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Tuatara and a New Morphometric Dataset for Rhynchocephalia: Comments on Herrera-Flores Et Al

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Tuatara and a New Morphometric Dataset for Rhynchocephalia: Comments on Herrera-Flores Et Al [Palaeontology, 2018, pp. 1–14] DISCUSSION TUATARA AND A NEW MORPHOMETRIC DATASET FOR RHYNCHOCEPHALIA: COMMENTS ON HERRERA-FLORES ET AL. by FELIX VAUX1,2 ,MARYMORGAN-RICHARDS1 , ELIZABETH E. DALY1 and STEVEN A. TREWICK1 1Wildlife & Ecology Group, Massey University, Private Bag 11222, Palmerston North, 4442, New Zealand; [email protected], [email protected], [email protected], [email protected] 2Current address: Coastal Oregon Marine Experiment Station, Department of Fisheries & Wildlife, Hatfield Marine Science Center, Oregon State University, 2030 SE Marine Science Drive, Newport, OR USA; [email protected] I T has recently been suggested that a ‘living fossil’ can be (Grandcolas et al. 2014; Grandcolas & Trewick 2016). identified because it is both morphologically conservative The amount of change considered to be ‘little’ is subjec- and exhibits a significantly slower rate of morphological tive as it varies wildly among organisms due to different evolution compared to related lineages (Herrera-Flores evolutionary rates, durations in the fossil record, and tax- et al. 2017a). As an exemplar, variation among known onomic paradigms employed (Vaux et al. 2016; Bennett rhynchocephalians was investigated, and it was concluded et al. 2017). Also, limited morphological change over time that the New Zealand tuatara Sphenodon punctatus Gray might not indicate evolutionary conservatism, but trait is a living fossil species (Herrera-Flores et al. 2017a). We fluctuation around a mean state due to constraining respond here to Herrera-Flores et al., questioning the bio- selection (Charlesworth & Lande 1982; Eldredge et al. logical meaning and basis for describing a ‘living fossil’, 2005; Hunt 2007; Davis et al. 2014; Grandcolas et al. and we draw attention to limitations in their previous 2014; Pennell et al. 2014a). The morphology of fossils investigation of morphological conservatism among represents only a fraction of the genetic, behavioural, eco- rhynchocephalians. To assist future studies of dentary- logical, internal morphological and physiological diversity shape-variation data among rhynchocephalians and that has evolved in a lineage (Grandcolas et al. 2014; morphological conservatism, we provide a new geometric Vaux et al. 2016, 2017a). morphometric dataset for tuatara. Most species gain recognition as ‘living fossils’ because they belong to groups that contained greater species diversity in the past (see references in Grandcolas et al. THEORETICAL PROBLEMS WITH A 2014) but this reasoning is barren because taxon rarity on ‘LIVING FOSSIL’ a lineage reflects rates of extinction. Indeed, almost all biodiversity is extinct. Phylogenetic relicts are not espe- Charles Darwin (1859, p. 486) recognized that living spe- cially uncommon, and the perception of rarity and the cies possessed seemingly primitive morphological features timing of species origination are sensitive to subjective and might, ‘fancifully be called living fossils’. As a con- taxonomy (Forey et al. 2004; Haile-Selassie & Simpson cept a ‘living fossil’ is an oxymoron that has been widely 2013; Vaux et al. 2016). Phylogenetic relicts at all system- criticized in scientific and education literature (Schopf atic levels from genera to phyla attract attention because 1984; Freeman & Herron 2007; Mobley et al. 2009; they represent the sole living representatives of taxonomic Casane & Laurenti 2013; Futuyma 2013; Mathers et al. clades known better in the fossil record (e.g. ginkgo 2013; Grandcolas et al. 2014; Werth & Shear 2014; Ben- Werth & Shear 2014; vampire squid Yokobori et al. 2007; nett et al. 2017). Putative living fossils are typically extant hoatzin Mobley et al. 2009; red panda Mallick 2015). For taxa that are considered to have undergone little morpho- example, humans could justifiably be considered to be logical and inferred ecological change from a temporally living fossils under current taxonomy due to the extinc- distant ancestor known from palaeontological evidence tion of ~16 other species within Homo (Schopf 1984). © The Palaeontological Association doi: 10.1111/pala.12402 1 2 PALAEONTOLOGY Some putative living fossils are inferred to have persisted relatives that are argued to have been omnivorous, insec- over many millions of years as a single evolutionary lineage tivorous and piscivorous (Herrera-Flores et al. 2017a). segment (e.g. Ginkgo biloba, see Werth & Shear 2014). Since tuatara were the ‘seventh most similar to the However, a taxon that appears to straddle a long period of average shape (out of 29 taxa)’, it was concluded that the evolutionary time almost certainly represents inadequate species is ‘conservative’ for dentary shape as well as diet sampling of the fossil record, where evolutionary change (Herrera-Flores et al. 2017a). and multiple evolutionary lineage segments have simply not Treating diet as a categorical variable is problematic, been documented thoroughly (Werth & Shear 2014; Vaux even for living species for which behavioural and dietary et al. 2016). Palaeontologists are acutely aware of this issue, data are available from observation, as habitats and feed- and it is one reason why rare specimens are not always ing opportunity can vary dramatically among individuals classified below the genus level and certain taxa are treated and time (Arthur et al. 2008; Lopez-Alfaro et al. 2014; as merely placeholders (e.g. Archaeopteryx Rauhut et al. Williams et al. 2015; Sato et al. 2016). Tuatara them- 2018; see discussion by Forey et al. 2004). selves are primarily insectivorous (Walls 1981; Cree et al. Morphological evolution is a crucial aspect of palaeon- 1999; Ussher 1999; cf. Herrera-Flores et al. 2017a) but tology and evolutionary biology, independent of any con- also feed on crustaceans, reptiles and fish, as well as the sideration of ‘living fossils’. Many recent studies have eggs and juveniles of small seabirds (Cartland-Shaw et al. taken model-based approaches to analyse morphological 1998; Cree et al. 1999; Ussher 1999). Many animal spe- change through time, and such methods are appropriate cies also change dietary preference with ontogeny. For to address morphological conservatism and the origin of example, juvenile Clevosaurus brasiliensis Bonaparte & relict lineages (e.g. paleoTS and Geiger; Hunt 2007; Har- Sues may have been restricted to insectivory, but it is mon et al. 2008; Pennell et al. 2014b; Hunt et al. 2015). likely that adults also ate plants (de Vivar Martınez & Soares 2015). Classifying both Sphenodon and Clevosaurus as omnivores (Herrera-Flores et al. 2017a) risks oversim- TUATARA AS A ‘LIVING FOSSIL’ plifying real intraspecific diversity, and introducing inaccu- racy where no controls or sampling restrictions are The tuatara (Sphenodon punctatus) is among the most fre- enforced for ontogeny. A compounding issue is the accu- quently cited animal phylogenetic relicts (Fraser 1993; racy of dietary estimates for fossil taxa, and inferences Sues et al. 1994; Jones 2008; Jones et al. 2009a; Meloro & about extinct rhynchocephalians have been criticized (de Jones 2012; Cree 2014; Grandcolas et al. 2014). It is often Vivar Martınez & Soares 2015). Studies of other reptiles described as a living fossil (Hay et al. 2008; Mitchell et al. also demonstrate that dentary morphology without addi- 2008; Benton 2014; Klein & Scheyer 2017), although fossil tional evidence, such as preserved stomach contents or remains of tuatara are all fewer than 10 000 years old coprolites, can mislead interpretation of diet (e.g. some (Hay et al. 2008). Fossil jaw fragments from New Zealand herbivory-adapted dinosaurs ate crustaceans and rotten Miocene deposits have been assigned to Sphenodontinae wood; Chin et al. 2017). (a putative subgroup within Rhynchocephalia), but the Beyond diet estimated from dentary morphology, little material is insufficient to identify to genus level (Jones consideration was given to other known differences et al. 2009a). Given the brief fossil record of tuatara, it is among the rhynchocephalians sampled by Herrera-Flores not possible to infer morphological stasis for the evolu- et al. (2017a). Predominant habitat differs among species tionary lineage it represents. of Rhynchocephalia from terrestrial to aquatic (de Vivar Herrera-Flores et al. (2017a) investigated morphologi- Martınez & Soares 2015; Klein & Scheyer 2017; Bever & cal variation in the dentary bones of rhynchocephalian Norell 2017) and there is a considerable size range reptiles and concluded that tuatara have a conservative (Apesteguıa et al. 2014; de Vivar Martınez & Soares morphology among their extinct relatives. The authors 2015). Notably, although tuatara exhibit similar dentary focused especially on the comparison of morphological shape to the earliest known species Brachyrhinodon (Tri- variation with inferred diet. A phenogram based on mor- assic, 250–200 million years ago), body shape differs with phology (Herrera-Flores et al. 2017a, fig. 1), indicated tuatara having a larger trunk and shorter limbs (Fraser & that tuatara are equally related to fossil genera that might Benton 1989). Overall, there are good reasons why the have eaten plants and insects (Clevosaurus Bonaparte & well-preserved relatives of tuatara, alive millions of years Sues; Fraser 1988), bivalves or crabs (Oenosaurus Rauhut ago, have been interpreted as representing diversity span- et al., 2012) and seaweed (Ankylosphenodon Reynoso, ning genera and higher ranks. Therefore, focusing on a 2000). A principal component analysis (PCA) scatterplot single
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