Bone microstructure and relative age of the holotype specimen of the diplodocoid sauropod dinosaur Suuwassea emilieae BRANDON P. HEDRICK, ALLISON R. TUMARKIN-DERATZIAN, and PETER DODSON Hedrick, B.P., Tumarkin-Deratzian, A.R., and Dodson, P. 2014. Bone microstructure and relative age of the holotype specimen of the diplodocoid sauropod dinosaur Suuwassea emilieae. Acta Palaeontologica Polonica 59 (2): 295–304. We present the first study of bone microstructure in Suuwassea emilieae, a diplodocoid sauropod from the Morrison Formation. Although the holotype of Suuwassea was recently identified as a subadult, bone histology demonstrates that this individual had reached sexual maturity at approximately 75–80% of maximum adult size. The smaller size of the holotype of Suuwassea relative to contemporary sauropods is due to ontogeny rather than a true reflection of adult size. A fully adult individual would have likely been similar in size to a fully adult specimen of Apatosaurus. Suuwassea has a number of plesiomorphic characters that might have been explained by its supposed early ontogenetic status, and would then have called the validity of the taxon into question. However, our demonstration that it was an adult confirms that these features represent retention of plesiomorphic character states or evolutionary reversals in a derived animal. Addi- tionally, the specimen shows extensive cortical drift and secondary osteon formation related to skeletal loading, which is often obscured by complete remodeling of Haversian systems in sauropod taxa. There are substantial differences in the microstructure across the bone, which could make histologic samples not based upon complete cross sections problem- atic. Suuwassea is one of four currently recognized dicraeosaurid taxa, and this study contributes potential taxonomic characters in sauropod bone microstructure. Key words: Dinosauria, Sauropoda, Suuwassea, histology, Jurassic, Morrison Formation, Montana, USA. Brandon P. Hedrick [[email protected]] and Allison R. Tumarkin-Deratzian [[email protected]], Department of Earth and Environmental Science, University of Pennsylvania, 251 Hayden Hall, 240 S 33rd Street, Philadelphia, PA 19104, USA; Peter Dodson [[email protected]] School of Veterinary Medicine, Department of Animal Biology, University of Pennsylvania, 3800 Spruce Street, Philadelphia, PA 19104, USA; and Department of Earth and Environmental Science, University of Pennsylvania, 251 Hayden Hall, 240 S 33rd Street, Philadelphia, PA 19104, USA. Received 19 April 2012, accepted 9 October 1012, available online 16 October 2012. Copyright © 2014 B.P. Hedrick et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. continent, extending the geographic range for dicraeosau- Introduction rids substantially (Whitlock and Harris 2010). Even though Suuwassea Suuwassea emilieae Harris and Dodson, 2004 is a diplodo- there is only one specimen, has been extensively coid sauropod from the Upper Jurassic Morrison Formation studied (Harris and Dodson 2004; Harris 2006a–c, 2007; of Montana. The holotype (ANS 21122), a partial skeleton Whitlock and Harris 2010). that includes numerous cranial, axial, and appendicular el- The holotype specimen of Suuwassea was originally char- ements, is currently the only known specimen (Harris and acterized by its small size and was estimated to be approxi- Dodson 2004). In phylogenetic analyses, Suuwassea orig- mately two-thirds the size of the fully adult contemporary inally created a trichotomous clade Flagellicaudata, as the Morrison sauropods, Apatosaurus and Diplodocus (Harris sister taxon to diplodocids and dicraeosaurids (Harris and 2006b, c). The complete humerus of Suuwassea (ANS 21122) Dodson 2004). However, a newly described dentary added measures 752 mm in length and the complete tibia (based on three dicraeosaurid characters to Suuwassea, so strengthen- the fibula) is 839 mm. Apatosaurus (CMNH 3018) has a hu- ing its placement as a basal dicraeosaurid (Salgado et al. merus measuring 1150 mm and a tibia of 1115 mm (Gilmore 2006; Whitlock and Harris 2010; Whitlock 2011). This made 1936). Diplodocus (CMNH 94) has a tibia 1006 mm long Suuwassea the first dicraeosaurid found on the Laurasian (Hatcher 1901). Therefore, based on the tibia, ANS 21122 Acta Palaeontol. Pol. 59 (2): 295–304, 2014 http://dx.doi.org/10.4202/app.2012.0049 296 ACTA PALAEONTOLOGICA POLONICA 59 (2), 2014 is actually 83% the size of Diplodocus and 75% the size of the National University of Comahue, Neuquén, Argentina; Apatosaurus rather than a mere 66%. However, it should NMNH, National Museum of Natural History, Washington, be noted that differing allometry of the tibia between these DC, USA; SMNS, Staatliches Museum für Naturkunde, taxa could cause discrepancies in determining absolute size Struttgart, Germany; T, Chongqing Natural History Muse- using simple ratios. Further, Suuwassea retains a number of um, Chongqing, China; UNPSJB,PV, Universidad Nacional plesiomorphic characters (amphiplatyan distal caudals, post- de la Patagonia “San Juan Bosco”, Paleovertebrados, Como- parietal foramen, elongate pedal phalanges with a small, un- doro Rivadavia, Argentina; YPM, Yale Peabody Museum, compressed ungual). These and other characters distinguish New Haven, Connecticut, USA; ZDM, Zigong Dinosaur it from contemporary sauropod taxa (Harris 2006a–c; 2007). Museum, Zigong, China; ZPAL, Institute of Paleobiology, Plesiomorphies are often found in juvenile specimens, which Polish Academy of Sciences, Warsaw, Poland. make it advisable to name new taxa based only on adult indi- Other abbreviations viduals (Rozhdestvensky 1965; Gould 1966; Sereno 2010). .—BOS, biological ontogenetic stage; Woodruff and Fowler (2012) suggested that Suuwassea HOS, histologic ontogenetic stages; LAG, line of arrested was a subadult specimen of another Morrison sauropod be- growth. cause its phylogenetic position was difficult to discern owing to plesiomorphic characters and that it appeared to be small in body size. The subadult status of ANS 21122 was inferred Material and methods on the basis of neurocentral fusion and some fusion in the skull and braincase (Harris 2006c). All of the cervical neural We took our histologic section from the mid-diaphysis of the arches are fused with the synostoses completely obliterated, right tibia. As there is presently only one specimen of Suu- the dorsal neural arches are fused with the synostoses visible, wassea, destructive sampling had to be kept to a minimum. and the caudal neural arches are unfused (Harris 2006b). The tibia was chosen because the bone was originally broken Although synostosis (fusion) is often used to infer on- at midshaft, allowing for minimal destruction and informa- togenetic status (Brochu 1996), the most precise meth- tion loss. It is 535 mm in preserved length and the section od for determining relative age is bone histology. Recent was taken 30 mm from the distal preserved end (Fig. 1). years have seen an explosion of studies of bone histology ANS 21122 also preserves a complete fibula and humerus, in Sauropodomorpha (see SOM: table S1, Supplementary but the data from the tibia were sufficient to make sectioning Online Material at http://app.pan.pl/SOM/app59-Hedrick_ these bones unnecessary. In other studies of Sauropodomor- etal_SOM.pdf; Chinsamy 1995; Curry 1999; Sander 1999, pha (Sander 2000; Klein and Sander 2008; Sander et al. 2000; Sander and Tückmantel 2003; Klein and Sander 2008; 2011), humeri and femora are preferentially used over tibiae. Woodward and Lehman 2009; Stein et al. 2010; Sander et However, other authors argue that the tibia is the ideal bone al. 2011). Not only does this large body of work reinforce to section (Horner and Padian 2004). the validity of characters distinguishing histologic ontoge- The terminology for ontogenetic stages (juvenile, sub- netic stages (Klein and Sander 2008) in sauropods, but it adult, adult) has not been standardized. Often sexual maturity also provides a basis for meaningful comparisons among the is used as the transition between subadult and adult. This is bone microstructures of groups within Sauropodomorpha. often defined in fossil taxa by the inflection point in a taxon To better understand the ontogenetic stage and phylogenetic growth curve (Erickson et al. 2001; Erickson 2005). How- position of Suuwassea, we sectioned the tibia to examine the ever, Lee and Werning (2008) suggest that the inflection internal microstructure of the bone. point may not demonstrate the onset on sexual maturity in dinosaurs based on data from extant animals. Other authors Institutional abbreviations.—AMNH, American Museum of define adulthood by the attainment of maximum body size Natural History, New York, USA; ANS, Academy of Natural (Scannella and Horner 2010). The distinction between ju- Sciences of Drexel University, Philadelphia, Pennsylvania, veniles and subadults is even less clear. As ANS 21122 is a USA; BYU, Brigham Young University, Provo, Utah, USA; single specimen and its ontogenetic stage cannot be related CMNH, Carnegie Museum of Natural History, Pittsburgh, to a growth curve, we use the biologic ontogenetic stage cor- Pennsylvania, USA; DMNH, Denver Museum of Natural related with its HOS as defined by Klein and Sander (2008), History, Denver, Colorado, USA; GCP, Grupo Cultural Pa- which defines growth stage