparative growth curves for several species, which plot body mass (calculated from femur size) against age in years (calculated from counting growth lines in histological section) (<i>8</i>) (Fig. 4B), show that large tyrannosaurids reached somatic maturity around 20 years old, though most rarely lived for more than 25 years. <i>T</i>. <i>rex</i> evidently attained its large size via acceleration of growth rates relative to closely related species, not by extending its life span.Its maximum growth rate may have exceeded 767 kg per year, equiv- alent to adding a remarkable 2 kg per day (<i>8</i>). Tyrannosaur skeletons changed substantially as individuals matured. Although less is known about the growth of small, basal tyrannosaurs, tyrannosaurids and their closest large-bodied relatives are united by a conservative pattern of growth in which the skulls of juveniles were en- tirely reshaped during ontogeny (<i>9</i>, <i>20</i>, <i>39</i>). This sequence has been reconstructed by cladistic analysis, based on the principle that ontogeny, like phylogeny, involves a hierarchically nested series of character changes (<i>19</i>, <i>39</i>). During the growth of an individual species, the skull and jaws deepened, pneumatic bones inflated, orna- mented structures enlarged and coarsened, sutural surfaces deepened and became more rugose, and the teeth became larger and thicker (<i>9</i>, <i>40</i>) (Fig. 1). Changes have also been documented in the post- cranial skeleton.Most notably, the forearm shortened and the long shin and foot of juveniles became shorter and stockier in adults (<i>40</i>). The differences between juvenile and adult tyrannosaurids are so great that different growth stages have often been mistaken for different species (<i>3</i>, <i>9</i>). Tyrannosaur Behavior 1 Division of Paleontologyι American Museum of Natural Historyι central Park West at 79th Streetι New Yorkι NY 10024ι USA. 2 Department of Earth and Environmental Sciencesι columbia Universityι New Yorkι NY 10964ι USA. 3 Department of Biologyι carthage collegeι Kenoshaι WI 53140ι USA. 4 Department of Biological Scienceι Florida State Universityι Tallahasseeι FL 32306ι USA. 5 Department of Veterinary Basic Sciencesι The Royal Veterinary collegeι Hatfieldι Herts AL9 7TAι UK. 6 Department of Geology and Geophysicsι Yale Universityι New Havenι cT 06520ι USA. 7 Department of Biological Sciencesι The George Washington Universityι Washingtonι Dc 20052ι USA. 8 Department of Geologyι The Field Museum of Natural Historyι chicagoι IL 60605ι USA. 9 Institute of Vertebrate Paleontology and Paleoanthropologyι chinese Academy of Sciencesι Beijing 100044ι People’ s Republic of china. *To whom correspondence should be addressed. E-mail: sbrusatte@amnh.org †These authors contributed equally to this work. Tyrannosaurs are a long-lived group that originated by the Middle Jurassic, ~165 million years ago (<i>5</i>) (Fig. 2). The oldest and most basal tyrannosaurs comprise a speciose subclade, Proceratosauridae, which includes mostly small-bodied animals no larger than a human, many of which possessed elaborate cranial crests (<i>22</i>). Progressively more derived tyrannosaurs form a pectinate series on the line toward Tyrannosauridae, the subclade of multi-ton, deep-skulled behemoths from the terminal Cretaceous (Campanian-Maastrichtian), including <i>Tyrannosaurus</i>, <i>Tarbosaurus</i>, <i>Albertosaurus</i>, and close relatives (<i>1</i>). Taxa phylogenetically intermediate between proceratosaurids and tyrannosaurids include a range of genera from the Late Jurassic–early Late Cretaceous of Asia, North America, and Europe, most of which have been recently discovered (<i>4</i>, <i>14</i>, <i>20</i>, <i>21</i>, <i>23</i>, <i>24</i>). These taxa run the gamut from small to medium size (~1.4 to 9.0 m in length), and few were likely apex predators in their ecosystems (<i>21</i>). Until recently, the prevailing notion was that tyrannosaur body size gradually, and progressively, increased over time, in concert with the piecemeal accumulation of signature tyrannosaur skeletal features (<i>14</i>, <i>21</i>). However, new discoveries have led to a reassessment. The Early Cretaceous proceratosaurid <i>Sinotyrannus</i> may have approached 10 m in body length, demonstrating that tyrannosaurs could attain a large size early in their history (<i>25</i>). More striking, the close tyrannosaurid outgroup <i>Raptorex</i> is only 2 to 3 m in length, suggesting that there was great size variability among close tyrannosaurid relatives and perhaps that the immediate ancestors of tyrannosaurids were small animals (<i>4</i>). Truly enormous size, however, is restricted to the latest Cretaceous tyrannosaurids, some of which grew to lengths of 13 m and masses of 5 to 8 tons (<i>8</i>). Therefore, for the first 80 million years of their history tyrannosaurs were mostly small- to mid-sized animals that lived in the shadow of other giant predators (e.g., allosauroids, megalosauroids), and only during the final 20 million years of the Mesozoic did they develop into some of the largest terrestrial carnivores to ever live (<i>26</i>). The dominance of tyrannosaurs as megapredators was purely a latest Cretaceous phenomenon. Tyrannosaur Anatomy The spate of new discoveries has prompted a renewed focus on tyrannosaur anatomy, including external, internal, and soft-tissue morphology (Fig. 3). All tyrannosaurs are bipedal predators and possess several unique features, including a small premaxilla with D-shaped “incisor”-like teeth, fused nasals, extreme pneumaticity in the skull roof and lower jaws, a pronounced muscle attachment ridge on the ilium, and an elevated femoral head (<i>6</i>, <i>27</i>, <i>28</i>). A number of derived specializations characterize the giant tyrannosaurids: a large and deep skull with powerful jaw muscles, robust teeth, reinforced sutures between skull bones, and tiny forelimbs (<i>6</i>, <i>29</i>), features often considered adaptations for a hypercarnivore to function at large size (<i>4</i>). Basal tyrannosauroids, in contrast, have smaller skulls and longer arms and generally resemble sleek, bird-like theropods more than their enormous tyrannosaurid cousins (<i>14</i>, <i>22</i>). New discoveries have shown, however, that the hallmark tyrannosaurid body plan (large and deep skull, robust teeth, etc.) does not uniquely or uniformly characterize the tyrannosaurid clade. Most of these features are now known to be present in <i>Raptorex</i>, the man-sized tyrannosaurid outgroup that lived 40 million years before tyrannosaurids originated (<i>4</i>). Furthermore, the gracile and long-snouted <i>Alioramus</i>, a tyrannosaurid that is about half the size of close relatives such as <i>Tarbosaurus</i> and <i>Tyrannosaurus</i>, lacks a deep and muscular skull and thick teeth (<i>19</i>). Thus, characteristic tyrannosaurid features did not evolve as a consequence of large body size, but likely originated in small animals, and not even all derived, Late Cretaceous tyrannosaurids are united by a characteristic morphotype (<i>4</i>, <i>19</i>). Much is also known about the internal anatomy of the tyrannosaur skull, thanks to the discovery of exceptionally preserved fossils and the application of digital techniques such as computerized tomography (CT) scanning (<i>7</i>, <i>30</i>, <i>31</i>). Tyrannosaurs possessed the required neuroanatomy to lead the active, predatory life-style expected of derived theropods (<i>7</i>, <i>30</i>). Their encephalization quotient <i>—</i> an estimate of relative brain size <i>—</i> varies between 2.0 and 2.4, larger than in basal theropods but lower than that of birds and their closest relatives (<i>19</i>). Large olfactory lobes indicate a strong sense of smell (<i>7</i>, <i>31</i>). Elongate cochlear and semicircular canals apparently support elevated sensitivity to low-frequency sound and highly coordinated head and eye movements (<i>7</i>). Several tyrannosaurid specimens have been reported to preserve integumentary structures and other soft tissues, which rarely fossilize in dinosaurs. Although impressions of scaly skin have been described for large tyrannosaurids (<i>32</i>), simple filamentous integument, interpreted as homologous to feathers, is clearly preserved in a specimen of the basal tyrannosauroid <i>Dilong</i> (<i>14</i>). These branched filaments appear to have extensively covered the body, as they are observed near the skull and tail. A recent study suggests that much larger tyrannosauroids were covered with elongate, broader integumentary structures (<i>33</i>), which were likely used for display (<i>34</i>). Several easily degraded soft tissues, such as cells, blood vessels, and collagen,have been reported from a specimen of <i>Tyrannosaurus</i> (<i>15</i>, <i>35</i>). Some of these findings have been met with skepticism (<i>36</i>), and they remain to be validated by other research groups. However, if correct, they promise to give radical new insight into the process of fossilization and may allow for molecular phylogenetic analysis of these extinct taxa (<i>37</i>). Tyrannosaur Growth Arguably we know more about tyrannosaur biology than that of any other dinosaurs (Figs. 3 and 4). Much of this knowledge has been gained over the past 20 years, through the collection of skeletons of both adults and juveniles, bones of their prey with bite marks, coprolites (fossil feces), stomach contents, and pathological specimens (<i>38</i>). Much attention has focused on how tyranno- saurs grew, especially on how giants such as <i>T. rex</i> achieved such massive size and how their skel- etons changed during the transition from embryo to multi-ton adult. Comparative growth curves for several species, which plot body mass (calculated from femur size) against age in years (calculated from counting growth lines in histological section) (<i>8</i>) (Fig. 4B), show that large tyrannosaurids reached somatic maturity around 20 years old, though most rarely lived for more than 25 years. <i>T</i>. <i>rex</i> evidently attained its large size via acceleration of growth rates relative to closely related species, not by extending its life span.Its maximum growth rate may have exceeded 767 kg per year, equiv- alent to adding a remarkable 2 kg per day (<i>8</i>). Tyrannosaur skeletons changed substantially as individuals matured
description: Published as part of <i>Stephen L. Brusatte, Mark A. Norell, Thomas D. Carr, Gregory M. Erickson, John R. Hutchinson, Amy M. Balanoff, Gabe S. Bever, Jonah N. Choiniere, Peter J. Makovicky &amp; Xing Xu, 2010, Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms, pp. 1481-1485 in Science 329</i> on pages 1481-1485, DOI: 10.1126/science.1193304, http://zenodo.org/record/3744287
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