Positive Allometry and the Prehistory of Sexual Selection

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. abstract: The function of the exaggerated structures that adorn many fossil vertebrates remains largely unresolved. One recurrent hypothesis is that these elaborated traits had a role in thermoregulation. This orthodoxy persists despite the observation that traits exaggerated to the point of impracticality in extant organisms are almost invariably sexually selected. We use allometric scaling to investigate the role of sexual selection and thermoregulation in the evolution of exaggerated traits of the crested pterosaur Pteranodon longiceps and the sail-backed eupelycosaurs Dimetrodon and Edaphosaurus. The extraordinarily steep positive allometry of the head crest of Pteranodon rules out all of the current hypotheses for this trait's main function other than sexual signaling. We also find interspecific patterns of allometry and sexual dimorphism in the sails of Dimetrodon and patterns of elaboration in Edaphosaurus consistent with a sexually selected function. Furthermore, small ancestral, sailbacked pelycosaurs would have been too small to need adaptations to thermoregulation. Our results question the popular view that the elaborated structures of these fossil species evolved as thermoregulatory organs and provide evidence in support of the hypothesis that Pteranodon crests and eupelycosaur sails are among the earliest and most extreme examples of elaborate sexual signals in the evolution of terrestrial vertebrates.

Selection towards larger body size in both herbivorous and carnivorous synapsids during the Carboniferous

“…The lack of overwhelming support for any particular model is likely due to the short time series. An increase in body size in Permo-Carboniferous non-mammalian synapsids has been qualitatively noted since specimens were first placed in a stratigraphic context (Romer and Price 1940;Olson 1962;Gould 1967;Pivorunas 1970;Gould and Littlejohn 1973;Bakker 1986;Tracy et al 1986;Berman et al 2001;Tomkins et al 2010;Reisz and Fröbisch 2014;Brink et al , 2015. Reisz and Fröbisch (2014) proposed that a trend towards increased body size can be observed in all herbivorous lineages, and also posited a body size increase in the Sphenacodontidae; although they suggested that this increase was less than that observed in herbivores.…”

“…Sphenacomorpha is the only group of medium-to-large bodied non-mammalian synapsids to have developed elongate neural spines forming a dorsal sail, the function of which has been subject to intense scrutiny (Romer 1927;Rodbard 1949;Pivorunas 1970;Bramwell and Felqett 1973;Haack 1986;Tracy et al 1986;Bennett 1996;Florides et al 2001;Huttenlocker et al 2010Huttenlocker et al , 2011Tomkins et al 2010;Rega et al 2012). It has been suggested that sphenacomorphs were the largest terrestrial vertebrates of the Permo-Carboniferous because of their dorsal sails, as the sail would act to strengthen the vertebral column at large body masses and increase the efficiency of thermoregulation (Romer 1927;Pivorunas 1970;Florides et al 2001).…”

“…It has been suggested that sphenacomorphs were the largest terrestrial vertebrates of the Permo-Carboniferous because of their dorsal sails, as the sail would act to strengthen the vertebral column at large body masses and increase the efficiency of thermoregulation (Romer 1927;Pivorunas 1970;Florides et al 2001). Recent studies have questioned the use of the dorsal sail as a thermoregulatory organ based on histological analyses of the neural spines (Huttenlocker et al 2010(Huttenlocker et al , 2011, the suggestion that Dimetrodon was nocturnal (Angielczyk and Schmitz 2014), and the positive allometry of neural spine height through evolutionary time as evidence that the sail was used for sexual selection (Bakker 1986;Tomkins et al 2010). However, the idea of whether or not dorsal sails are related to body size in sphenacomorphs has never been quantitatively tested.…”

Response to Kellner (2017) 'Rebuttal of Martin-Silverstone, E., J.R.N. Glasier, J.H. Acorn, S. Mohr, and P.J. Currie, 2017'

“…What matters is that there was significant variation within Pteranodon sternbergi. Additionally, if the cranial crests of Pteranodon were sexually dimorphic (Bennett 1992;Tomkins et al 2010;Hone et al 2012;Knell et al 2013), then they were probably under the influence of sexual selection. In modern animals, sexually selected or "signal" traits are more variable than those of non-signal traits, with respect to their size, shape, colour, and presence/absence (e.g., Alatalo et al 1988;Wiens 2001;Cuervo and Møller 2009;Tazzyman and Iwasa 2010;Emlen et al 2012).…”

Premaxillary crest variation within the Wukongopteridae (Reptilia, Pterosauria) and comments on cranial structures in pterosaurs

“…However, there is no direct association of the skulls and pelves that could back this hypothesis (e.g., Kellner and Tomida 2000). Furthermore, it has been suggested that, at least theoretically, the differences found by Bennett (1992) could be accounted for ontogeny and taxonomy (Tomkins et al 2010). In fact, a re-evaluation of several specimens attributed to Pteranodon has shown that in some cases there are sufficient morphological differences other than the shape and size of the cranial crest, supporting a larger taxonomic diversity within what can be called the Pteranodon-complex (Kellner 2010).…”