Facing the facts: adaptive trade-offs along body size ranges determine mammalian craniofacial scaling

The mammalian cranium (skull without lower jaw) is representative of mammalian diversity and is thus of particular interest to mammalian biologists across disciplines. One widely retrieved pattern accompanying mammalian cranial diversification is referred to as 'craniofacial evolutionary allometry' (CREA). This posits that adults of larger species, in a group of closely related mammals, tend to have relatively longer faces and smaller braincases. However, no process has been officially suggested to explain this pattern, there are many apparent exceptions, and its predictions potentially conflict with well-established biomechanical principles. Understanding the mechanisms behind CREA and causes for deviations from the pattern therefore has tremendous potential to explain allometry and diversification of the mammalian cranium. Here, we propose an amended framework to characterise the CREA pattern more clearly, in that 'longer faces' can arise through several kinds of evolutionary change, including elongation of the rostrum, retraction of the jaw muscles, or a more narrow or shallow skull, which all result in a generalised gracilisation of the facial skeleton with increased size. We define a standardised workflow to test for the presence of the pattern, using allometric shape predictions derived from geometric morphometrics analysis, and apply this to 22 mammalian families including marsupials, rabbits, rodents, bats, carnivores, antelopes, and whales. Our results show that increasing facial gracility with size is common, but not necessarily as ubiquitous as previously suggested. To address the mechanistic basis for this variation, we then review cranial adaptations for harder biting. These dictate that a more gracile cranium in larger species must represent a structural sacrifice in the ability to produce or withstand harder bites, relative to size. This leads us to propose that facial gracilisation in larger species is often a product of bite force allometry and phylogenetic niche conservatism, where more closely related species tend to exhibit more similar feeding ecology and biting behaviours and, therefore, absolute (size-independent) bite force requirements. Since larger species can produce the same absolute bite forces as smaller species with less effort, we propose that relaxed bite force demands can permit facial gracility in response to bone optimisation and alternative selection pressures. Thus, mammalian facial scaling represents an adaptive by-product of the shifting importance of selective pressures occurring with increased size. A reverse pattern of facial 'shortening' can accordingly also be found, and is retrieved in several cases here, where larger species incorporate novel feeding behaviours involving greater bite forces. We discuss multiple exceptions to a bite force-mediated influence on facial proportions across mammals which lead us to argue that ecomorphological specialisation of the cranium is likely to be the primary driver of facial scaling patterns, with some developmental constraints as possible secondary factors. A potential for larger species to have a wider range of cranial functions when less constrained by bite force demands might also explain why selection for larger sizes seems to be prevalent in some mammalian clades. The interplay between adaptation and constraint across size ranges thus presents an interesting consideration for a mechanistically grounded investigation of mammalian cranial allometry.

Flexible conservatism in the skull modularity of convergently evolved myrmecophagous placental mammals

Background

The skull of placental mammals constitutes one of the best studied systems for phenotypic modularity. Several studies have found strong evidence for the conserved presence of two- and six-module architectures, while the strength of trait correlations (integration) has been associated with major developmental processes such as somatic growth, muscle-bone interactions, and tooth eruption. Among placentals, ant- and termite-eating (myrmecophagy) represents an exemplar case of dietary convergence, accompanied by the selection of several cranial morphofunctional traits such as rostrum elongation, tooth loss, and mastication loss. Despite such drastic functional modifications, the covariance patterns of the skull of convergently evolved myrmecophagous placentals are yet to be studied in order to assess the potential consequences of this dietary shift on cranial modularity.

Results

Here, we performed a landmark-based morphometric analysis of cranial covariance patterns in 13 species of myrmecophagous placentals. Our analyses reveal that most myrmecophagous species present skulls divided into six to seven modules (depending on the confirmatory method used), with architectures similar to those of non-myrmecophagous placentals (therian six modules). Within-module integration is also similar to what was previously described for other placentals, suggesting that most covariance-generating processes are conserved across the clade. Nevertheless, we show that extreme rostrum elongation and tooth loss in myrmecophagid anteaters have resulted in a shift in intermodule correlations in the proximal region of the rostrum. Namely, the naso-frontal and maxillo-palatine regions are strongly correlated with the oro-nasal module, suggesting an integrated rostrum conserved from pre-natal developmental processes. In contrast, the similarly toothless pangolins show a weaker correlation between the anterior rostral modules, resembling the pattern of toothed placentals.

Conclusions

These results reveal that despite some integration shifts related to extreme functional and morphological features of myrmecophagous skulls, cranial modular architectures have conserved the typical mammalian scheme.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02030-9.

Taxonomic revision of maned sloths, subgenus Bradypus (Scaeopus), Pilosa, Bradypodidae, with revalidation of Bradypus crinitus Gray, 1850

We present a taxonomic revision of maned sloths, subgenus Bradypus (Scaeopus), a taxon endemic to the Brazilian Atlantic Forest and currently composed of a single species, the vulnerable Bradypus torquatus. Our review is based on coalescent species delimitation analyses using mitochondrial and nuclear DNA, morphological analyses, and field observations. Our integrative approach demonstrates that two species of maned sloth can be recognized: the northern maned sloth (Bradypus torquatusIlliger, 1811) occurring in the Brazilian states of Bahia and Sergipe, and the southern maned sloth (Bradypus crinitusGray, 1850), occurring in Rio de Janeiro and Espirito Santo states. The two species diverged in the Early Pliocene and are allopatrically distributed. We discuss the biogeographic pattern of the two maned sloth species, comparing it with other Atlantic Forest mammals. We also suggest that the conservation status of both maned sloths needs to be reassessed after this taxonomic rearrangement.

Applied Functional Biology: Linking Ecological Morphology to Conservation and Management

Many researchers work at the interface of organisms and environment. Too often, the insights that organismal, or functional, biologists can bring to the understanding of natural history, ecology, and conservation of species are overlooked. Likewise, natural resource managers are frequently focused on the management of populations and communities, while ignoring key functional traits that might explain variation in abundance and shifts in species composition at these ecological levels. Our intention for this symposium is two-fold: (1) to bring to light current and future research in functional and ecological morphology applicable to concerns and goals of wildlife management and conservation and (2) to show how such studies can result in measurable benchmarks useful to regulatory agencies. Symposium topics reveal past, present, and future collaborations between functional morphologists/biomechanists and conservation/wildlife biologists. During the SICB 2020 Annual Meeting, symposium participants demonstrated how data gathered to address fundamental questions regarding the causes and consequences of organismal form and function can also help address issues of conservation and wildlife management. Here we review how these, and other, studies of functional morphology, biomechanics, ecological development morphology and performance can inform wildlife conservation and management, principally by identifying candidate functional traits that have clear fitness consequences and population level implications.

Skull shape variation in extant pangolins (Pholidota: Manidae): allometric patterns and systematic implications

Pangolins are among the most endangered groups of mammals, comprising eight extant species delineated into three genera. Despite several studies dedicated to their skeletal anatomy, the potential taxonomic insight from cranial morphological variation in extant Pholidota is yet to be assessed with modern geometric morphometric methods. We present the first comprehensive study on the cranial morphology of extant pangolins and discuss its implications for the taxonomy and evolution of the group. We performed landmark-based morphometric analyses on 241 museum specimens to describe the variation in skull shape in seven of the eight extant species. Our analyses revealed genus- and species-level morphological discrimination, with Asian species (Manis spp.) being grouped together, whereas African pangolins present distinct skull shapes between small (Phataginus spp.) and large (Smutsia spp.) species. Analyses of allometry also identified a set of traits whose allometric trajectories distinguish Asian from African specimens. Finally, we uncovered intraspecific variation in skull shape in white-bellied pangolins (Phataginus tricuspis) that partly corroborates recent DNA-based differentiation among biogeographically distinct populations. Overall, our results shed light on the morphological diversity of the skull of these enigmatic myrmecophagous mammals and confirm the genus-level classification and cryptic diversity within the white-bellied pangolin revealed by molecular phylogenetics.

Variación no alométrica en el cráneo del perezoso bayo

El perezoso bayo Bradypus variegatus (Schinz, 1825) es un mamífero monomórfico, de ontogenia craneal poco conocida. En este estudio analizamos la alometría estática entre tamaño y forma, utilizando 21 especímenes diferentes de edades diversas. El tamaño y la forma fueron determinados mediante técnicas de morfometría geométrica. De los resultados obtenidos se desprende que la variación en la forma craneal queda muy poco explicada por la variación en el tamaño. Muchos estudios han señalado características morfológicas únicas en los perezosos en relación al resto de mamíferos, características fenotípicas que van de la estructura esquelética a tejidos blandos. En este caso, el escalado no alométrico del cráneo debería ser visto como otra característica única de este grupo taxonómico.

The earliest well-documented occurrence of sexual dimorphism in extinct sloths: evolutionary and palaeoecological insights

Sexual dimorphism (SD) is extremely common in species that have reproductive roles segregated into separate sexes, and it has been recognized in several mammalian lineages, both extant and extinct. Sexual dimorphism is low to moderate in living sloths, but it had a more important role for extinct sloth taxa. The presence of SD in extinct sloths was first suggested at the end of the 19th century and it is now commonly advocated as a possible explanation of high intraspecific variation in many extinct sloth species. In this paper, we report the presence of SD in Simomylodon uccasamamensis, a Late Miocene to Late Pliocene sloth from the Bolivian Altiplano. We present evidence of SD in the morphology of cranial and postcranial remains, representing the earliest unequivocal occurrence of size-based SD in an extinct sloth species. Differences between sexes are mainly observed in the morphology of the feeding apparatus and general body size. Comparisons with extant large mammals allow us to hypothesize different food selection between the two sexes, with probable divergent habitat use and concomitant niche separation. This, in turn, could have represented an important selective factor for adaptation to environmental changes experienced by the Bolivian Altiplano in Late Neogene times.

Body mass predicts isotope enrichment in herbivorous mammals

Carbon isotopic signatures recorded in vertebrate tissues derive from ingested food and thus reflect ecologies and ecosystems. For almost two decades, most carbon isotope-based ecological interpretations of extant and extinct herbivorous mammals have used a single diet–bioapatite enrichment value (14‰). Assuming this single value applies to all herbivorous mammals, from tiny monkeys to giant elephants, it overlooks potential effects of distinct physiological and metabolic processes on carbon fractionation. By analysing a never before assessed herbivorous group spanning a broad range of body masses—sloths—we discovered considerable variation in diet–bioapatite δ¹³C enrichment among mammals. Statistical tests (ordinary least squares, quantile, robust regressions, Akaike information criterion model tests) document independence from phylogeny, and a previously unrecognized strong and significant correlation of δ¹³C enrichment with body mass for all mammalian herbivores. A single-factor body mass model outperforms all other single-factor or more complex combinatorial models evaluated, including for physiological variables (metabolic rate and body temperature proxies), and indicates that body mass alone predicts δ¹³C enrichment. These analyses, spanning more than 5 orders of magnitude of body sizes, yield a size-dependent prediction of isotopic enrichment across Mammalia and for distinct digestive physiologies, permitting reconstruction of foregut versus hindgut fermentation for fossils and refined mean annual palaeoprecipitation estimates based on δ¹³C of mammalian bioapatite.

Creating diversity in mammalian facial morphology: a review of potential developmental mechanisms

Mammals (class Mammalia) have evolved diverse craniofacial morphology to adapt to a wide range of ecological niches. However, the genetic and developmental mechanisms underlying the diversification of mammalian craniofacial morphology remain largely unknown. In this paper, we focus on the facial length and orofacial clefts of mammals and deduce potential mechanisms that produced diversity in mammalian facial morphology. Small-scale changes in facial morphology from the common ancestor, such as slight changes in facial length and the evolution of the midline cleft in some lineages of bats, could be attributed to heterochrony in facial bone ossification. In contrast, large-scale changes of facial morphology from the common ancestor, such as a truncated, widened face as well as the evolution of the bilateral cleft possessed by some bat species, could be brought about by changes in growth and patterning of the facial primordium (the facial processes) at the early stages of embryogenesis.

Cranial osteology of the pampathere Holmesina floridanus (Xenarthra: Cingulata; Blancan NALMA), including a description of an isolated petrosal bone

The present study entails descriptions of several well-preserved skulls from the pampathere species Holmesina floridanus, recovered from Pliocene localities in central Florida and housed in the collections of the Florida Museum of Natural History. Bone by bone descriptions have allowed detailed reconstructions of cranial morphology. Cranial foramina are described and illustrated in detail, and their contents inferred. The first ever description of an isolated pampathere petrosal is also included. Cranial osteology of Holmesina floridanus is compared to that of Pleistocene species of Holmesina from both North and South America (Holmesina septentrionalis, Holmesina occidentalis), as well as to the other well-known pampathere genera, to closely related taxa among glyptodonts (Propalaehoplophorus), and to extinct and extant armadillos (Proeutatus, Euphractus). This study identifies a suite of apomorphic cranial features that serve to diagnose a putative, progressive series of more inclusive monophyletic groups, including the species Holmesina floridanus, the genus Holmesina, pampatheres, pampatheres plus glyptodonts, and a clade formed by pampatheres, glyptodonts, and Proeutatus. The study highlights the need for further anatomical investigations of pampathere cranial anatomy, especially those using modern scanning technology, and for analyses of pampathere phylogenetic relationships.

Beyond the carapace: skull shape variation and morphological systematics of long-nosed armadillos (genus Dasypus)

Background

The systematics of long-nosed armadillos (genus Dasypus) has been mainly based on a handful of external morphological characters and classical measurements. Here, we studied the pattern of morphological variation in the skull of long-nosed armadillos species, with a focus on the systematics of the widely distributed nine-banded armadillo (Dasypus novemcinctus).

Methods

We present the first exhaustive 3D comparison of the skull morphology within the genus Dasypus, based on micro-computed tomography. We used geometric morphometric approaches to explore the patterns of the intra- and interspecific morphological variation of the skull with regard to several factors such as taxonomy, geography, allometry, and sexual dimorphism.

Results

We show that the shape and size of the skull vary greatly among Dasypus species, with Dasypus pilosus representing a clear outlier compared to other long-nosed armadillos. The study of the cranial intraspecific variation in Dasypus novemcinctus evidences clear links to the geographic distribution and argues in favor of a revision of past taxonomic delimitations. Our detailed morphometric comparisons detected previously overlooked morphotypes of nine-banded armadillos, especially a very distinctive unit restricted to the Guiana Shield.

Discussion

As our results are congruent with recent molecular data and analyses of the structure of paranasal sinuses, we propose that Dasypus novemcinctus should be regarded either as a polytypic species (with three to four subspecies) or as a complex of several distinct species.

Orientation of the lateral semicircular canal in Xenarthra and its links with head posture and phylogeny

The orientation of the semicircular canals of the inner ear in the skull of vertebrates is one of the determinants of the capacity of this system to detect a given rotational movement of the head. Past functional studies on the spatial orientation of the semicircular canals essentially focused on the lateral semicircular canal (LSC), which is supposedly held close to horizontal during rest and/or alert behaviors. However, they generally investigated this feature in only a few and distantly related taxa. Based on 3D-models reconstructed from µCT-scans of skulls, we examined the diversity of orientations of the LSC within one of the four major clades of placental mammals, that is, the superorder Xenarthra, with a data set that includes almost all extant genera and two extinct taxa. We observed a wide diversity of LSC orientations relative to the basicranium at both intraspecific and interspecific scales. The estimated phylogenetic imprint on the orientation of the LSC was significant but rather low within the superorder, though some phylogenetic conservatism was detected for armadillos that were characterized by a strongly tilted LSC. A convergence between extant suspensory sloths was also detected, both genera showing a weakly tilted LSC. Our preliminary analysis of usual head posture in extant xenarthrans based on photographs of living animals further revealed that the LSC orientation in armadillos is congruent with a strongly nose-down head posture. It also portrayed a more complex situation for sloths and anteaters. Finally, we also demonstrate that the conformation of the cranial vault and nuchal crests as well as the orientation of the posterior part of the petrosal may covary with the LSC orientation in Xenarthra. Possible inferences for the head postures of extinct xenarthrans such as giant ground sloths are discussed in the light of these results.

Mitogenomics phylogenetic relationships of the current sloth's genera and species (Bradypodidae and Megalonychidae)

We sequenced the complete mitogenome of 39 sloths (19 Bradypus variegatus, 4 B. tridactylus, 1 B. pygmaeus, 1 B. torquatus, 4 Choloepus didactylus, and 10 C. hoffmanni). A Bayesian tree (BI) indicated a temporal split between Bradypus and Choloepus around 31 million years ago (MYA, Oligocene) and the other major splits within each genera during the Miocene and Pliocene. A haplotype network (MJN) estimated a lower temporal split between the sloth genera (around 23.5 MYA). Both methods detected the ancestor of B. torquatus as the first to diverge within Bradypus (21 for BI and 19 MJN), followed by that of the ancestor of B. tridactylus. The split of B. pygmaeus from the common ancestor with B. variegatus was around 12 MYA (BI) or 4.3 MYA (MJN). The splits among the previous populations of B. variegatus began around 8 MYA (BI) or 3.6 MYA (MJN). The trans-Andean population was the first to diverge from the remaining cis-Andean populations of B. variegatus. The genetic differentiation of the trans-Andean B. variegatus population relative to the cis-Andean B. variegatus is similar to that found for different species of sloths. The mitogenomic analysis resolved the differentiation of C. hoffmanni from the C. didactylus individuals of the Guiana Shield. However, one C. didactylus from the Colombian Amazon specimen was inside the C. hoffmanni clade. This could be the first example of possible natural hybridization in the Amazon of both Choloepus taxa or the existence of un-differentiable phenotypes of these two species in some Amazonian areas.

The hidden teeth of sloths: evolutionary vestiges and the development of a simplified dentition

Xenarthrans are unique among mammals in retaining simplified teeth that are rootless and homodont, which makes it difficult to determine dental homologies. We apply computerized tomography to prenatal developmental series of extant sloths, Bradypus and Choloepus, to further elucidate the patterns of morphological variation in their dentition. We also propose new criteria based on sequences of dental mineralization, and the presence of vestigial teeth, to distinguish between caniniforms and postcaniniforms. We report for the first time the presence of vestigial incisors in Bradypus. We also show the presence of a vestigial tooth in front of the lower caniniform in both extant sloth genera and the existence of two generations for the upper caniniform in Choloepus. The study of their sequence of mineralization indicates that the lower and upper caniniform teeth are not homologous in sloths, and suggests that upper caniniforms are not homologous between the two extant sloth genera. Our results show that assessing the developmental processes and functional constraints remains crucial to understand the dental variations observed in sloths, and more generally, tooth class homology issues in mammals. Applied to the tooth row of all extinct sloths, these developmental data illuminate a potentially ancestral dental formula for sloths.

Morphological diversity of the bony labyrinth (inner ear) in extant Xenarthrans and its relation to phylogeny

We present a survey of the morphological diversity of the bony labyrinth of the inner ear in Xenarthra, including the fossil ground sloth Megatherium. Using a combination of traditional and geometric morphometrics, correlation analyses, and qualitative observations, we attempt to extract independent and informative phylogenetic characters of the bony labyrinth for the superorder. Geometric morphometric analyses demonstrate a strong imprint of phylogenetic history on the shape of the bony labyrinth of xenarthrans and a weak influence of allometry. Discrete characters mapped on a consensus cladogram for xenarthrans show support for many traditional nodes within the superorder and may also provide critical information for problematic nodes within Cingulata. A relatively large lateral semicircular canal may, for instance, represent a synapomorphy for the molecular clade allying fairy armadillos (Chlamyphorinae) to the Tolypeutinae. Striking convergences were detected when comparing Megatherium, the giant ground sloth, with extant armadillos and Chlamyphorus, the pink fairy armadillo, with the extant three- and two-toed sloths. These findings have the potential to help understand the phylogenetic relationships of fossil xenarthrans.

Presentamos un estudio de la diversidad morfológica del laberinto óseo del oído interno de los xenartros, incluyendo el perezoso fósil Megatherium. Utilizamos una combinación de morfométrica tradicional y geométrica, análisis de correlación y observaciones cuantitativas para intentar extraer caracteres filogenéticos independientes e informativos del laberinto óseo para el superorden. Los análisis geométricos morfométricos muestran una fuerte impronta de la historia filogenética de la forma del laberinto óseo de los xenartros y una baja influencia de la alometría. Los caracteres discretos mapeados en un cladograma de consenso para xenartros apoyan varios nodos tradicionales dentro del superorden y podrían también brindar información importante para los nodos problemáticos dentro de los Cingulata. Un canal semicircular lateral relativamente largo podría, por ejemplo, representar una sinapomorfía que apoye el clado molecular que une a los pichiciegos con los Tolypeutinae. Se hallaron notables convergencias al comparar Megatherium con los armadillos actuales, y Chlamyphorus con los perezosos actuales. Estos hallazgos tienen el potencial para ayudar a entender las relaciones filogenéticas de los xenartros fósiles.