This article was downloaded by: [Dr Daniel Marty] On: 08 October 2012, At: 23:57 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Historical Biology: An International Journal of Paleobiology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ghbi20 The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour Timothy E. Isles a a Palaeobiology Research Group, School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, Hampshire, PO1 3QL, UK Version of record first published: 08 Jan 2010. To cite this article: Timothy E. Isles (2009): The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour, Historical Biology: An International Journal of Paleobiology, 21:3-4, 139-214 To link to this article: http://dx.doi.org/10.1080/08912960903450505 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. Historical Biology Vol. 21, Nos. 3–4, September–December 2009, 139–214 The socio-sexual behaviour of extant archosaurs: implications for understanding dinosaur behaviour Timothy E. Isles* Palaeobiology Research Group, School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth, Hampshire PO1 3QL, UK Dinosaur behaviour has little legacy in the fossil record and the rarity of fossil soft tissues makes it difficult to evaluate. Indirect evidence from bonebeds, trackways, nesting traces and in-group comparisons with extant Archosauria suggests that the only substantive arguments to be made for dinosaur sociality concern cranial ornamentation and herding behaviour. There is currently no reliable method to determine gender from skeletal remains. Dinosaur reproductive anatomy was a unique combination of crocodilian and avian characters and extant models indicate that dinosaurs copulated using a reptilian ‘leg over back’ posture. Reliable evidence for post-hatching care in dinosaurs is lacking and extant archosaurs yield little insight. A hypothesis is proposed that for the majority of dinosaurs there was no post-hatching care provided which would have allowed adults energy acquisition that would otherwise have been required for defence and provisioning to be redirected towards growth and increased fecundity, both traits for which there is fossil evidence. Arguments suggesting that the more advanced aspects of extant avian care boasting an explicit coelurosaurian theropod origin are rejected as these behaviours appear unique to the Neornithes. Three ancestral care hypotheses are tested and none conform in a satisfactory manner with body fossil and ichnological evidence. Keywords: dinosauria; mating; parental care; aves; crocodilians; reproduction 1. Introduction nestbound altricial neonates have been suggested for Recently, it has become fashionable to portray a range of hadrosaurids (e.g. Horner and Makela 1979; Horner 2000). advanced avian post-hatching and nesting behaviours as The phylogenetic relationships between extant avians having been explicitly present in theropod dinosaurs in and coelurosaurian theropods, in particular, have been both popular culture and science. Novels (e.g. Bakker well documented (e.g. Gauthier 1986; Holtz 1996; 1996) and television documentaries (e.g. the Discovery Dingus and Rowe 1998; Prum 2002), resulting in Channel’s 2003 four-part series Dinosaur Planet) investigations of the origin of Neoaves parental-care systems. Extant archosaurs share ancestral characters such represent many dinosaur taxa as maintaining lifelong as calcareous eggs, nest construction and unique oviduct pair bonds in multigenerational family units that live, learn morphology (e.g. Mateus 1998; Carpenter 1999; Sato et al. and hunt in stable long-term groups. These views tend to 2005). Many investigators have applied these observations be inspired by hypotheses, albeit presented in a less to extinct archosaurs, especially dinosaurs. It is argued that visually dramatic manner, commonly found in the coelurosaurians, and Troodon and the oviraptorids in scientific literature. Bakker (1997) argued that Allosaurus particular, employed direct contact incubation, used Downloaded by [Dr Daniel Marty] at 23:57 08 October 2012 parents provisioned their offspring with carcasses dragged delayed incubation of their clutches and exhibited over long distances to ‘dens’ where the young could feed male-only care of nests. Furthermore, according to these safely. Dromaeosaurids are often exclusively described as views the most derived aspects of neognath parental the dinosaurian equivalent of wolves with apparently behaviour and nest attendance were already present in and sophisticated social structures deemed necessary for the thus originated in theropods (e.g. see Larson 1998; Prum effective pack hunting of larger prey (Bakker 1986; 2002; Varricchio and Jackson 2004b; Varricchio et al. Maxwell and Ostrom 1995). A similar scenario was 2008a; Zelenitsky and Therrien 2008). In this emerging formally proposed for the tyrannosaurids Albertosaurus consensus, theropods do not simply have an evolutionary (Currie 1998) and Daspletosaurus (Currie et al. 2005a) in relationship with avians, but are considered to be more or which both employed pack hunting and a sophisticated less interchangeable from both a social and behavioural division of labour, with smaller and apparently faster perspective. juvenile members targeting faster moving prey. Similar This modern tendency to propose behavioural patterns ideas concerning long-term parental feeding and and social organisations that go far beyond what can *Email: [email protected] ISSN 0891-2963 print/ISSN 1029-2381 online q 2009 Taylor & Francis DOI: 10.1080/08912960903450505 http://www.informaworld.com 140 T.E. Isles be reasonably extrapolated from the fossil record, is in stark Henderson 2006; Alexander 2008), it is obvious that the contrast to classic paleobiological investigations in which a majority of dinosaur behaviour has no legacy in the fossil more conservative approach was advanced (e.g. see Boucot record. Furthermore, the rarity of soft tissue preservation 1990). But is there any evidence supporting the presence of makes it extremely difficult to evaluate putative socio- sophisticated social behaviours observed in derived sexual behaviours in extinct dinosaurs. However, indirect mammals such as Canidae and Primates? Does the fossil evidence and clues can be gleaned from such varied sources record support notions of dinosaurs enjoying advanced as anatomy, bonebeds, trackways, paleopathology, nesting social organisation that included extended family struc- traces and in-group comparisons with extant Archosauria tures? Is there any reason to consider that theropods were (e.g. Molnar 1977; Weishampel 1995a; Witmer 1995; the behavioural and social equivalent of Neoaves? What Sampson 1997a). This evidence and any competing reliable and unequivocal evidence is there for dinosaur hypotheses of dinosaur behaviour are reviewed. socio-sexual (courtship, mating and parenting) behaviour? Investigating these types of activity in extinct organisms is hampered by a general lack of fossil preservation of either 2.1 Evidence for gender determination in dinosaurs identifiable behaviour or soft tissues. Therefore, studies The question of how to positively identify gender from often tend to focus on trackways, mass accumulations and other incomplete or damaged skeletons is a pressing concern trace evidence (Molnar 1977; Weishampel 1995a). Further in any discussion of reproductive behaviour in extinct insights can be attained by means of in-group comparisons vertebrates. The positive identity of gender is a key using extant phylogenetic bracketing, a method of component in the understanding of possible social inference in which an extinct taxon is compared to its organisations, demographics and population dynamics and nearest extant relatives based upon their position on a as such remains a ‘holy grail’ of dinosaur paleontology (e.g. phylogenetic tree (see Witmer 1995). Erickson et al. 2005). Distinguishing the gender of extinct This paper critically evaluates the current state of taxa based wholly on fossil is by no means straightforward, evidence by first reviewing the essential aspects of socio- as the entire process is plagued by confounding variables sexual characteristics of extinct archosaurs and comparing ranging from variations in preservation and taphonomy, them to those described for their extant relatives. [see Chapman et al. (1997) for a detailed discussion],