The many faces of synapsid cranial allometry

Abstract

Previous studies of cranial shape have established a consistent interspecific allometric pattern relating the relative lengths of the face and braincase regions of the skull within multiple families of mammals. In this interspecific allometry, the facial region of the skull is proportionally longer than the braincase in larger species. The regularity and broad taxonomic occurrence of this allometric pattern suggests that it may have an origin near the base of crown Mammalia, or even deeper in the synapsid or amniote forerunners of mammals. To investigate the possible origins of this allometric pattern, we used geometric morphometric techniques to analyze cranial shape in 194 species of nonmammalian synapsids, which constitute a set of successive outgroups to Mammalia. We recovered a much greater diversity of allometric patterns within nonmammalian synapsids than has been observed in mammals, including several instances similar to the mammalian pattern. However, we found no evidence of the mammalian pattern within Therocephalia and nonmammalian Cynodontia, the synapsids most closely related to mammals. This suggests that the mammalian allometric pattern arose somewhere within Mammaliaformes, rather than within nonmammalian synapsids. Further investigation using an ontogenetic series of the anomodont Diictodon feliceps shows that the pattern of interspecific allometry within anomodonts parallels the ontogenetic trajectory of Diictodon. This indicates that in at least some synapsids, allometric patterns associated with ontogeny may provide a “path of least resistance” for interspecific variation, a mechanism that we suggest produces the interspecific allometric pattern observed in mammals.

Effects of taphonomic deformation on geometric morphometric analysis of fossils: a study using the dicynodontDiictodon feliceps(Therapsida, Anomodontia)

“…We used empirical specimens as the starting points for our simulated datasets, but the specimens chosen were relatively extreme shapes (e.g., very large and very small undistorted empirical specimens for the simulated datasets with an ontogenetic signal), which could result in an exaggerated "biological" signal. However, the simulated ontogenetic signal in our lateral view dataset is quite similar to that documented by Krone, Kammerer & Angielczyk (2019) for Diictodon (larger specimens have proportionally deeper snouts and are more dorsoventrally constricted near the level of the orbits), so we do not think we have seriously mischaracterized the biological signals included in the simulated datasets. Instead, we consider it more likely that better preserved biological signals in the simulated deformed datasets is a reflection of the simpler style of deformation applied in the simulations.…”

“…In contrast to the strong signal from deformation style, Diictodon specimens categorized by the biological variables of sex and size class (a proxy for age) exhibit seemingly random occupation of morphospace. Krone, Kammerer & Angielczyk (2019) were able to recover an ontogenetic signal for Diictodon skulls in lateral view using a smaller sample of more or less undeformed specimens and a slightly different configuration of landmarks in lateral view. The absence of an obvious size-shape relationship in the empirical data here suggests that it was overprinted by deformation, mirroring the conclusion of Hedrick & Dodson (2013) that their Psittacosaurus dataset did not preserve an original allometric signal.…”

Including Distorted Specimens in Allometric Studies: Linear Mixed Models Account for Deformation

“…All living multicellular organisms grow and change through time, whether solely in absolute size or with changing proportions of individual features ( Thompson 1917 ; Huxley 1932 ; Gould 1968 , 1977 ; Gatsuk et al 1980; Hochuli 2001; Rowe 2004 ; Tanner et al 2010 ; Chagnon et al 2013; Griffin et al 2021 ). These patterns of growth can also influence evolutionary trajectories and allmetries among species, as they can reflect constraints on the variation available to evolutionary change ( Gould 1966 ; Cheverud 1982 ; Alexander 1985 ; Klingenberg 1996a ; Marroig and Cheverud 2005 ; Voje et al 2014 ; Cardini 2019 ; Krone et al 2019 ). Changes in the patterns of growth, or the study of allometry, has been explored extensively across the tree of Life (see Gould 1966 ), including extinct animals (e.g., Abdala and Giannini 2002 ; Kilbourne and Makovicky 2010 ; Jasinoski et al 2015 ; Griffin and Nesbitt 2016a , 2016b ; Krone et al 2019 ).…”

“…These patterns of growth can also influence evolutionary trajectories and allmetries among species, as they can reflect constraints on the variation available to evolutionary change ( Gould 1966 ; Cheverud 1982 ; Alexander 1985 ; Klingenberg 1996a ; Marroig and Cheverud 2005 ; Voje et al 2014 ; Cardini 2019 ; Krone et al 2019 ). Changes in the patterns of growth, or the study of allometry, has been explored extensively across the tree of Life (see Gould 1966 ), including extinct animals (e.g., Abdala and Giannini 2002 ; Kilbourne and Makovicky 2010 ; Jasinoski et al 2015 ; Griffin and Nesbitt 2016a , 2016b ; Krone et al 2019 ). Allometric studies tend to focus on either identifying scaling patterns present in entire clades or reconstructing allometric patterns of a single population or species ( Cheverud 1982 ; Pélabon et al 2014 ; Klingenberg 2016 ).…”

Computed Tomography elucidates ontogeny within the basal therapsid clade Biarmosuchia

“…Previous research on ontogenetic series of some non-biarmosuchian therapsid taxa provided insight on their palaeobiology. Allometric studies have been undertaken on various therapsid groups to evaluate and study ontogenetic patterns ( Huttenlocker & Abdala, 2015 ; Jasinoski, Abdala & Fernandez, 2015 ; Jasinoski & Abdala, 2017a ; Krone, Kammerer & Angielczyk, 2019 ). Using allometric and multivariate analyses Jasinoski & Abdala (2017a) identified ontogenetic modifications and sexual dimorphism in the cynodont Galesaurus planiceps and found that a few craniomandibular features, including the shape of the sutures, change during ontogeny.…”