Acrocanthosaurus Inside and Out
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How can paleontologists know what a living dinosaur was like more than a hundred million years ago, particularly when only partial skeletons remain? Focusing on one large carnivorous dinosaur, Acrocanthosaurus ("high-spined lizard"), paleontologist Kenneth Carpenter explains the process, pairing scholarly findings with more than 75 color illustrations to reconstruct "Acro" before readers' eyes. In Acrocanthosaurus Inside and Out, he offers the most complete portrait possible of this fascinating dinosaur's appearance, biology, and behavior. Acrocanthosaurus--similar in size to its later cousin Tyrannosaurus rex, but studded with large spines--roamed what is now the south-central United States 110 to 115 million years ago, during the Early Cretaceous. Carpenter worked on the most complete of the Acrocanthosaurus skeletons (nicknamed "Fran") that has been found. Here he describes the techniques that tell us about Acro's biological makeup, movements, and habits. Studies of joints reveal the range of possible motion, while bumps, ridges, and scars on the bones show where muscles, ligaments, and tendons attached. CT scans allow us to peer into the braincase, while microscopes afford a cross-sectional view of bones. These findings in turn offer an idea of how Acro stalked and ate its prey. Scientific evidence beyond the fossils provides avenues for further inquiry: What does the sedimentary rock encasing Fran's bones tell us about Acro's environment? What does our knowledge of Acro's distant relatives, such as crocodilians and birds, imply about its heart and other soft tissues? Can our understanding of other animals explain Acro's huge spines? Carpenter distills all this information into a clear, accessible, engaging account that will appeal to general readers and scholars alike. As the first book-length work on Acrocanthosaurus, this volume introduces a prehistoric giant that once stalked Texas and Oklahoma and offers a rare, firsthand glimpse into the trials and triumphs of paleontology.
ISBN-13: 9780806191546
Media Type: Paperback
Publisher: University of Oklahoma Press
Publication Date: 07-05-2022
Pages: 152
Product Dimensions: 6.12(w) x 9.25(h) x 0.58(d)
Kenneth Carpenter is the Director and Curator of the Prehistoric Museum of Utah State University–Eastern. He received the Ph.D. in geology from the University of Colorado in 1996. From 1989 until 2010, he worked at the Denver Museum of Nature and Science. He has written hundreds of both scholarly and popular articles and encyclopedia entries on dinosaurs. Among other books, he is the author of Eggs, Nests, and Baby Dinosaurs: A Look at Dinosaur Reproduction.
Discovering Acro "Acro" is the nickname for the large carnivorous dinosaur Acrocanthosaurus atokensis (ack-row-can-thoughsoar-us ah-toke-en-sis). A cast, or replica, of its skeleton is on display at the Museum of the Red River in Idabel, Oklahoma, and is based on a partial skeleton now at the North Carolina Museum of Natural History in Raleigh (fig. 1.1). Acrocanthosaurus atokensis was officially named in a 1950 article by J. Willis Stovall and Wann Langston, Jr. The description of the new species was based on parts of two skeletons from Atoka County, Oklahoma; these were the first of five specimens of Acrocanthosaurus that we know of today (figs. 1.2 and 1.3). The bones of the first skeleton were reported in late March or early April 1940 by Joe Southern, of the Works Progress Administration, to Stovall, who was then director of the University of Oklahoma Museum. (Later this museum became the Oklahoma Museum of Natural History; now it's the Sam Noble Oklahoma Museum of Natural History, or SNOMNH.) The bones were eroding out of sandstone in what is now called the Antlers Formation. The site looked promising to Stovall, so he returned a short time later with Langston and a small crew from the Works Progress Administration (WPA). The WPA was a federal program created by President Franklin Roosevelt in 1935 to give jobs to the unemployed during the Great Depression. SNOMNH was one of many museums nationwide that benefited from the labor (fig. 1.4). The excavation site was on the Arnold farm in the southeastern corner of Atoka County, about 7 miles (11.25 km) south-southeast of Farris, Oklahoma. While some WPA workers excavated bones (fig. 1.5), others prospected for other sites in the area by walking the landscape and looking for fossil bone eroding out of the bedrock. Because random digging rarely produces a fossil, prospecting is a more time- and cost-effective means of discovering new sites and specimens. Unfortunately, erosion, which uncovers a fossil, also destroys it. As flowing water erodes the soft rock, the harder, more resistant bone is exposed. However, fossilized bone is usually brittle and full of cracks, so the exposed part tends to break into pieces. These pieces may then be transported downslope by gravity and runoff. By tracing the trail of pieces upslope, a prospector can pinpoint the source of the fossil bone eroding out of the rock. Where the landscape is covered by vegetation, such as in southeastern Oklahoma, erosion is limited mostly to the banks of rivers and streams. It was here that further exploration produced a second skeleton, 0.75 miles (1.2 km) east of the first discovery, on the adjacent Cochran farm. One specimen was found in relatively soft, yellow sandstone of the Antlers Formation, although many of the bones were encased in a rind of much harder stone. Preservation in a rind of harder stone is common for dinosaur bones throughout the world and is due to conditions at the original burial setting (more about this in chapter 3). The other skeleton was slightly more complete than the first and, most importantly, included parts of the skull (fig. 1.6). The tall spines on the vertebrae and the shapes of the pelvic bones, although a little smaller, matched those of the other skeleton, which is how Stovall and Langston were able to show that the two skeletons were of the same type of dinosaur. This specimen was found in dark, reddish-brown, sandy shale, also in the Antlers Formation. The bones lay only a few inches below the surface, and some were heavily damaged by plant roots. Plants often grow into and around fossil bones because of the high concentration of minerals they find there. In Utah I once collected a fossil bone that was little more than a long, brown stain and root mesh. It was only the presence of more recognizable bones inches away, as well as the shape of the root mesh and stain, that enabled me to identify the bone as a dinosaur rib. Fortunately, the Oklahoma bones were pristine, though damaged by erosion. The success of these early discoveries led the Oklahoma crew to continue prospecting in the area. The result was the discovery of nine other sites containing dinosaur bones, including the skull of a plant-eating dinosaur called Tenontosaurus (ten-on-toe-soar-us; see chapter 8). World War II and the subsequent dissolution of the WPA programs brought an abrupt end to further work in Atoka County just as new discoveries were being made. After the war, Langston returned to the University of Oklahoma and described the two specimens that would become Acrocanthosaurus in his master's thesis (1947). This thesis was published almost verbatim in 1950, with Langston's thesis advisor, Willis Stovall, listed as lead author. This publication officially recognized Acrocanthosaurus as a new type of dinosaur. It would be more than thirty years before another specimen of Acrocanthosaurus was discovered. This third specimen was found about 70 miles (112.6 km) east-southeast of the Cochran site, or about 15 miles (24 km) northeast of Idabel in McCurtain County, Oklahoma (fig. 1.7). Kristi Silvey, a resident of McCurtain County, collected a few bones around 1982. In April 1987 she took Dr. Rich Cifelli, the paleontologist at what was then called the Oklahoma Museum of Natural History, to the site. It was clear to Cifelli that someone had been actively digging up the bones, but he did not know at the time that the site was being excavated by amateur paleontologists Cephis Hall and Sid Love. They'd begun their work around 1983 (fig. 1.8). It was here that Fran was unearthed. The skeleton was found partially in soft, coarse-grained, salt-and-pepper-colored sandstone in the middle of the Antlers Formation area (Miser 1954). The sandstone also contained a lot of plant debris and balls of mudstone. Many of the bones were encased in a hard rind of rock, just like the first Acro specimen excavated by the WPA. Most significantly, the skull was recovered. Most of the bones of this specimen were cleaned of their encasing rock at the Black Hills Institute in Hill City, South Dakota (fig. 1.9), and it was there that I studied the skeleton with my colleague, Phil Currie (Currie and Carpenter 2000). Fran was ultimately purchased by the North Carolina Museum of Natural History in Raleigh, where a plastic cast (an exact replica of the skeleton) is on display. Another cast of this same specimen, is displayed at the Museum of the Red River in Idabel. In 1990 another Acrocanthosaurus specimen was collected, this time on the Hobson Ranch in Parker County, west of Fort Worth, Texas. The bones were excavated by Southern Methodist University, Dallas. The specimen was found in medium- to fine-grained sandstone with the sand grains cemented together into concrete by the mineral calcite. This sandstone occurs in the Twin Mountains Formation, which was deposited at the same time (around 110 million years ago) as the Antlers Formation in Oklahoma. The partial skeleton, including bits of the skull, was widely scattered, although some vertebrae lay in sequence as they did in life (fig. 1.10). Cleaning the skeleton was difficult, but when it had been completed, Southern Methodist University graduate student Jerry Harris described it in his master's thesis (1997). Jerry published his thesis a year later, two years before Currie and I published our description of Fran. The specimen Jerry studied was important because it contained parts of the skeleton that had not been available to Stovall and Langston, including parts of the skull and lower jaw, neck vertebrae, and shoulder blade. The fifth and most recently collected Acrocanthosaurus specimen consists of a handful of bones from the Cloverly Formation of northeastern Big Horn County, Wyoming. The specimen was found in 2008 by paleontology student Mike D'Emic from the University of Michigan. This partial skeleton is from an Acrocanthosaurus that was about three-quarters grown (D'Emic, Melstrom, and Eddy 2012) and thus represents the smallest of the five known specimens. The find occurred in dark, brick-red, silt-rich mud rock (mudstone) with green and yellow splotches caused by subtle chemical differences in the rock. The fossil bones were badly crushed from compaction of the original layers of mud during the process of becoming rock (fig. 1.11). This Acrocanthosaurus, about 1,000 miles (1,600 km) northwest of the Oklahoma specimens, is the northernmost Acro find. Fossil footprints that may have been made by Acrocanthosaurus suggest that the species might have lived as far north as southern Canada (Currie, personal communication, 2015). Big teeth and other isolated bones in Lower Cretaceous rocks (145–100.5 million years old; fig. 1.9) from a scattering of localities, including Utah and Maryland (Lipka 1998), have been tentatively identified as Acrocanthosaurus, but a reexamination of the specimens by one of Wilson's former students, Mike D'Emic, casts doubt on these identifications (D'Emic, Melstrom, and Eddy 2012). Five partial skeletons may not seem like much, but for dinosaur paleontologists they are a treasure trove. Most of the dinosaurs we study are known from far fewer bones. As I will show you in the rest of the chapters, these five specimens have revealed a lot about Acrocanthosaurus, and I hope that along the way you'll be able to imagine Acro as a living, breathing creature, not just a collection of old bones. Who Is Acrocanthosaurus? When Langston studied the first Acrocanthosaurus specimens in the late 1940s, he knew the bones were those of a carnivorous dinosaur. All carnivorous dinosaurs belong to a group called theropods, though a few members of this group have at various times appeared to have given up chasing and killing prey to become vegans, eating various plants. Two of the most famous carnivorous theropods are Tyrannosaurus and Allosaurus, which are well known scientifically from numerous fairly complete skeletons. We know that they were carnivores based on their long, narrow, blade-like teeth, which are well adapted for slicing flesh. Neither of the two specimens of Acro available to Langston at the time had teeth, but he was still able to determine that the skeletons were those of a theropod because the skull bones more closely matched those of Tyrannosaurus and Allosaurus than those of any other dinosaur. This was especially true for the bones surrounding the brains in these three dinosaurs. Other similar features were the Allosaurus-like ball-and-socket neck vertebrae (in Tyrannosaurus and many other dinosaurs these vertebrae are flat-faced); the spines on the vertebrae, which are similar to those of Allosaurus in shape and structure, though much taller (fig. 2.1); and the pelvic bones, which are similar to those of Tyrannosaurus and especially Allosaurus. Despite these similarities in Acro to Tyrannosaurus and Allosaurus, there were also major differences, which convinced Langston (1947) and Stovall and Langston (1950) that they were dealing with a new dinosaur. When coining new names, scientists are guided by the International Code of Zoological Nomenclature (ICZN), which ensures that a new scientific name is valid. The code requires that the name have two parts: the first is the genus name and the second is the species name. Stovall and Langston named this new dinosaur Acrocanthosaurus atokensis. In creating the genus name, they combined the Greek words akra, meaning "high," akantha, meaning "spine," and sauros, meaning "lizard," to construct the name "high-spined lizard" in reference to the particularly tall spines on the vertebrae (fig. 2.2). The species name refers to Atoka County, where the specimens were found (the root ensis means "from"). Another ICZN requirement is that one specimen, called a holotype, be selected as the name-bearer. This one specimen becomes the standard for the species; all similar specimens must be compared with it to determine whether they belong to the same species or a different one. The holotype must have at least one unique characteristic that defines or diagnoses the new species and makes it different from all others. If more unique characteristics can be identified, they will strengthen the case for the specimen belonging to a distinct species. Stovall and Langston had two specimens from which to work. They selected the second specimen, which had skull parts, as the holotype and made the other a paratype. A paratype is a supporting specimen that complements the holotype in rounding out the description of the new species. Usually it contains parts missing in the holotype. In the case of these first two Acrocanthosaurus specimens, the foot bones of the paratype were used in the scientific description because they were missing in the holotype. Stovall and Langston (1950) gave a rather long list of characteristics in the diagnosis of Acrocanthosaurus, not all of which are unique to the species. Forexample, they mentioned that it was a "carnivorous saurischian of gigantic size and heavy proportions" (p. 700). Even in 1950, these terms were true of other dinosaurs, such as Tyrannosaurus (named by Osborn 1905). Also, some carnivorous dinosaurs that have been discovered since the original description have characteristics that were originally thought to be unique to Acro. One of these characteristics was the prominent ball-and-socket connection of the first few neck vertebrae. Such characteristics no longer form part of the diagnosis for Acrocanthosaurus, but they are still helpful in determining how closely related Acrocanthosaurus is to other theropods. I now give you a scientific definition of Acrocanthosaurus atokensis. Instead of "Large theropod with elongate neural spines that are more than 2.5 times corresponding presacral, sacral and proximal caudal lengths of the centra" (p. 211), as Phil Currie and I gave it in our scientific description (Currie and Carpenter 2000), I'll give it in plain language. However, I'll have to use the scientific names of bones because most bones have no common name. I'll discuss and illustrate the bones of Acrocanthosaurus in chapter 4; for now, just know that its skull and lower jaw are made up of multiple bones. One of these, on the back of the skull, is a wedge-shaped bone called the supraoccipital; it sits above the opening through which Acro's spinal cord passed to the brain. The supraoccipital has two V-shaped ridges that end in little flat surfaces, and the neck muscles that tilt the head back were attached to these. Also along the back of the skull is a vertical bone called the quadrate, to which the lower jaw attaches in a hinge-like joint. On the inner side of the quadrate is a pocket that was probably connected in some manner to the air system that was spread throughout the body. In the lower jaw, one of the bones that makes up the rear half, called the surangular, has a ridge or shelf to which jaw-closing muscles attached. (Chapter 6discusses more about this and the air system.) This ridge or shelf has a localized thickening, or knob ("bump"). Below the shelf is a large opening through which a nerve passed. All of the vertebrae of the neck, back, and pelvis have depressions and smaller openings into which some of the air system passed. One of the most distinctive features, and what gave Acrocanthosaurus its name, are the tall spines that project upward (I discuss these at greater length in chapter 5). The height of these spines is about two and a half times the length of their individual vertebrae. Those on the neck also have an odd, triangular spur that inserts into a pocket on the backside of the next vertebrae in front. Finally, the vertebrae from the middle of the tail have a secondary flange that projects out to the side. Read an Excerpt
Acrocanthosaurus
Inside and Out
By Kenneth Carpenter UNIVERSITY OF OKLAHOMA PRESS
Copyright © 2016 University of Oklahoma Press, Norman, Publishing Division of the University
All rights reserved.
ISBN: 978-0-8061-5637-8
CHAPTER 1
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Excerpted from Acrocanthosaurus by Kenneth Carpenter. Copyright © 2016 University of Oklahoma Press, Norman, Publishing Division of the University. Excerpted by permission of UNIVERSITY OF OKLAHOMA PRESS.
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Preface ix 1 Discovering Acro 3 2 Who Is Acrocanthosaurus? 15 3 How to Fossilize an Acrocanthosaurus 23 4 The Skeleton of Acrocanthosaurus 36 5 Why Tall Back Spines? 49 6 Soft, Squishy Stuff: The Internal Anatomy 59 7 The Next Generation: Reproduction and Growing Up 81 8 Acro the Hunter 92 9 The World of Aero 107 10 A Final Pew Words 120 Annotated References 123 Index 133Table of Contents