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Species Concept in North American Stegosaurs

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Species Concept in North American Stegosaurs Swiss J Geosci (2010) 103:155–162 DOI 10.1007/s00015-010-0020-6 Species concept in North American stegosaurs Kenneth Carpenter Received: 14 July 2009 / Accepted: 25 May 2010 / Published online: 21 August 2010 Ó Swiss Geological Society 2010 Abstract The plated thyreophoran or stegosaurian dino- Institutional abbreviations saur Stegosaurus armatus was named in 1877 by Marsh for DMNH Denver Museum of Natural History (now the fragmentary remains from the Morrison Formation (Upper Denver Museum of Nature & Science), Denver, Jurassic) of Colorado, USA. Subsequent discoveries from CO, USA the same formation in Wyoming and Colorado (USA) have HMNH Hayashibara Museum of Natural History, been assigned to separate stegosaurian genera and species, Okayama, Japan but most of these are no longer considered valid. More SMA Sauriermuseum Aathal, Switzerland recently, a partial stegosaurian skeleton from Wyoming was USNM United States National Museum (now the National named Hesperosaurus mjosi. However, the validity of this Museum of Natural History, Smithsonian genus has been questioned recently, raising the question: Institution), Washington, DC, USA how much osteological difference among stegosaur taxa is YPM Yale Peabody Museum of Natural History, New needed to separate genera from species? The question Haven, CT, USA is examined vis-a`-vis species and genus recognition in other dinosaurs, including iguanodonts, lambeosaurine iguanodontids, chasmosaurine ceratopsians, tyrannosaurid theropods, and diplodocid sauropods. The basis for taxo- Introduction nomic distinction is largely philosophical: if the species are morphologically distinct enough, they should be treated as The stegosaurid Hesperosaurus mjosi CARPENTER,MILES & separate genera. Based on these criteria, Hesperosaurus CLOWARD (2001), was named for a nearly complete skeleton mjosi is a distinct taxon. from very low in the Morrison Formation in north-central Wyoming, USA. Carpenter and Galton (2001) reviewed the Keywords Stegosaurus Á Hesperosaurus Á species of Stegosaurus named by Marsh and concluded that Morrison Formation Á Late Jurassic Á S. armatus could not be diagnosed at the species level with Palaeontological species Á Taxonomy the available material, although two species (S. stenops, S. ungulates) were valid (S. longispinus was not considered). Maidment et al. (2008) challenged these conclusions, con- sidering Hesperosaurus mjosi as a species of Stegosaurus (as S. mjosi), along with S. armatus from North America, and S. homheni (for Wuerhosaurus homheni) from China. They noted that Stegosaurus mjosi differed from S. armatus ‘‘as Editorial handling: Jean-Paul Billon-Bruyat & Daniel Marty. the former possesses the following primitive characters: atlas neural arches not fused to intercentrum in ontogeneti- & K. Carpenter ( ) cally mature individuals, postzygapophyses not elevated Prehistoric Museum, Utah State University-College of Eastern Utah, 155 Main Street, Price, UT 84501, USA significantly on posterior cervical vertebrae, neural arches e-mail: [email protected] of dorsal vertebrae not elongated above the neural canal, 156 K. Carpenter ossified epaxial tendons present, ribs distally expanded, and encasing larger ones in plaster of Paris jackets. caudal neural spines not bifurcated, enlargement of the distal Regardless, these pioneers were faced with fossils of end of the pubis. It also has the following autapomorphies: organisms unlike anything living today. Clearly, the 11 dorsal vertebrae, fourth sacral vertebra not fused to organisms were morphologically different and hence, to sacrum, dorsal dermal plates longer anteroposteriorly than their thinking, warranted a distinct name; this same phi- tall dorsoventrally.’’ (Maidment et al. 2008, p. 379). losophy holds today in the naming of specimens as is The number of North American stegosaur genera and evidenced by the ‘‘diagnosis.’’ species raises the age old question of the species concept in It was inevitable that the principles of taxonomy as palaeontology. The subject for dinosaurs was addressed in applied to vertebrate fossils in the 1800s and early 1900s Carpenter and Currie (1990), and more recently by Paul would be questioned as specimens accumulated. Much of (2008) for iguanodontids. The argument for species rec- this questioning came during the 1930s to 1940s, and again ognition in palaeontology is mostly philosophical because in the 1960s, by such workers as Simpson, Dobzhansky and species recognition is limited to only a portion of the entire Mayr, and it has been revived yet again with the expanded biological system, namely the hard parts, which among use of cladistics (e.g. Wheeler and Meier 2000). The vertebrates is the skeleton (Raup and Stanley 1978; problem, of course, is due to the limitations of only having Donoghue 1985; see also Ereshefsky 2001). The applica- hard parts, usually fragmented and incomplete, and which tion of the species concept in palaeontology, including are often distorted due to crushing during sediment com- dinosaurs, has long struggled with what criteria to use in paction. It is the unfortunate nature of vertebrate fossils, taxonomy. particularly dinosaurs, that does not allow the use of Early palaeontologists, especially Marsh and Cope, are modern techniques to identify species, including molecu- frequently derided today for naming taxa based on scrappy, lar, colour or scale patterns of skin, behaviour, reproductive fragmentary material. For example, Marsh (1889) named potential, vocalizations, etc. Therefore, all methodologies Triceratops galeus for a nasal horn core collected from the for the identification of dinosaur species must rely upon Late Cretaceous near Denver, CO, USA. Leidy (1856) skeletal differences. But how much difference must two named four species of dinosaurs from among a hand-full of specimens show or have to warrant separation at the spe- teeth surface collected by Hayden in the Late Cretaceous cies level, let alone at the generic level? Unfortunately, badlands along the Judith River of Montana (Leidy 1859). there is no consensus to this conundrum (i.e. the species Then, of course, there is Iguanodon named by Mantell problem in palaeontology). As Raup and Stanley (1978) (1825) for teeth discovered by his wife. Some of the noted, sibling species differ very slightly from one another, fragmentary materials named by these pioneers are bor- whereas other species are so very different that their sep- derline as to whether they are sufficiently unique for the aration is obvious. But what level of ‘‘obvious’’ is name to be valid. For example, Cope (1876) named acceptable? Barrett et al. (2005) diagnosed two small Monoclonius crassus for material collected along the ornithischians from China, each with a single autapomorphy: Judith River. Dodson (1996) accepted the taxon as valid, Hexinlusaurus multidens (‘‘marked concavity that extends whereas Sampson et al. (2008) did not. over the lateral surface of the postorbital’’) and Xiaosaurus It is easy today to ridicule these pioneers for naming dashanpensis (‘‘proximally straight humerus’’). Are these taxa from scrappy material. However, we need to view single autapomorphies sufficient? If so, then the situation is their actions in the light of the times. The 1800s is when the as Raup and Stanley (1978, p. 108) stated: ‘‘A species is a natural world was just beginning to be studied scientifi- species if a competent specialist says it is.’’ This use of cally. The large majority of the animals and plants we only autapomorphies to diagnose taxa may work some of know today were given their scientific names during this the time, but other times it may not. For example, Farke time. The principles of zoological and plant nomenclature, (2007: 253) noted that ‘‘few characters are truly unique to roughed out previously by Linnaeus (1735), were being the species T[orosaurus] latus, but the combination of utilized in scientific descriptions. It is important to consider characters is unique. Only the combination of features that many of these early palaeontological pioneers in diagnoses the taxon’’ (emphasis by Farke 2007). dinosaurs also described modern organisms. Cope is especially noted for his work on reptiles, amphibians and fishes (Osborn 1931). Thus, these early workers had an Lessons learned from species concept in dinosaur understanding of modern organisms and the methodology palaeontology for describing them. It was with this concept of organisms that they drew upon when faced with fragmentary speci- There is little question that Tyrannosaurus rex is a distinct mens. These specimens became less fragmented once taxon from Allosaurus fragilis because each can be iden- techniques were developed for hardening crumbly fossils tified based on very distinct morphological (typological) Stegosaur species concept 157 criteria. However, such a distinction is less apparent among others. Lambeosaurine hadrosaurs are also conservative in several other different groups of dinosaurs. As Sampson their postcrania, with the biggest differences also occurring et al. (2008) have noted, ceratopsid taxa are notoriously in the skull (Fig. 2; Dodson 1975;Paul2007, 2008); even conservative in their postcrania, with almost all of the the well known tyrannosaurids are remarkably similar generic and specific criteria being based on differences in (Fig. 3). The reverse may also be true where the skull is the skull architecture (Fig. 1); this point has been also conservative, with the biggest differences in the postcrania. noted by Dodson (1996) and
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