Biology subjects

Grossnickle, D. M.

Publications and source records attributed to Grossnickle, D. M..

6 recordsLinked to original sources

bioRxiv2026

Dietary specializations are captured by jaw muscle proportions in mammals

Mammalian diet and feeding ecology are often reflected by craniofacial skeleton specializations, but feeding requires skeletal actuation by a complex suite of muscles with varying sizes, lines of action, and mechanical function. While muscles play a critical role in feeding mechanics, and hence diet, it remains unclear how well variation in jaw muscle morphology predicts diet in mammals. We quantified the evolutionary interplay between mammalian muscle morphology and diet using a large and taxonomically broad sample. We measured the relative proportions and putative force production capacity, quantified as muscle physiological cross-sectional area (PCSA), for the major adductor complexes, along with a key jaw depressor, in 91 mammalian species (30 chiropterans, 33 primates, and 28 ungulates, carnivorans, rodents, and marsupials). We recovered clear dietary signals for several muscle complexes, with the medial pterygoid (larger in herbivores) and temporalis (larger in carnivores) performing best as dietary predictors. The medial pterygoid is particularly relevant for the mechanical innovation in mammals of moving the mandible along non-orthal, medio-lateral trajectories during mastication. Our findings underscore the intuitive, yet previously unquantified, importance of muscles in the evolution of mandibular roll, yaw, and lateral translation, all mammalian hallmarks of processing diverse types of food.

zoology↗

bioRxiv2025

Reexamining the evolutionary history of the mammalian medial pterygoid muscle

In non-mammalian synapsids, feeding and hearing are closely linked because some jaw bones are involved in both functions. The evolutionary decoupling of these two systems in early mammals likely catalyzed greater specialization of feeding and hearing. Although fossil osteological changes during this process are well documented, the corresponding evolutionary changes to soft tissue anatomy are less certain. The medial pterygoid muscle is a jaw adductor that is central to this evolutionary transition because in many fossil lineages it inserted near or possibly on jaw bones involved in both feeding and hearing. In therians (placentals and marsupials), the medial pterygoid muscle develops medial to Meckels cartilage and inserts on the mandibular angular process. Similarly, non-mammalian cynodonts are often reconstructed with a medial pterygoid muscle passing medial to the ossified Meckels cartilage, inserting on the dentary angular (i.e., pseudangular) process. Thus, the traditional interpretation is that the medial pterygoid remained medial to Meckels cartilage through the evolutionary transitions from early cynodonts to therians. Here we highlight issues with that interpretation: the medial pterygoid muscle inserts lateral (not medial) to Meckels cartilage in monotremes and, presumably, early mammal groups (e.g., spalacotherioids) that lacked an angular process. This suggests at least two possible explanatory hypotheses: 1) the medial pterygoid muscle is evolutionarily labile, shifting in position relative to Meckels cartilage multiple times or 2) the medial pterygoid muscle did not insert on the pseudangular process of non-mammalian cynodonts and instead inserted on the mandibular medial ridge, dorsal to Meckels cartilage. We advocate for the latter hypothesis, proposed by Patterson (1956), which suggests that the medial pterygoid did not shift medial to Meckels cartilage until the complete separation of the ear and jaw in cladotherians (therians and close relatives), with the shift in position possibly triggering the evolution of the therian angular process as an insertion site. Pattersons hypothesis is in line with a growing body of evidence that indicate concomitant evolutionary changes of muscles, ears, and jaws at the cladotherian node were important catalysts for the evolution of hearing and feeding specializations in extant mammals.

evolutionary biology↗

bioRxiv2025

Locomotor Trends Among Early Mammals Illuminated by Predictors of Arboreality from the Mammalian Appendicular Skeleton

Arboreal locomotion is a common trait among extant mammals, and it likely evolved independently in numerous Mesozoic and Cenozoic mammalian lineages. We review evidence for the evolution of arboreality within lineages of early mammals and their stem relatives (Synapsida) while also emphasizing the uncertainty of locomotor inferences of fossil lineages. An improved understanding of the evolutionary origins and prevalence of arboreality in early mammals requires robust links between morphological traits and locomotor behaviors that can be applied to fossils. To this end, we use a large comparative dataset of linear measurements taken on extant mammalian postcranial specimens to evaluate the skeletal proxies that best differentiate arboreality from other locomotor modes. Consistent with previous research, we find that measures of digit length and elbow proportions are especially powerful predictors of climbing. Locomotor predictions based on single measurements or ratios are improved when phylogeny is incorporated into models. Nonetheless, combining multiple functionally relevant traits into our models yields robust predictions of locomotor mode even without incorporating phylogenetic information--this is especially encouraging for fossil groups with unstable phylogenies. We test these locomotor proxies in three early-mammalian case studies--Multituberculata, Plesiadapiformes, and early Theria. We find that multituberculates had a wide locomotor breadth whereas plesiadapiforms all had a high probability of climbing, consistent with previous studies. However, some Mesozoic therians in our sample are predicted to be ground-dwellers, conflicting with the hypothesis that the ancestral therian was scansorial or arboreal. The predictive tools presented here will improve locomotor inferences for small mammals, further illuminating the prevalence and importance of arboreality in early mammalian evolution.

paleontology↗

bioRxiv2023

On the cusp of adaptive change: the hierarchical radiation of phyllostomid bats

Adaptive radiations are bursts in biodiversity that lead to the origin of new evolutionary lineages and phenotypes. However, adaptive radiations typically occur over millions of years and it is unclear how the macroevolutionary dynamics that underpin them vary through time and among groups of organisms. Phyllostomid bats radiated extensively for diverse diets -from insects to vertebrates, fruit, nectar, and blood- and we use their molars as a model system to examine the dynamics of adaptive radiations. Three-dimensional shape analyses of lower molars of Noctilionoidea (Phyllostomidae and close relatives) indicate that different diet groups exhibit distinct morphotypes. Comparative analyses further reveal that phyllostomids are a striking example of a hierarchical radiation; their initial, higher-level diversification involved an early burst in molar morphological disparity as lineages invaded new diet-affiliated adaptive zones, followed by subsequent lower-level diversifications within adaptive zones involving less dramatic morphological changes. We posit that strong selective pressures related to initial shifts to derived diets may have freed molars from morpho-functional constraints associated with the ancestral molar morphotype. Then, lineages with derived diets (frugivores and nectarivores) diversified considerably within broad adaptive zones, likely reflecting finer-scale niche partitioning. The observed early burst pattern is only evident when examining molar traits that are strongly linked to diet, highlighting the importance of ecomorphological traits in comparative studies. Our results support the hypothesis that adaptive radiations are commonly hierarchical and involve different tempos and modes at different phylogenetic scales, with early bursts being more common at broader scales. SIGNIFICANCE STATEMENTMany groups of organisms are exceptionally diverse in their ecology, morphology, and number of species. But there is debate as to whether these groups commonly achieved this diversity through bursts in diversification early in their history. Phyllostomid bats are one of the most ecologically diverse mammalian families and a classic example of an adaptive radiation. We use their molar shapes, which correlate with diet, as a model for examining macroevolutionary patterns during diversifications. We find that phyllostomids experienced a two-step process of diversification; the first step involved a rapid burst, whereas the second involved finer-scale changes as lineages filled ecological niches. We posit that this is a common, yet underappreciated, pattern during the early histories of many diverse clades.

evolutionary biology↗

bioRxiv2022

A cautionary note on quantitative measures of phenotypic convergence

Tests of phenotypic convergence can provide evidence of adaptive evolution, and the popularity of such studies has grown in recent years due to the development of novel, quantitative methods for identifying and measuring convergence. These methods include the commonly applied C1-C4 measures of Stayton (2015), which measure morphological distances between lineages, and Ornstein-Uhlenbeck (OU) evolutionary model-fitting analyses, which test whether lineages convergently evolved toward adaptive peaks. We test the performance of C-measures and other convergence measures under various evolutionary scenarios and reveal a critical issue with C-measures: they often misidentify divergent lineages as convergent. We address this issue by developing novel convergence measures--Ct1-Ct4-measures--that measure distances between lineages at specific points in time, minimizing the possibility of misidentifying divergent taxa as convergent. Ct-measures are most appropriate when focal lineages are of the same or similar geologic ages (e.g., extant taxa), meaning that the lineages evolutionary histories include considerable overlap in time. Beyond C-measures, we find that all convergence measures are influenced by the position of focal taxa in phenotypic space, with morphological outliers often statistically more likely to be measured as strongly convergent by chance. Further, we mimic scenarios in which researchers assess convergence using OU models with a priori regime assignments (e.g., classifying taxa by ecological traits), and we find that multiple-regime OU models with phenotypically divergent lineages assigned to a shared selective regime often outperform simpler models. This highlights that model support for these multiple-regime OU models should not be assumed to always reflect convergence among focal lineages of a shared regime. Our new Ct1-Ct4-measures provide researchers with an improved comparative tool, but we emphasize that all available convergence measures are imperfect, and researchers should recognize the limitations of these methods and use multiple lines of evidence when inferring and measuring convergence.

evolutionary biology↗

bioRxiv2022

Gliding towards an understanding of the origin of flight in bats

Bats are the only mammals capable of powered flight and have correspondingly specialized body plans, particularly in their limb morphology. The origin of bat flight is still not fully understood due to an uninformative fossil record but, from the perspective of a functional transition, it is widely hypothesized that bats evolved from gliding ancestors. Here, we test predictions of the gliding-to-flying hypothesis of the origin of bat flight by using phylogenetic comparative methods to model the evolution of forelimb and hindlimb traits on a dataset spanning four extinct bats and 231 extant mammals with diverse locomotor modes. Our results reveal that gliders exhibit adaptive trait optima (1) toward relatively elongate forelimbs that are intermediate between those of bats and non-gliding arborealists, and (2) toward relatively narrower but not longer hindlimbs that are intermediate between those of non-gliders and bats. We propose a hypothetical adaptive landscape based on limb length and width optimal trends derived from our modeling analyses. Our results support a hypothetical evolutionary pathway wherein glider-like postcranial morphology precedes a bat-like morphology adapted to powered-flight, setting a foundation for future developmental, biomechanical, and evolutionary research to test this idea.

evolutionary biology↗