Ann. Zool. Fennici 51: 80–94 ISSN 0003-455X (print), ISSN 1797-2450 (online) Helsinki 7 April 2014 © Finnish Zoological and Botanical Publishing Board 2014 Old world ruminant morphophysiology, life history, and fossil record: exploring key innovations of a diversification sequence Marcus Clauss1,* & Gertrud E. Rössner2,3,4 1) Clinic of Zoo Animals, Exotic Pets and Wildlife, Vetsuisse Faculty, University of Zurich, Winterthurerstr. 260, CH-8057 Zurich, Switzerland (*corresponding author’s e-mail: mclauss@ vetclinics.uzh.ch) 2) SNSB-Bayerische Staatssammlung für Paläontologie und Geologie, Richard-Wagner-Str. 10, D-80339 Munich, Germany 3) Department für Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität München, Richard-Wagner-Strasse 10, D-80333 München, Germany 4) GeoBio-Center LMU, Richard-Wagner-Strasse 10, D-80333 München, Germany Received 3 Apr. 2013, final version received 4 Sep. 2013, accepted 4 Sep. 2013 Clauss, M. & Rössner, G. E. 2014: Old world ruminant morphophysiology, life history, and fossil record: exploring key innovations of a diversification sequence. — Ann. Zool. Fennici 51: 80–94. The omasum of pecoran ruminants (which is absent in tragulids) and shorter gesta- tion periods in non-giraffid crown pecorans (as opposed to giraffids) could represent cases of key innovations that caused disparity in species diversity in extant ruminants. Literature suggests that the different ruminant groups inhabited similar niche spectra at different times, supporting the ‘increased fitness’ interpretation where a key innova- tion does not mainly open new niches, but allows more efficient use of existing ones. In this respect, we explored data on fossil species diversity of Afro-Eurasian ruminants from the Neogene and Quaternary. Tragulid and giraffid diversity first increased during the Early/Middle Miocene with subsequent declines, whereas bovid and cervid diver- sity increased distinctively. Our resulting narrative, combining digestive physiology, life history and the fossil record, thus provides an explanation for the sequence of diversity patterns in Old-World ruminants. Introduction from the structure of the first terrestrial tetra- pod’s limb to the unguligrade extremity of larger Evolutionary progress and key herbivores (Shubin et al. 2006, O’Leary et al. innovations 2013), from the addition of the hypocone to the molar morphology of therian mammals (Hunter That progress occurred during evolutionary his- & Jernvall 1995) — the concept of progress in tory at a macroevolutionary level is usually not functional morphology appears intuitive. What an issue of debate (e.g. Rosenzweig & McCord is often debated is progress at a microevolution- 1991). From the anatomy of the first multicel- ary level: why does a certain taxonomic group lular organisms to complex plants and animals, appear to be more successful in terms of spe- ANN. ZOOL. FENNICI Vol. 51 • Tragulid and pecoran diversity 81 ciation than another, like certain spider clades the plausibility of the argument is the major indi- (Bond & Opell 1998) or a certain clade among cator of its quality, but the underlying hypothesis phyllostomid bats (Dumont et al. 2012)? Can cannot be tested statistically. we explain this in terms of evolutionary progress Hence, phenotypic traits without a potential and a hypothetical, more efficient functionality, to fossilize such as soft tissue anatomy, physiol- maybe even linked to a certain (and potentially ogy and life history characteristics provide ideal new) niche, in the sense of ‘directional evolu- bases to explore key innovations, because they tion’ (Liem 1990), or do we choose to consider are usually not used to reconstruct phylogenetic morphophysiological variety at low taxonomic relationships. Under the assumption that those levels as random variety solving the same prob- traits observed in extant species are representa- lems in different ways (of similar efficiency) in tive for the entire clade including fossil spe- the sense of neutral evolution (ibid.)? cies, the adaptive value of such features can be The concept of key innovation plays an assessed in comparative and even experimental important role in the more general concept of studies on extant species. A prominent example evolutionary progress; key innovations may where soft tissue anatomy and physiology was explain competitive displacement (Nitecki 1990, used to explain the evolutionary diversification Rosenzweig & McCord 1991, Heard & Hauser is the digestive physiology of ungulate her- 1995, Hunter 1998). Morphophysiological, bivores. Using the concept of a difference in behavioural and life history peculiarities of a the digestive function of hindgut and foregut certain clade do not only help to define that clade fermenters (Janis 1976, Duncan et al. 1990), taxonomically, but represent potential candidates Janis et al. (1994) have explained the apparent for innovations that helped shape that clade’s displacement of equids by ruminants, conclud- evolutionary success. However, apart from rare ing a primarily digestion-driven evolutionary exceptions when fossils also reveal details, e.g. advantage for ruminants. Similarly, the differ- of copulation (Joyce et al. 2012), pregnancy, ence in species diversity between Tylopoda and precociality and birth (Gingerich et al. 2009), or Ruminantia was speculated to result from the sociality (Bibi et al. 2012), mostly only a certain differences in functionality of the sorting mecha- subset of morphological attributes fossilizes. The nism that prevents Tylopoda from achieving the emphasis in the concept of key innovations has higher food intakes observed in many Ruminan- therefore traditionally been on hard tissue mor- tia (Clauss et al. 2010a). This latter hypothesis phological aspects (Burggren & Bemis 1990). correlates well with the apparent replacement One perceived problem with the concept of of camelids by ruminants in Janis et al. (1994). key innovation is the tautology amounting from Because of their prominence in specimen and the likewise usage of morphological characters species number in the fossil record, and detailed to identify a (diverse) clade and as a reason knowledge about their comparative anatomy and for its success (diversity). The recent develop- physiology (Clauss et al. 2008), ungulate her- ment of deriving phylogenetic estimates from bivores appear as promising test cases for the genetic information alleviates this problem and exploration of evolutionary success and related facilitates statistical approaches to test for the key innovations. effect of certain morphological characteristics on diversification rates in extant species clusters (e.g. Dumont et al. 2012). For most fossil taxa, Tragulidae, Pecora, and ruminant however, the lack of genetic material means that stomach anatomy the problem is not readily alleviated. Another solution is to use a character considered to be Compelling evidence of extended comparative apomorphic or convergent in several different analyses including phenotypic as well as molec- taxa, which can be compared to closely related ular data (Janis & Scott 1987, Gentry & Hooker taxa in which it is absent (e.g. Hunter & Jernvall 1988, Hernández-Fernández & Vrba 2005, Has- 1995). Thus, studies of key innovations in the sanin et al. 2012) supports the view that Traguli- fossil record often represent narratives, in which dae are the sister group of Pecora, and the most 82 Clauss & Rössner • ANN. ZOOL. FENNICI Vol. 51 equator to subpolar regions (Wilson & Mitter- meier 2011), whereas all of the tragulids are of small size, and inhabit exclusively dense forest undergrowth or thickets within these forests in Africa and southeast Asia (Meijaard 2011). Although very few peer-reviewed reports on their natural diet exist (but see Dubost 1984), extant tragulids are commonly considered to be selective feeders with a major component of fruit and additional browse (Meijaard 2011). This is supported by a correlation of tragulid density with the abundance of fruit (Heydon & Bolloh 1997). In contrast, the fossil record of tragulids points to a diverse evolutionary history with a substantial diversification at the beginning of the Miocene or even end of the Oligocene (Geraads 2010, Sánchez et al. 2010). It documents a wide geographical distribution covering vast parts of Afro-Eurasia, large ranges of body sizes (more than twice the size of the largest extant species), skeletodental morphologies, preferred diets, and habitats as well as a common sympatric occur- rence of up to four species in the Miocene Fig. 1. Schematic representation of the stomach of (23.03 to 5.3 mya) (Mottl 1961, Fahlbusch 1985, tragulids (top) and Pecora (bottom). Rum = rumen, Ret Pickford 2001, Rössner 2004, 2007, Barry et = reticulum, Om = omasum (lacking in tragulids), Abom al. 2005, Eronen & Rössner 2007, Kaiser & = abomasum. Drawing by Jeanne Peter, after Schmidt Rössner 2007, Ungar et al. 2012). In particular, (1911) and Hofmann (1969). dietary reconstructions for fossil tragulids indi- cate a spectrum that ranges from fruit-dominated basal living ruminant group. The branch-off of to pure browse diets and mixed diets with a dis- the tragulid clade from the ruminant stem lin- tinctive monocot component (Kaiser & Rössner eage is biostratigraphically/biochronologically 2007, Ungar et al. 2012); tragulids have there- dated for the Late Eocene in southeast Asia fore recently been considered ‘ecological precur- (Métais et al. 2001). Molecular clock analyses sors’ of bovid ruminants (Ungar et al. 2012). produced