Ontogenetic growth in the crania ofExaeretodon argentinus(Synapsida: Cynodontia) captures a dietary shift

Abstract

Background An ontogenetic niche shift in vertebrates is a common occurrence where ecology shifts with morphological changes throughout growth. How ecology shifts over a vertebrate’s lifetime is often reconstructed in extant species—by combining observational and skeletal data from growth series of the same species—because interactions between organisms and their environment can be observed directly. However, reconstructing shifts using extinct vertebrates is difficult and requires well-sampled growth series, specimens with relatively complete preservation, and easily observable skeletal traits associated with ecologies suspected to change throughout growth, such as diet. Methods To reconstruct ecological changes throughout the growth of a stem-mammal, we describe changes associated with dietary ecology in a growth series of crania of the large-bodied (∼2 m in length) and herbivorous form, Exaeretodon argentinus (Cynodontia: Traversodontidae) from the Late Triassic Ischigualasto Formation, San Juan, Argentina. Nearly all specimens were deformed by taphonomic processes, so we reconstructed allometric slope using a generalized linear mixed effects model with distortion as a random effect. Results Under a mixed effects model, we find that throughout growth, E. argentinus reduced the relative length of the palate, postcanine series, orbits, and basicranium, and expanded the relative length of the temporal region and the height of the zygomatic arch. The allometric relationship between the zygomatic arch and temporal region with the total length of the skull approximate the rate of growth for feeding musculature. Based on a higher allometric slope, the zygoma height is growing relatively faster than the length of the temporal region. The higher rate of change in the zygoma may suggest that smaller individuals had a crushing-dominated feeding style that transitioned into a chewing-dominated feeding style in larger individuals, suggesting a dietary shift from possible faunivory to a more plant-dominated diet. Dietary differentiation throughout development is further supported by an increase in sutural complexity and a shift in the orientation of microwear anisotropy between small and large individuals of E. argentinus. A developmental transition in the feeding ecology of E. argentinus is reflective of the reconstructed dietary transition across Gomphodontia, wherein the earliest-diverging species are inferred as omnivorous and the well-nested traversodontids are inferred as herbivorous, potentially suggesting that faunivory in immature individuals of the herbivorous Traversodontidae may be plesiomorphic for the clade.

Evolution of postcanine complexity in Gomphodontia (Therapsida: Cynodontia)

“…Extant herbivorous mammals of similar size are known to consume large amounts of structural plant material such as leaves and stems (Pineda‐Munoz et al, 2016), a similar type of diet that might have been shared by these large‐bodied gomphodontosuchines. Representatives of this group show conspicuous cranial and functional adaptations to chewing hard plant material such a strictly palinal (i.e., backward) movement of the lower jaw (Kubo et al, 2017) as well as robust and high horizontal rami, a huge, tall coronoid process with the dorsal margin located above the zygomatic arch, deep zygomatic arches (more massive than in the majority of other traversodontids), and enlarged temporal fenestrae (Bonaparte, 1962; Liparini et al, 2013; Wynd et al, 2022). These features are indicative of the development of a powerful occlusal musculature (Crompton & Attridge, 1986; Reisz & Sues, 2000).…”

“…Filippini et al (2022) recently hypothesized the fact that small‐bodied traversodontids (i.e., skull length of <150 mm) like Pascualgnathus and Andescynodon had skull and dental morphologies suggestive of highly nutritious food items and may have, therefore, been generalist cynodonts. An exclusively herbivorous feeding ecology, however, appears to have been present in the larger and more derived traversodontids Massetognathinae and Gomphodontosuchinae (e.g., Melo et al, 2019; Wynd et al, 2022), which were hypothesized to have had a generalized and specialized herbivorous diet, respectively, by Filippini et al (2022).…”

Old fossil findings in the Upper Triassic rocks of southern Brazil improve diversity of traversodontid cynodonts (Therapsida, Cynodontia)

“…The number of upper incisors is unknown in the holotype of Proexaeretodon vincei whereas the holotype of ‘ Ischignathus sudamericanus ’ seems to have two upper incisors in the left premaxilla (PVL 2564; contra Bonaparte, 1963c; a third alveolus with a particularly small incisor is not obvious at all in the original material. However, this taxon is considered a junior synonym of Exaeretodon argentinus ; see Liu, 2007; Wynd et al, 2022).…”

“…In traversodontids whose growth series is relatively well known (e.g., Massetognathus , Abdala & Giannini, 2000; Exaeretodon , Abdala et al, 2002; Liu, 2007; Wynd et al, 2022; Andescynodon , Liu & Powell, 2009; and Dadadon , Kammerer et al, 2012), some ontogenetic changes were described, namely: The reduction of the number of postcanines, the length of the palate, orbits and the basicranium and the increase of the temporal region length and the height of the zygomatic arches in E. argentinus (Wynd et al, 2022), the presence/absence of the parietal foramen and the increase of the number of postcanines in Massetognathus sp. (Abdala & Giannini, 2000), the increase of the height of the canines in Andescynodon (Liu & Powell, 2009), and increase of the length of the snout and the size of the temporal fenestra, reduction of the size of the orbits and the increase of the number of postcanines in Dadadon (Kammerer et al, 2012).…”

Functional reorganisation of the cranial skeleton during the cynodont–mammaliaform transition

“…The change in zygomatic arch height parallels ontogenetic niche shifts observed in gomphodontian cynodonts. For example, smaller individuals (inferred to be juveniles) of Exaeretodon argentinus and E xaeretodon riograndensis had a low zygoma adapted to crushing-dominated feeding, whereas larger individuals (inferred to be adults/older ontogenetic stages) showed a high zygoma adapted to a chewing-dominated feeding style 47 . A similar trend has been found tentatively also in the more basal cynodonts Galesaurus and Thrinaxodon 48 , 49 .…”