Cranial ontogeny ofCaluromys philander(Didelphidae: Caluromyinae): a qualitative and quantitative approach

Morphological traits explain the individual position within resource-consumer networks of a Neotropical marsupial

Knowledge regarding the influence of individual traits on interaction patterns in nature can help understand the topological role of individuals within a network of intrapopulation interactions. We tested hypotheses on the relationships between individuals' positions within networks (specialization and centrality) of 4 populations of the mouse opossum Gracilinanus agilis and their traits (i.e., body length, body condition, tail length relative to body length, sex, reproductive condition, and botfly parasitism) and also seasonal effects in the Brazilian savanna. Individuals with lower body length, better body condition, and relatively shorter tail were more specialized (i.e., less connected within the network). Individuals were also more specialized and less connected during the warm-wet season. The relationship between individuals' position in the network and body traits, however, was independent of season. We propose that specialization may arise not only as a result of preferred feeding strategies by more capable individuals (i.e., those with better body condition and potentially prone to defend and access high-quality food resources) but also because of morphological constraints. Smaller/younger individuals (consequently with less experience in foraging) and short-tailed individuals (less skilled to explore the vertical strata of the vegetation) would feed only on a subset of the available food resources and consequently become more specialized. Moreover, individuals are more specialized during the warm-wet season because of high competition (population-dense period) and higher ecological opportunities (resource-rich period). Therefore, our study reveals the relevance of individual traits in shaping interaction patterns and specialization in populations.

Patterns of ontogenetic evolution across extant marsupials reflect different allometric pathways to ecomorphological diversity

The relatively high level of morphological diversity in Australasian marsupials compared to that observed among American marsupials remains poorly understood. We undertake a comprehensive macroevolutionary analysis of ontogenetic allometry of American and Australasian marsupials to examine whether the contrasting levels of morphological diversity in these groups are reflected in their patterns of allometric evolution. We collate ontogenetic series for 62 species and 18 families of marsupials (n = 2091 specimens), spanning across extant marsupial diversity. Our results demonstrate significant lability of ontogenetic allometric trajectories among American and Australasian marsupials, yet a phylogenetically structured pattern of allometric evolution is preserved. Here we show that species diverging more than 65 million years ago converge in their patterns of ontogenetic allometry under animalivorous and herbivorous diets, and that Australasian marsupials do not show significantly greater variation in patterns of ontogenetic allometry than their American counterparts, despite displaying greater magnitudes of extant ecomorphological diversity.

Vestigial structures and variation in the evolution of the marsupial mammal dental development—a study of the woolly opossum Caluromys philander

The pattern of dental replacement in marsupial mammals has received much attention for its derived nature and potential relationship to the life history of the group. However, few species have been studied thoroughly, and little is known about the embryonic structures and their use in addressing issues of homology and dental evolution in general. We studied a developmental series of ten individuals of pouch young Caluromys philander to thoroughly document dental development with histological sections and 3D models of dental series. We report that the successor P3 arises from a lingual successional lamina from its predecessor dP3. The germs of vestigial, unerupted deciduous incisors and canines are present alongside their respective permanent successors. These discoveries demonstrate significant differences from the developmental patterns reported for Didelphis and Monodelphis and illustrate that an unsuspected diversity of dental ontogeny is not reflected in the adult pattern of mineralised, erupted or almost erupted teeth.

Postnatal cranial growth of Risso’s dolphin (Grampus griseus)

The short-nosed Risso’s dolphin (

Phylogeny explains better than ecology or body size the variation of the first lower molar in didelphid marsupials

Tribosphenic molars are considered great innovations in mammals and are related to several structures and variables that can explain adaptation. The aim of this study was to investigate the importance of body size and habitat relation, using a phylogenetic approach, in the first lower molar shape in didelphid marsupials. Geometric morphometric analyses of the lower molar’s shape were performed on 261 specimens, 130 females and 131 males, covering 14 genera and 37 species of the Didelphidae family. The molar conformation showed a larger talonid in relation to the trigonid in more arboreal genera, and narrower and longer molars in genera with a larger body size. Phylogeny was the variable with the highest explanation for both females and males (16.17% and 9.02%, respectively). The body size was significant in males, presenting an important influence on molar shape, while the body size in females was not significant when phylogenetic relationship was controlled for. In both sexes, habitat presents a strong effect of phylogeny, with no direct effect on molar shape. Didelphid molar shape is another result of its phylogenetic history and does not respond very much to environmental pressures. Male body size influences molar shape in didelphids, even in the presence of a strong phylogenetic signal.

A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials

Marsupial mammal relatives (stem metatherians) from the Mesozoic Era (252-66 million years ago) are mostly known from isolated teeth and fragmentary jaws. Here we report on the first near-complete skull remains of a North American Late Cretaceous metatherian, the stagodontid Didelphodon vorax. Our phylogenetic analysis indicates that marsupials or their closest relatives evolved in North America, as part of a Late Cretaceous diversification of metatherians, and later dispersed to South America. In addition to being the largest known Mesozoic therian mammal (node-based clade of eutherians and metatherians), Didelphodon vorax has a high estimated bite force and other craniomandibular and dental features that suggest it is the earliest known therian to invade a durophagous predator-scavenger niche. Our results broaden the scope of the ecomorphological diversification of Mesozoic mammals to include therian lineages that, in this case, were linked to the origin and evolution of marsupials.

Mammalian development does not recapitulate suspected key transformations in the evolutionary detachment of the mammalian middle ear

The ectotympanic, malleus and incus of the developing mammalian middle ear (ME) are initially attached to the dentary via Meckel's cartilage, betraying their origins from the primary jaw joint of land vertebrates. This recapitulation has prompted mostly unquantified suggestions that several suspected--but similarly unquantified--key evolutionary transformations leading to the mammalian ME are recapitulated in development, through negative allometry and posterior/medial displacement of ME bones relative to the jaw joint. Here we show, using µCT reconstructions, that neither allometric nor topological change is quantifiable in the pre-detachment ME development of six marsupials and two monotremes. Also, differential ME positioning in the two monotreme species is not recapitulated. This challenges the developmental prerequisites of widely cited evolutionary scenarios of definitive mammalian middle ear (DMME) evolution, highlighting the requirement for further fossil evidence to test these hypotheses. Possible association between rear molar eruption, full ME ossification and ME detachment in marsupials suggests functional divergence between dentary and ME as a trigger for developmental, and possibly also evolutionary, ME detachment. The stable positioning of the dentary and ME supports suggestions that a 'partial mammalian middle ear' as found in many mammaliaforms--probably with a cartilaginous Meckel's cartilage--represents the only developmentally plausible evolutionary DMME precursor.

Cranial Ontogeny of the Early Triassic Basal Cynodont Galesaurus planiceps

Ontogenetic changes in the skull and mandible of thirty-one specimens of Galesaurus planiceps, a basal non-mammaliaform cynodont from the Early Triassic of South Africa, are documented. The qualitative survey indicated eight changes in the craniomandibular apparatus occurred during growth, dividing the sample into three ontogenetic stages: juvenile, subadult, and adult. Changes in the temporal region, zygomatic arch, occiput, and mandible occurred during the transition from the subadult to adult stage at a basal skull length of 90 mm. At least four morphological and allometric differences divided the adult specimens into two morphs, indicating the presence of sexual dimorphism in Galesaurus. Differences include extensive lateral flaring of the zygomatic arches in the "male" morph resulting in a more anterior orientation of the orbits, and a narrower snout in the "female". This is the first record of sexual dimorphism in a basal cynodont, and the first time it is quantitatively documented in a non-mammaliaform cynodont. An ontogenetic comparison between Galesaurus and the more derived basal cynodont Thrinaxodon revealed differences in the timing and extent of sagittal crest development. In Galesaurus, the posterior sagittal crest, located behind the parietal foramen, developed relatively later in ontogeny, and the anterior sagittal crest rarely formed suggesting the anterior fibres of the temporalis were less developed than in Thrinaxodon. In contrast, craniomandibular features related to the masseters became more developed during the ontogeny of Galesaurus. The development of the adductor musculature appears to be one of the main factors influencing skull growth in these basal non-mammaliaform cynodonts. Anat Rec, 300:353-381, 2017. © 2016 Wiley Periodicals, Inc.

Morphological diversity in tenrecs (Afrosoricida, Tenrecidae): comparing tenrec skull diversity to their closest relatives

It is important to quantify patterns of morphological diversity to enhance our understanding of variation in ecological and evolutionary traits. Here, we present a quantitative analysis of morphological diversity in a family of small mammals, the tenrecs (Afrosoricida, Tenrecidae). Tenrecs are often cited as an example of an exceptionally morphologically diverse group. However, this assumption has not been tested quantitatively. We use geometric morphometric analyses of skull shape to test whether tenrecs are more morphologically diverse than their closest relatives, the golden moles (Afrosoricida, Chrysochloridae). Tenrecs occupy a wider range of ecological niches than golden moles so we predict that they will be more morphologically diverse. Contrary to our expectations, we find that tenrec skulls are only more morphologically diverse than golden moles when measured in lateral view. Furthermore, similarities among the species-rich Microgale tenrec genus appear to mask higher morphological diversity in the rest of the family. These results reveal new insights into the morphological diversity of tenrecs and highlight the importance of using quantitative methods to test qualitative assumptions about patterns of morphological diversity.

Statistical support for the hypothesis of developmental constraint in marsupial skull evolution

Background

In contrast to placental neonates, in which all cranial bones are ossified, marsupial young have only the bones of the oral region and the exoccipital ossified at birth, in order to facilitate suckling at an early stage of development. In this study, we investigated whether this heterochronic shift in the timing of cranial ossification constrains cranial disparity in marsupials relative to placentals.

Methods

We collected three-dimensional (3D) landmark data about the crania of a wide range of extant placentals and marsupials, and from six fossil metatherians (the clade including extant marsupials and their stem relatives), using a laser scanner and a 3D digitizer. Principal components analysis and delta variance tests were used to investigate the distribution and disparity of cranial morphology between different landmark sets (optimizing either number of landmarks or number of taxa) of the whole skull and of individual developmental or functional regions (neurocranium, viscerocranium, oral region) for extant placentals and marsupials. Marsupial and placental data was also compared based on shared ecological aspects including diet, habitat, and time of peak activity.

Results

We found that the extant marsupial taxa investigated here occupy a much smaller area of morphospace than the placental taxa, with a significantly (P<0.01) smaller overall variance. Inclusion of fossil taxa did not significantly increase the variance of metatherian cranial shape. Fossil forms generally plotted close to or within the realm of their extant marsupial relatives. When the disparities of cranial regions were investigated separately, significant differences between placentals and marsupials were seen for the viscerocranial and oral regions, but not for the neurocranial region.

Conclusion

These results support the hypothesis of developmental constraint limiting the evolution of the marsupial skull, and further suggest that the marsupial viscerocranium as a whole, rather than just the early-ossifying oral region, is developmentally constrained.

Ontogeny of the cranial system in Laonastes aenigmamus

Rodents, together with bats, are among the ecologically most diverse and most speciose groups of mammals. Moreover, rodents show elaborate specializations of the feeding apparatus in response to the predominantly fore-aft movements of the lower jaw. The Laotian rock rat Laonastes aenigmamus was recently discovered and originally thought to belong to a new family. The difficulties in classifying L. aenigmamus based on morphological characters stem from the fact that it presents a mixture of sciurognathous and hystricognathous characteristics, including the morphology of the jaw adductors. The origin of the unusual muscular organization in this species remains, however, unclear. Here, we investigate the development of the masticatory system in Laonastes to better understand the origin of its derived morphology relative to other rodents. Our analyses show that skull and mandible development is characterized by an overall elongation of the snout region. Muscle mass increases with positive allometry during development and growth, and so does the force-generating capacity of the jaw adductor muscles (i.e. physiological cross-sectional area). Whereas fetal crania and musculature are more similar to those of typical rodents, adults diverge in the elongation of the rostral part of the skull and the disproportionate development of the zygomaticomandibularis. Our data suggest a functional signal in the development of the unusual cranial morphology, possibly associated with the folivorous trophic ecology of the species.