Forelimb Myology of Carnivorous Marsupials (Marsupialia: Dasyuridae): Implications for the Ancestral Body Plan of the Australidelphia

Origin and distribution of the brachial plexus in red-necked wallaby (Notamacropus rufogriseus, Marsupialia: Macropodidae)

Notamacropus rufogriseus (red-necked wallaby) are in the family Macropodidae, which is the second largest family of marsupials after the family Didelphidae. This study was conducted with the aim of providing a detailed description of the origin and distribution of the brachial plexus in N. rufogriseus. Two-year-old male and 3-year-old female red-necked wallabies were used for the study. The brachial plexus was formed by ventral rami of C4, C5, C6, C7, C8, and T1 spinal nerves. It is composed of three trunks that give rise to 12 principal nerves. The cranial trunk is formed by the combination of the rami C4-C7; the middle trunk is formed by the combination of the rami C6 and C7; and the caudal trunk is formed by the combination of the rami C8 and T1. Differences between left and right side of the plexus brachialis were not observed. C6 ventral spinal rami contribute the most to brachial plexus nerve formation, while C4 contributes the least. The formation and distribution of the plexus in N. rufogriseus exhibited more resemblance to the patterns observed in marsupial animals rather than placental mammals. Marsupial mammals demonstrate the involvement of C4 in the development of the brachial plexus. The formation and branching of the brachial plexus sequentially adapt in accordance with changes in their thoracic limb activities and innervation points. Anatomical data from brachial plexus studies optimizes thoracic limb clinical and surgical treatments. This work can provide baseline data for future marsupial brachial plexus studies and fill gaps in the scarce literature.

The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I-Forelimb

This paper is the first in a two-part series that charts the evolution of appendicular musculature along the mammalian stem lineage, drawing upon the exceptional fossil record of extinct synapsids. Here, attention is focused on muscles of the forelimb. Understanding forelimb muscular anatomy in extinct synapsids, and how this changed on the line to mammals, can provide important perspective for interpreting skeletal and functional evolution in this lineage, and how the diversity of forelimb functions in extant mammals arose. This study surveyed the osteological evidence for muscular attachments in extinct mammalian and nonmammalian synapsids, two extinct amniote outgroups, and a large selection of extant mammals, saurians, and salamanders. Observations were integrated into an explicit phylogenetic framework, comprising 73 character-state complexes covering all muscles crossing the shoulder, elbow, and wrist joints. These were coded for 33 operational taxonomic units spanning >330 Ma of tetrapod evolution, and ancestral state reconstruction was used to evaluate the sequence of muscular evolution along the stem lineage from Amniota to Theria. In addition to producing a comprehensive documentation of osteological evidence for muscle attachments in extinct synapsids, this work has clarified homology hypotheses across disparate taxa and helped resolve competing hypotheses of muscular anatomy in extinct species. The evolutionary history of mammalian forelimb musculature was a complex and nonlinear narrative, punctuated by multiple instances of convergence and concentrated phases of anatomical transformation. More broadly, this study highlights the great insight that a fossil-based perspective can provide for understanding the assembly of novel body plans.

Description of the Pliocene marsupial Ambulator keanei gen. nov. (Marsupialia: Diprotodontidae) from inland Australia and its locomotory adaptations

Diprotodontids were the largest marsupials to exist and an integral part of Australian terrestrial ecosystems until the last members of the group became extinct approximately 40 000 years ago. Despite the frequency with which diprotodontid remains are encountered, key aspects of their morphology, systematics, ecology and evolutionary history remain poorly understood. Here we describe new skeletal remains of the Pliocene taxon Zygomaturus keanei from northern South Australia. This is only the third partial skeleton of a late Cenozoic diprotodontid described in the last century, and the first displaying soft tissue structures associated with footpad impressions. Whereas it is difficult to distinguish Z. keanei and the type species of the genus, Z. trilobus, on dental grounds, the marked cranial and postcranial differences suggest that Z. keanei warrants genus-level distinction. Accordingly, we place it in the monotypic Ambulator gen. nov. We, also recognize the late Miocene Z. gilli as a nomen dubium. Features of the forelimb, manus and pes reveal that Ambulator keanei was more graviportal with greater adaptation to quadrupedal walking than earlier diprotodontids. These adaptations may have been driven by a need to travel longer distances to obtain resources as open habitats expanded in the late Pliocene of inland Australia.

Behavioral correlates of fascicular organization: The confluence of muscle architectural anatomy and function

Muscle is a complex tissue that has been studied on numerous hierarchical levels: from gross descriptions of muscle organization to cellular analyses of fiber profiles. In the middle of this space between organismal and cellular biology lies muscle architecture, the level at which functional correlations between a muscle's internal fiber organization and contractile abilities are explored. In this review, we summarize this relationship, detail recent advances in our understanding of this form-function paradigm, and highlight the role played by The Anatomical Record in advancing our understanding of functional morphology within muscle over the past two decades. In so doing, we honor the legacy of Editor-in-Chief Kurt Albertine, whose stewardship of the journal from 2006 through 2020 oversaw the flourishing of myological research, including numerous special issues dedicated to exploring the behavioral correlates of myology across diverse taxa. This legacy has seen the The Anatomical Record establish itself as a preeminent source of myological research, and a true leader within the field of comparative anatomy and functional morphology.

Evolutionary and terminological analysis of the flexor digitorum superficialis, interflexorii and palmaris longus muscles in kinkajou (Potos flavus) and crab-eating racoon (Procyon cancrivorus)

The kinkajou (Potos flavus) and crab-eating raccoon (Procyon cancrivorus) are carnivores belonging to the family Procyonidae, but both species are characterized by different types of locomotion. Differences can be found in the adaptations that these two species present in the forearm muscles, such as the flexor digitorum superficialis (FDS), palmaris longus (PL) and interflexorii (IF), which have been described confusingly in previous studies. Therefore, the aim of this study was to describe these muscles in both species together with their innervations to carry out an evolutionary and terminological analysis among carnivorans. Both thoracic limbs were dissected from five P. flavus and three P. cancrivorus that had died of natural causes in Wildlife Care Centers. Two PL muscles (m. palmaris longus lateralis, PLL, and m. palmaris longus medialis, PLM) were found in P. flavus, and the IF were the only superficial flexors of the digits, whereas P. cancrivorus presented the IF and two bellies homologous to the two PLs of P. flavus, where the homologous belly of the PLM sent tendons to digits II-IV. Therefore, it was considered as the FDS due to its similarity to other carnivorans, and the lateral belly is the only PL present in P. cancrivorus. The topology, attachments and innervation of these muscles in P. flavus and P. cancrivorus allowed homologies to be established, hypothesizing their evolutionary derivation from the FDS. It also allowed the differences among PL, FDS and IF muscles to be described, concluding that most carnivorans do not have a PL.

Extreme bilateral polydactyly in a wild-caught western grey kangaroo

Polydactyly is a congenital malformation resulting from an autosomal dominant mutation manifesting as supernumerary digits of the hands or feet. It is most commonly reported in humans and domestic mammals, though there have also been isolated examples across a range of wild vertebrate species. Here we report a case of extremely unusual bilateral preaxial polydactyly on the pectoral limbs of a male western grey kangaroo (Macropus fuliginosus) from the South West region of Western Australia, in which two supernumerary digits were present on each manus. A supernumerary digit I on each manus was rudimentary in morphology without extrinsic muscular connections. However, supernumerary digit II present on each manus had fully developed extrinsic and intrinsic muscular connections, suggesting that these digits possessed normal function in flexion and extension. An alternative hypothesis is that the two supernumerary digits are both representatives of the most radial digit I, though this would then require the true digit I to have taken on the appearance of digit II by acquiring an additional phalanx and modified muscular attachments. The carpal bones exhibited a number of subtle differences in morphology when compared to normal pentadactyl individuals. The presence of a distal, rather than proximal, epiphysis on the first metacarpal was unexpected but further investigation suggested that this characteristic is perhaps more variable (in this species at least) than has been previously recognized. This case provides an unusual example to be considered within the broader context of limb development.

The extraordinary osteology and functional morphology of the limbs in Palorchestidae, a family of strange extinct marsupial giants

The Palorchestidae are a family of marsupial megafauna occurring across the eastern Australian continent from the late Oligocene through to their extinction in the Late Pleistocene. The group is known for their odd 'tapir-like' crania and distinctive clawed forelimbs, but their appendicular anatomy has never been formally described. We provide the first descriptions of the appendicular skeleton and body mass estimates for three palorchestid species, presenting newly-identified, and in some cases associated, material of mid-Miocene Propalorchestes, Plio-Pleistocene Palorchestes parvus and Pleistocene Palorchestes azael alongside detailed comparisons with extant and fossil vombatiform marsupials. We propose postcranial diagnostic characters at the family, genus and species level. Specialisation in the palorchestid appendicular skeleton evidently occurred much later than in the cranium and instead correlates with increasing body size within the lineage. We conclude that palorchestid forelimbs were highly specialised for the manipulation of their environment in the acquisition of browse, and that they may have adopted bipedal postures to feed. Our results indicate palorchestids were bigger than previously thought, with the largest species likely weighing over 1000 kg. Additionally, we show that P. azael exhibits some of the most unusual forelimb morphology of any mammal, with a uniquely fixed humeroulnar joint unlike any of their marsupial kin, living or extinct.