Skeletal Characters Research Articles

The axial skeleton of Tiktaalik roseae

The axial columns of the earliest limbed vertebrates show distinct patterns of regionalization as compared to early tetrapodomorphs. Included among their novel features are sacral ribs, which provide linkage between the vertebral column and pelvis, contributing to body support and propulsion by the hindlimb. Data on the axial skeletons of the closest relatives of limbed vertebrates are sparce, with key features of specimens potentially covered by matrix. Therefore, it is unclear in what sequence and under what functional context specializations in the axial skeletons of tetrapods arose. Here, we describe the axial skeleton of the elpistostegalian Tiktaalik roseae and show that transformations to the axial column for head mobility, body support, and pelvic fin buttressing evolved in finned vertebrates prior to the origin of limbs. No atlas-axis complex is observed; however, an independent basioccipital-exoccipital complex suggests increased mobility at the occipital vertebral junction. While the construction of vertebrae in Tiktaalik is similar to early tetrapodomorphs, its ribs possess a specialized sacral domain. Sacral ribs are expanded and ventrally curved, indicating likely attachment to the expanded iliac blade of the pelvis by ligamentous connection. Thus, the origin of novel rib types preceded major alterations to trunk vertebrae, and linkage between pelvic fins and axial column preceded the origin of limbs. These data reveal an unexpected combination of post-cranial skeletal characters, informing hypotheses of body posture and movement in the closest relatives of limbed vertebrates.

Boring systematics: A genome skimmed phylogeny of ctenostome bryozoans and their endolithic family Penetrantiidae with the description of one new species.

Ctenostomes are a group of gymnolaemate bryozoans with an uncalcified chitinous body wall having few external, skeletal characters. Hence, species identification is challenging and their systematics remain poorly understood, even more so when they exhibit an endolithic (boring) lifestyle. Currently, there are four Recent families of endolithic bryozoans that live inside mineralized substrates like mollusk shells. In particular, Penetrantiidae Silén, 1946 has received considerable attention and its systematic affinity to either cheilostomes or ctenostomes has been debated. Species delimitation of penetrantiids remains difficult, owing to a high degree of colonial and zooidal plasticity. Consequently, an additional molecular approach is essential to unravel the systematics of penetrantiids, their phylogenetic placement and their species diversity. We therefore sequenced the mitochondrial (mt) genomes and two nuclear markers of 27 ctenostome species including nine penetrantiids. Our phylogeny supports the Penetrantiidae as a monophyletic group placed as sister taxon to the remaining ctenostomes alongside paludicellids, arachnidioids and terebriporids. The boring family Terebriporidae d'Orbigny, 1847 were previously considered to be among vesicularioids, but our results suggest an arachnidioid affinity instead. Ctenostome paraphyly is supported by our data, as the cheilostomes nest within them. A Multiporata clade is also well supported, including the former victorelloid genus Sundanella. Altogether, this study provides new insights into ctenostome systematics, assists with species delimitation and contributes to our understanding of the bryozoan tree of life.

Skeletal traits and otoliths can unravel the relationships within European Gobiidae (Gobius lineage sensu lato)

Abstract The high diversity of extant gobiids (Gobiidae: Teleostei) makes taxonomic and phylogenetic interpretation of fossil members of the clade a difficult task. To facilitate future taxonomic and systematic work on the group, we have assembled a morphological reference database encompassing skeletal characters, an otolith atlas and otolith morphometric data of 25 present-day species from the European Gobius lineage (s.l.) that represent 18 different genera and include all nine sublineages. We show that: (1) skeletal traits and morphometric otolith variables can be diagnostic for a sublineage; (2) otolith morphology allows identification at the genus and species levels; and (3) the number of anal-fin rays and details of the otolith margins can be used to discriminate closely related dwarf gobies. The skeletal and otolith characters are largely stable in the marine gobies analysed here, whereas freshwater gobies (Padogobius, Ponto-Caspian gobies) are far more variable. This might be related to the conquest by Padogobius and Ponto-Caspian gobies of freshwater and low-salinity habitats, in which environmental conditions can fluctuate widely. We anticipate that the database presented here can be used as a valuable reference tool to assess the relationships of fossil gobiids and increase our knowledge of the evolutionary history of the group as a whole.

A new sponge genus from the Chengjiang biota with an intriguing combination of skeletal characters

The development of fossil sponge systematics is hindered by factors such as their low anatomical complexity and the low preservation potential of many characters used in the definition of extant sponge clades. Here we describe Calliospongia chunchengia gen. et sp. nov., a new sponge from the Lower Cambrian Chengjiang biota, which possesses a unique combination of characters. The skeleton of C. chunchengia is similar to those of leptomitid protomonaxonids in being composed of large spicules arranged longitudinally. However, it is distinct enough to belong in its own genus and even question a leptomitid affinity, because, unlike the members of this group, which usually have two categories of diactine spicules, its skeleton is entirely composed of triactines. The new taxon shares the presence of large megascleres with demosponges, hexactinellids and the ascosponges, suggesting an affinity to the total group of Silicea. It is also reminiscent of the hexactinellid-like reticulosans, showing a potential connection between two major types of Early Palaeozoic sponge body plans. A more accurate definition of the Cambro-Ordovician groups and the creation of a total evidence framework that integrates extinct and extant sponge morphology into the phylogenetic scenario derived from phylogenomics seem to be essential steps for the improvement of our understanding of early sponge evolution.

The importance of the appendicular skeleton for the phylogenetic reconstruction of lamniform sharks (Chondrichthyes: Elasmobranchii).

Lamniform sharks are one of the more conspicuous groups of elasmobranchs, including several emblematic taxa as the white shark. Although their monophyly is well supported, the interrelationships of taxa within Lamniformes remains controversial because of the conflict among various previous molecular-based and morphology-based phylogenetic hypotheses. In this study, we use 31 characters related to the appendicular skeleton of lamniforms and demonstrate their ability to resolve the systematic interrelationships within this shark order. In particular, the new additional skeletal characters resolve all polytomies that were present in previous morphology-based phylogenetic analyses of lamniforms. Our study demonstrates the strength of incorporating new morphological data for phylogenetic reconstructions.

Morphology and phylogenetic significance of the pelvic articular region in elasmobranchs (Chondrichthyes).

The morphology of paired fins is commonly overlooked in morphological studies, particularly the pelvic girdle and fins. Consequently, previous phylogenetic studies incorporating morphological data used few skeletal characters from this complex. In this paper, the phylogenetic significance of pelvic articular characters for elasmobranchs is discussed in light of the morphological variation observed in 130 species, the most comprehensive study exploring the morphology of the pelvic girdle done so far. The 10 morphological characters proposed herein for the pelvic articulation were incorporated into a molecular matrix of NADH2 sequences and submitted to an analysis of maximum parsimony employing extended implied weighting. The most stable tree was selected based on the distortion coefficients, SPR distances (subtree pruning and regrafting) and fit values. Some of the striking synapomorphies recovered within elasmobranchs include the presence of an articular surface for the first enlarged pelvic radial supporting Elasmobranchii and the pelvic articular region for the basipterygium extending from the posterolatral margin of the pelvic girdle over its lateral surface in Echinorhinus + Hexanchiformes. Additionally, the proposed characters and their distributions are discussed considering the relationships recovered and also compared with previous morphological and molecular phylogenetic hypotheses.

Morphology of hybrid marine turtles Caretta caretta x Chelonia mydas

We report on Caretta caretta x Chelonia mydas hybrid hatchlings that emerged from successive nests laid by a female Loggerhead Turtle (C. caretta) at Mon Repos, south-east Queensland, during the 1990–91 breeding season. We provide details of the parent female’s breeding history, which includes multiple paternity and genetic confirmation of her hybrid offspring. Three hybrid hatchlings were raised in captivity to assess whether scalation could be used to identify hybrid turtles in field studies. One captive hybrid died at 29 years of age, allowing an assessment of its reproductive status and examination of its skeletal anatomy. Two of the captive individuals were determined to be infertile. The reproductive status of the third individual was not determined. The external and skeletal morphology of the hybrids is discussed and illustrated. We show that atypical symmetrical scalation is diagnostic for C. caretta x C. mydas hybrids, as are various buccal and skeletal characters.

Further evidence in support of the recognition of the Freshwater Turtle Elseya oneiros (Testudines: Chelidae) from the Nicholson and Gregory Rivers of Northern Queensland

Thomson et al. (1997) proposed that the extant Elseya from the Nicholson-Gregory drainages in northwestern Queensland was conspecific with the holotype of Elseya lavarackorum, which comprised a fossil carapace and associated plastron excavated from the late Pleistocene Terrace Site at Riversleigh. Analysis of additional fossil material, and examination of a suite of 16 scute characters (eight for each of the carapace and plastron) by Joseph-Ouni et al. (2020) concluded that the two species were distinct and represented separate Elseya lineages and proposed the new name Elseya oneiros for the extant lineage. Recently, the Turtle Taxonomy Working Group (TTWG) in their 9th edition Checklist of Turtles of the World (TTWG 2021) questioned the status of E. oneiros and placed it in the synonymy of E. lavarackorum, making the claim, amongst others, that the skeletal characters of Thomson et al. (1997) were not addressed. Here we fully address the claims made and evaluate those skeletal characters. We also assess an additional nine thoracic skeletal characters of the pertinent Elseya species, including E. dentata sensu stricto. The results again do not support the conclusions of Thomson et al. (1997) of the holotype of Elseya lavarackorum being conspecific with the extant Elseya from the Nicholson-Gregory Rivers, reaffirms the proposal offered by Joseph-Ouni et al. (2020) for the placement of E. lavarackorum in the subgenus Elseya, and the status of E. oneiros in the Nicholson-Gregory drainages as a distinct species.

Interactive identification key to all brittle star families (Echinodermata; Ophiuroidea) leads to revised morphological descriptions

Ophiuroidea is the largest class among extant echinoderms, with over 2000 described species assigned to 33 families. Here, the first identification key to the recently revised classification was developed, and revised morphological descriptions were derived from it, expanding the previous short diagnoses. The key was built by analyzing internal and external skeletal characters of predominantly the type species of each family, including at least two mutually exclusive attributes per family. Various numeric and multistate characters were used to create a traditional as well as an interactive key using the DELTA and Xper software programs­. Illustrations (SEM and digital photos) are included in the key to facilitate the assessment of character states by users. Not only is it the first identification key to the families, according to the recently proposed new classification and the examined species, but this interactive key also assists users in understanding the family level taxonomy of brittle stars. The interactive key allows new characters and states to be added, when more species will have been analyzed, without the need to reconfigure the complete key (as may be necessary with conventional keys).

Apprehending novel biodiversity redux – thirteen new genera and three new families of Zealandian Bryozoa, with the first living species of the Eocene‒Miocene genus Vincularia (Vinculariidae)

AbstractThirteen new genera and three new families of cheilostome Bryozoa are described from the New Zealand biogeographic region, centred on Zealandia, between 26.42° (northern Norfolk Ridge) and 54.02°S latitude (south-east Campbell Plateau) from coastal waters to bathyal depths (46‒1676 m). The new genera, comprising 15 new and one previously described Zealandian species, are: Elementella (Elementellidae n. fam.), Niwapora, Quasicallopora, Quitocallopora, Judyella (Calloporidae), Ellisantropora (Antroporidae), Rhizellisina (Ellisinidae), Radixenia (Calloporoidea incertae sedis), Granomurus (Granomuridae n. fam.), Carolanna (Bugulidae), Borioplebs (Borioplebidae n. fam.), Seabournea (Cribrilinidae) and Waeschenbachia (Romancheinidae). A new Recent species of the otherwise Eocene‒Miocene genus Vincularia is also described from deep water on the Three Kings Ridge. Two new combinations are created – Judyella precocialis (Gordon, 1984) and Ellisantropora aggregata (Gordon, 1984). Ellisantropora tilbrooki sp. nov. is also introduced for a species from Torres Strait. Four of the deep-sea genera (three from one station on the northern Norfolk Ridge, one from a station on the Three Kings Ridge) have species with plesiomorphies or distinctive skeletal characters that suggest they are relict. Almost half of the new species form spot-like colonies on hard substrata.

Redefining the study of sexual dimorphism in bats: following the odour trail

Abstract Sexual dimorphism is the condition in which males and females of the same species are easily distinguished by specific traits (secondary sex characteristics), often related to body size, colour patterns, weapons, and ornaments. Males of many mammal species tend to be larger or more ornamented than females, and these characteristics tend to be more pronounced in polygynous, diurnal, and open‐habitat species. Bats have long been considered a largely non‐sexually dimorphic group due to lack of conspicuous differences in body size and other cranial and skeletal characters. However, bats, like many mammals, exhibit a diverse array of soft‐tissue integumentary glands and non‐glandular odour‐producing structures with intense odorous substances that have not been thoroughly investigated, although postulated functions include facilitating mate selection in their generally polygynous associations. To date, there has been no systematic assessment of the occurrence or expression of sexually dimorphic traits in bats, many of which show intriguing sexual dimorphism in soft tissue, and most of which involve intense odours. In this study, we review evidence of integumentary glands and non‐glandular odour‐producing structures known in bats, as a first step towards identifying future research pathways to study sexual dimorphism in bats. Highly variable glands and non‐glandular odour‐producing structures have been noticed in ten different regions of the body, but are most frequently found on the head and the ventral region of the neck. They have been described in nearly 9% of bat species and in 70% of 21 extant bat families. Our review, based on extremely scattered and unevenly detailed literature, unveils the extraordinary sexual dimorphism that has been observed in Chiroptera to date, identifying not only target body parts where sexually dimorphic traits are likely to be found, but also critical avenues for future investigation and discoveries, and stressing the importance of the timing of secondary sexual trait observations, behavioural studies, and chemical analyses.

Static allometry of a small-bodied omnivore: body size and limb scaling of an island fox and inferences for Homo floresiensis

Island dwarfing is a paraphyletic adaptation across numerous mammalian genera. From mammoths to foxes, extreme body size reduction is shared by diverse organisms that migrate to an island environment. Because it largely occurs owing to ecological variables, not phylogenetic ones, skeletal characters in a dwarfed taxon compared with its ancestor may appear abnormal. As a result, allometric patterns between body size and morphological traits may differ for an island dwarf compared with its ancestor. The diminutive Late Pleistocene hominin, Homo floresiensis, displays a unique character suite that is outside of the normal range of variation for any extinct or extant hominin species. To better explain these as ecological traits due to island dwarfing, this research looks at how dwarfing on islands influences limb scaling and proportions in an organism in a similar ecological niche as H. floresiensis. Here, I analyze absolute limb lengths and static allometry of limb lengths regressed on predicted body mass of dwarfed island foxes and their nondwarfed relatives. Dwarfed island foxes have significantly smaller intercepts but steeper slopes of all limb elements regressed on predicted body mass than the mainland gray fox. These allometric alterations produce limbs in the island fox that are significantly shorter than predicted for a nondwarfed gray fox of similar body mass. In addition, the humerofemoral, intermembral, and brachial indices are significantly different. These results provide a novel model for understanding skeletal variation of island endemic forms. Unique body size and proportions of H. floresiensis are plausible as ecological adaptations and likely not examples of symplesiomorphies with Australopithecus sp. Caution should be exerted when comparing an island dwarf with a closely related species as deviations from allometric expectations may be common.

Intraspecific Variation in the External and Skeletal Morphology of Hynobius setouchi Matsui, Okawa, Tanabe et Misawa, 2019 (Amphibia: Urodela: Hynobiidae).

The hynobiid salamander genus Hynobius is a basal taxon in the urodeles that plays a key role in understanding the evolution of sexual dimorphism in urodeles. We examined the age (juveniles vs. adults) and sexual differences in the external and skeletal morphology of a Japanese lentic breeding salamander, Hynobius setouchi. We found that juveniles had larger heads than those of adults, and most characters were larger in adult males than in females, except for trunk size. This was also observed in skeletal morphology, although some tail characters differed only in the skeleton. We suggest that intra- and interspecific comparisons of the external and skeletal characters of salamanders should consider age, sexual, and seasonal differences.

Skeletal development and adult osteology of Hypsiboas pulchellus (Anura: Hylidae)

Osteological and skeletal characters have long been proven to be particularly informative in taxonomic and systematic research. Furthermore, ossification sequences are assumed to be a potential tool to investigate developmental states and developmental modes of fossil and extant skeletal specimens. Herein, we provide a detailed account on adult osteology and skeletogenesis in the Montevideo treefrog, Hypsiboas pulchellus (Anura: Hylidae) based on evaluation of a series of cleared and stained specimens. A consensus sequence of ossification, i.e., the order of appearance of mineralized elements until early metamorphosis could be determined as (parasphenoid, presacral vertebrae I-VII, frontoparietal, exoccipital) – transverse processes of presacral vertebrae I-VIII – sacral vertebra – (humerus, radioulna, ilium, femur, tibiofibula, scapula) – (cleithrum, clavicle, coracoids, metacarpals, tarsals, metatarsals, phalanges, hypochord) – (prootic, angulosplenial, dentary, maxilla, premaxilla, squamosal). Comparing the state of mineralized elements in individual specimens, a number of skeletal elements, including the exoccipital, frontoparietal, parasphenoid and prootic, as well as elements of the shoulder and pelvic girdles, and the phalanges, were found to vary intraspecifically regarding the relative time of their ossification within the ossification sequence.

In search of predictive models for stenolaemate morphometry across the skeletal–polypide divide

AbstractMarine bryozoans have been members of benthic skeletal faunas since the Ordovician. These small suspension feeders collect particles in the range of 10 to 100 μm. Specific details of their feeding depend on the morphology of the feeding apparatus, which may be reflected in skeletal characters. While several studies have described the link between the skeletal and soft-body traits of gymnolaemate bryozoans, stenolaemates have received less attention. To fill this gap, we conducted a detailed analysis of morphometry within and across species and attempted to develop robust predictive models that can be used to infer the soft-body morphology from skeletal data. This, in turn, will help with extracting data on ecology of Paleozoic communities of suspension feeders from the extensive bryozoan fossil record. Characters of polypide morphology among New Zealand cyclostomates (single Recent order in Stenolaemata) displayed staggering variability and almost without exception were not connected to skeletal characters at the species level. When this variability is reduced to its central tendency, interspecific trends are more apparent. The relationship is positive, linear, and moderately strong, but the resulting models have wide predictive intervals (plus/minus hundreds of micrometers). A precise estimate of the characters of the feeding apparatus of modern, and especially fossil, stenolaemates may be difficult to attain, at least on the basis of the skeletal traits used here.

The comparative and functional anatomy of appendicular musculature of Humboldt’s woolly monkey (Lagothrix lagotricha): What can a (mediocre) suspensory monkey tell us about and human locomotor evolution?

Similar to other members of the platyrrhine subfamily Atelinae, Humboldt’s wooly monkey (Lagothrix lagotricha) isa ‘tail‐assisted’ brachiator that more frequently engages in non‐suspensory forms of locomotion (i.e. quadrupedal climbing). As a group the atelines share a suite of derived forelimb skeletal characters that are specialized for brachiation and that are convergent with ‘richochetal’ brachiating apes (Hylobates, Symphalangus), quadrumanus suspensory apes (Pongo) and secondarily terrestrial apes retaining suspensory capabilities (Pan, Gorilla). The skeletal anatomy of lagothrix is similar to more suspensory atelines (Ateles, Brachyteles) and living apes, but is typically less derived for suspension and many skeletal characters are intermediate between highly suspensory and non‐suspensory anthropoids. While the functional morphology of the ateline skeleton is well documented, relatively little is known about the comparative and functional morphology of the forelimb and hindlimb musculature. Specifically, can locomotor differences between Lagothrix and more suspensory atelines and hylobatids be explained, in part, as a result of differences in muscletendon architecture?This study describes preliminary quantitative data on the muscle–tendon architecture (relative muscle mass and relative physiological cross‐sectional area or PCSA) of the fore‐ and hind‐limbs of Lagothrix lagotricha and Macacasp. (n=9). Despite infrequent brachiation, Lagothrix forelimb relative muscle mass and PCSA is identical to similar results reported for hominoids. In contrast, the relative mass and PCSA of the hind‐limb musculature is most similar to quadrupedal monkeys (i.e. Macaca). Specifically, Lagothrix resembles hylobatids in the concentration of muscle mass and force in the flexor compartments of the arm and forearm but is more similar to Macaca in similar measures associated with the extensor and flexor thigh and leg. This suggests that despite a reduced reliance on suspensory postures and brachiation, Lagothrix forelimb muscle‐tendon architecture may be, at least in part, a response to the minimum functional demands of brachiation but that similar selection pressures are not as prevalent in the hind‐limb.

Bashkirian Rugosa (Anthozoa) from the Donets Basin (Ukraine). Part 9. The Subfamily Dirimiinae, subfam. nov.

A new Subfamily Dirimiinae of the Family Kumpanophyllidae Fomichev, 1953 is introduced on the basis of Dirimia gen. nov., which is represented by six new named species and three species left in open nomenclature. The new species are Dirimia multiplexa, D. similis, D. recessia, D. composita, D. extrema, D. nana, Dirimia sp. 1, Dirimia sp. 2 and Dirimia sp. 3. The progressing atrophy of the columnotheca, leading to its total reduction in extreme species, and the occurrence of an axial structure instead of a compact pseudocolumella established in these species are accepted as differences exceeding the genus level. All specimens assigned to this subfamily were derived from the same Limestone F1 of the Donets Basin, and mostly from the same locality. The reasons for their split into a relatively large number of species are: 1) an increased radiation typical for faunal turnover times; 2) a delay in the appearance of differentiated skeletal characters relative to the appearance of genetic differences large enough to characterise different species; 3) a bias in preservation of fossil remnants by comparison to living populations, amplified by biases in the collections available for study by comparison to the total number of specimens fossilised.

Appendicular skeleton of Protoceratops andrewsi (Dinosauria, Ornithischia): comparative morphology, ontogenetic changes, and the implications for non-ceratopsid ceratopsian locomotion.

Protoceratops andrewsi is a well-known ceratopsian dinosaur from the Djadokhta Formation (Upper Cretaceous, Mongolia). Since the 1920s, numerous skeletons of different ontogenetic stages from hatchlings to adults, including fully articulated specimens, have been discovered, but the postcranial anatomy of Protoceratops has not been studied in detail. A new, mostly articulated subadult individual provides an excellent opportunity for us to comprehensively describe the anatomy of the limb skeleton, to compare to other ceratopsian dinosaurs, and to study the ontogenetic and intraspecific variation in this species. New data provided by the specimen shed light on the lifestyle of P. andrewsi. The young subadult individuals present an array of morphological characters intermediate between the bipedal Psittacosaurus and fully quadrupedal adult P. andrewsi. We compare these observations with a broad range of non-ceratopsid Neoceratopsia (of various locomotor adaptations) and Psittacosauridae (obligate bipeds), which gives us insight into the evolution of the skeletal characters informative for the postural change in ceratopsian dinosaurs.

Mosaic of plesiomorphic and derived characters in an Eocene myliobatiform batomorph (Chondrichthyes, Elasmobranchii) from Italy defines a new, basal body plan in pelagic stingrays

BackgroundEnd-Cretaceous niche-filling by benthic Mesozoic survivors resulted in a prominent increase of durophagous fish families, resulting in the appearance of the earliest representatives of several extant fish lineages, including the pelagic durophagous stingrays, a monophyletic clade of myliobatiform batoids that is characterized by a derived swimming mode and feeding habits. Although the earliest members appeared in the Late Cretaceous, most of the crown genera date back to the Eocene.ResultsIn this study, we re-examine the anatomy of the Eocene eagle ray Promyliobatis gazolai (de Zigno), represented by two nearly complete and articulated specimens from the world-famous Ypresian Konservat-Lagerstätte of Bolca, in detail. This taxon exhibits a mosaic of plesiomorphic and derived characters (e.g. tail sting displaced posteriorly on the tail, at about 50–60% of tail length; pectoral fins joining in front of the head; anterior and posterior pectoral fin margins nearly straight; compagibus laminam absent; single, unfragmented mesopterygium) that clearly define a new body plan within the pelagic durophagous stingrays.ConclusionsThe significant morphological differences between Promyliobatis and extant representatives of Myliobatidae, Aetobatidae, Rhinopteridae, and Mobulidae, support its placement as separate stem group member. The phylogenetic placement of Promyliobatis, based on skeletal and dental characters, strongly supports its basal position within pelagic stingrays. However, its position within the Myliobatiformes becomes unstable when stingray taxa known by fossil teeth only are included. A comparative analysis of the skeletal and tooth morphologies, as well as of the evolutionary trends of pelagic stingrays is also discussed.

The comparative and functional anatomy of the fore‐ and hind‐limb musculature of Humboldt's woolly monkey (Lagothrix lagotricha): New insights into ape and human evolution from a (mediocre) suspensory platyrrhine

Similar to other members of the platyrrhine subfamily Atelinae, Humboldt's wooly monkey (Lagothrix lagotricha) is a ‘tail‐assisted’ brachiator that more frequently engages in non‐suspensory forms of locomotion (i.e. quadrupedal climbing). As a group the atelines share a suite of derived forelimb skeletal characters that are specialized for brachiation and that are convergent with ‘richochetal’ brachiating apes (Hylobates, Symphalangus), quadrumanus suspensory apes (Pongo) and secondarily terrestrial apes retaining suspensory capabilities (Pan, Gorilla). The skeletal anatomy of lagothrix is similar to more suspensory atelines (Ateles, Brachyteles) and living apes, but is typically less derived for suspension and many skeletal characters are intermediate between highly suspensory and non‐suspensory anthropoids. While the functional morphology of the ateline skeleton is well documented, relatively little is known about the comparative and functional morphology of the forelimb and hindlimb musculature. Specifically, can locomotor differences between Lagothrix and more suspensory atelines and hylobatids be explained, in part, as a result of differences in muscle‐tendon architecture?This study describes preliminary quantitative data on the muscle–tendon architecture (relative muscle mass and relative physiological cross‐sectional area or PCSA) of the fore‐ and hind‐limbs of Lagothrix lagotricha and Macaca sp. (n=9). Despite infrequent brachiation, Lagothrix forelimb relative muscle mass and PCSA is identical to similar results reported for hominoids. In contrast, the relative mass and PCSA of the hind‐limb musculature is most similar to quadrupedal monkeys (i.e. Macaca). Specifically, Lagothrix resembles hylobatids in the concentration of muscle mass and force in the flexor compartments of the arm and forearm but is more similar to Macaca in similar measures associated with the extensor and flexor thigh and leg. This suggests that despite a reduced reliance on suspensory postures and brachiation, Lagothrix forelimb muscle‐tendon architecture may be, at least in part, a response to the minimum functional demands of brachiation but that similar selection pressures are not as prevalent in the hind‐limb.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.