The morphology of female bopyrids is adapted to parasitism, but understanding the function of their thoracic and mouth appendages is hindered by their small size and cryptic lifestyle, limiting detailed examination. This study aimed to clarify the function of the first oostegites and maxillipeds in bopyrid isopods infesting the branchial chamber of caridean shrimp through behavioural observations and morphological examination. We tested whether the movement of these structures was exclusive to ovigerous female parasites during brood ventilation. The results revealed that the beating of the maxillipeds and flapping of the first oostegites were not restricted to ovigerous females. However, the frequency of these movements was significantly higher in ovigerous females than in non-ovigerous females. The frequency of maxilliped beating increased with embryonic development, whereas that of flapping the first oostegites exhibited the opposite trend. Microscopic observation using dye showed that the movements of the maxillipeds and the first oostegites expelled residual dye from the female brood chamber through the dorsal surface or beneath the first oostegites. The dye was then transported by the water current generated by the scaphognathite of the host shrimp. These findings suggest that these structures not only facilitate ventilation but also serve as a grooming mechanism for female parasites, which is critical for embryonic survival. The results of the present study represent the first observation of embryo grooming in bopyrid isopods. This study also provides new information on the functional morphology of bopyrid isopods, which is important for understanding their ecological dynamics and adaptation to parasitism.

The form–function conceptual framework, which assumes a strong relationship between the structure of a particular trait and its function, has been crucial for understanding morphological variation and locomotion among extant and fossil species across many disciplines. In biological anthropology, it is the lens through which many important questions and hypotheses have been tackled with respect to relationships between morphology and locomotor kinematics, energetics and performance. However, it is becoming increasingly evident that the morphologies of fossil hominins, apes and humans can confer considerable locomotor diversity and flexibility, and can do so with a range of kinematics depending on soft tissue plasticity and environmental and cultural factors. This complexity is not built into traditional biomechanical or mathematical models of relationships between structure and kinematics or energetics, limiting our interpretation of what bone structure is telling us about behaviour in the past. The nine papers presented in this Special Collection together address some of the challenges that variation in the relationship between form and function pose in evolutionary biomechanics, to better characterise the complexity linking structure and function and to provide tools through which we may begin to incorporate some of this complexity into our functional interpretations.

The aquatic diplocaulid nectridean Keraterpeton galvani is the commonest taxon represented in the Jarrow Coal assemblage from Kilkenny, Ireland. The Jarrow locality has yielded the earliest known Carboniferous coal-swamp fauna in the fossil record and is, therefore, of importance in understanding the history and diversity of the diplocaulid clade. The morphology of Keraterpeton is described in detail with emphasis on newly observed anatomical features. A reconstruction of the palate includes the presence of interpterygoid vacuities and new morphological details of the pterygoid, parasphenoid and basicranial region. The hyoid apparatus comprising an ossified basibranchial element has not been reported previously in nectrideans. The structure of the scapulocoracoid and primitive nature of the humerus is described and the presence of a five-digit manus confirmed. Previously unrecognised accessory dermal ossifications are present in the pectoral girdle. Keraterpeton longtoni from the Bolsovian in Staffordshire, England, is also described and newly figured. The primitive condition in diplocaulids is defined on the basis of the earliest occurrence at Jarrow and discussed in relation to functional morphology and mode of life. The evolution of the diplocaulid clade is assessed in relation to the revised diagnoses that define the primitive condition in Keraterpeton.

In this research, we compared the anatomy and biomechanics of two species of mudskipper vs an aquatic sandgoby in view of terrestrial locomotion. Of particular interest was the relationship (if any) of pectoral fin movement with pelvic fin movement. We show that the pelvic fins of the terrestrial mudskippers studied herein, are retractable and move antagonistically with the pectoral fins. The pelvic fin of the sandgoby studied here is contrarily non-retractable and drags on any underlying substrate that the sandgoby tries to crawl across. We find that the pelvic and pectoral fin muscles of all fish are separated, but that the pectoral fins of the mudskipper species have bulkier radial muscles than the sandgoby. By coupling a detailed morphological investigation of pectoral-pelvic fins musculature with finite element simulations, we find that unlike sandgobies, the mudskipper species are able to mechanically push the pelvic fins downward as pectoral fins retract. This allows for an instant movement of pelvic fins during the pectoral fin backward stroke and as such the pelvic fins stabilize mudskippers through Stefan attachment of their pelvic fins. This mechanism seems to be efficient and energy saving and we hypothesize that the piston-like action might benefit pelvic–pectoral fin antagonism by facilitating a mechanical down-thrust. Our research on the biomechanics of tree-climbing fish provides ideas and greater potential for the development of energetically more efficient systems of ambulation in biomimetic robots.

The bivalve ligament provides the thrust for shell opening, acting as the resistance in a lever system against which adductor muscle effort is applied. Usually, its outer lamellar layer is subjected to tensile stress, while the inner fibrous layer is compressed, with the pivotal axis located between them. However, opisthogyrate rostrate bivalves display a concave dorsal margin, and both the umbo and the postero-dorsal angle of the shell project dorsally to the ligament, which then fails to act as pivotal axis. Three opisthogyrate rostrate genera of unrelated lineages show somewhat different solutions to this morpho-functional challenge. In Cuspidaria (Anomalodesmata), the ligament is internal, subjected only to compression and ventral to the pivotal axis, a thickened periostracum develops, forcing the dorsal margins of the valves to act as pivotal axis, and the posterior parts of the shell's dorsal margins gape dorsally. In Nuculana (Palaeotaxodonta), the inner layer of the ligament is internal, the outer layer is external but reduced, and some species develop a dorsal ridge parallel to the commissural plane, on a level with the rostrum and acting as pivotal axis. In Pterotrigonia (Palaeoheterodonta) and other rostrate trigoniides, the ligament is external opisthodetic, but is allometrically reduced. Trigoniides may also develop a dorsal ridge.

Articular surfaces reflect the relative movements between adjacent bones, and the ones involved in the elbow joint provide information about forelimb movements and may be useful for making inferences about the substrate use. The proximal articular surface of the ulna was examined through 3-D geometric morphometrics, in order to assess its usefulness as a proxy for paleoecological interpretations; particularly for two small mammals from the early Miocene of Patagonia. The sample was composed of 22 extant small mammals (rodents, carnivorans and primates) and two extinct typotheres: Hegetotherium mirabile (Hegetotheriidae) and Interatherium robustum (Interatheriidae). Forty-five landmarks were taken and principal component analysis (PCA) was used to explore the morphospace structure. The results of PCA for the whole surface were inconclusive; therefore, successive analyses were made, subdividing the surface into sub-units. The PCA for the proximal part of the trochlear notch was the most informative, allowing the recognition of morphospaces with functional value: one for digging rodents and another for most climbers. Neither typothere would have had a specialisation for climbing or digging in the features analysed. This study allows morphological patterns on different parts of a joint to be detected; interpreted, at least partially, as differential responses to different kinds of mechanical stress.

The hind limbs of birds have long been considered a key feature in the conquest of different environments. However, the high level of morphological diversity encountered complicates the foundation of a good theoretical correlation between morphology, locomotor habits and substrate preference and this, in turn, complicates palaeobiological interpretations. Phorusrhacids (Aves, Cariamiformes) are a good example, since they have been unequivocally categorised as terrestrial birds due to their reduced forelimbs; and as apex predators with the ability to pursue prey based only on their hind limb morphology. Multivariate techniques (PCA and discriminant analysis), based on traditional metrics and geomorphometrics of the hind limb and pelvis, were applied in order to explore terrestriality and cursoriality in phorusrhacids. Although several groups of birds could be identified, when looking solely at hind limb metrics, some phorusrhacids appear to be associated with walking birds, while others are associated with cursorial birds. However, the pelvis separates cursorial birds and phorusrhacids from walking and wading birds. This scenario is complicated further by a lack of clear definition of the different locomotor modes and substrate preferences in extant birds, and this makes it difficult to confirm phorusrhacid cursoriality based solely on morphometrics. However, some qualitative features of the pelvis and foot make the picture a little clearer. To study limb adaptations in fossil birds, a more holistic study, with an emphasis on qualitative features of the whole posterior locomotor module, is necessary, since morphometrics leaves some issues unresolved. A comparison with the wings is also needed, in order to make a more complete analysis of locomotor behaviour.

The deep brine pools of the Red Sea comprise extreme, inhospitable habitats yet house microbial communities that potentially may fuel adjacent fauna. We here describe a novel bivalve from a deep-sea (1525 m) brine pool in the Red Sea, where conditions of high salinity, lowered pH, partial anoxia and high temperatures are prevalent. Remotely operated vehicle (ROV) footage showed that the bivalves were present in a narrow (20 cm) band along the rim of the brine pool, suggesting that it is not only tolerant of such extreme conditions but is also limited to them. The bivalve is a member of the Corbulidae and named Apachecorbula muriatica gen. et sp. nov. The shell is atypical of the family in being modioliform and thin. The semi-infaunal habit is seen in ROV images and reflected in the anatomy by the lack of siphons. The ctenidia are large and typical of a suspension feeding bivalve, but the absence of ‘guard cilia’ and the greatly reduced labial palps suggest that it is non-selective as a response to low food availability. It is proposed that the low body mass observed is a consequence of the extreme habitat and low food availability. It is postulated that the observed morphology of Apachecorbula is a result of paedomorphosis driven by the effects of the extreme environment on growth but is in part mitigated by the absence of high predation pressures.

Eophliantidae are poorly studied marine algal-dwelling amphipods with a wide distribution. A new species was found to excavate burrows across the main stem of Carpophyllum maschalocarpum (Turner) Grev. in the New Zealand subtidal, and a detailed morphological description of this amphipod is given. Bircenna macayai sp. nov. can be distinguished from other Bircenna species by a combination of the following characters: bilobed coxa 1, merus and carpus of pereopods 5–7 strongly extended posteriorly, crenulate basis of pereopod 7 and smooth posterior margin of epimeron 3, pereopod 7 basis longer than wide. A key to the fourteen world species of Eophliantidae is provided. Taxonomy, evolutionary sequences, functional morphology and biogeography of the Eophliantidae are briefly discussed. New Zealand and Australian shallow waters show the highest species diversity of Eophliantidae, containing both species bearing plesiomorphic and highly derived characters, suggesting that Australasia is an evolutionary centre for this amphipod family.

Siphonotretid brachiopod distribution in the Lower Ordovician of the Prague Basin is described and discussed; the new species Alichovia cometa is established. The preservation and distribution of the group indicate a preference for shallow-water offshore, high-energy environments: siphonotretids are abundant shelly fossils in the shallower-water, mostly marginal sites of the basin but are absent in the deeper parts. Their environmental limits were complementary to those of the graptolites; siphonotretids co-occur with epiplanktonic graptolites at only a few localities. Deeper offshore deposits with biramose dichograptids and diverse, generally delicate benthic dendroids lack siphonotretids. Indeed, siphonotretids were intolerant to dysoxia and preferred a well-aerated siliciclastic, firm sea floor in the shallower parts of the basin. They were tolerant to suspended coarser detritus, using their spines which functioned as an effective sieve. The density and ramification of spines indicate three sifting strategies. Suggested life styles above the seabed on elevated surfaces (algae, sponges, dendroids) are not supported by direct evidence. They more likely lived within more supple and soft matrices (sponges, tufts of algae) stabilised by their spines, with the spinose basket maintaining free space for inhalant and exhalant currents.

Although the mathematical principles underpinning population-level evolution are now well studied, the origin and evolution of morphological novelties has received far less attention. Here, a broad but general theory for how this sort of change takes place is outlined, relying on functional continuity, least-constrained components of morphology, redundancy and preadaptation. At least four distinct sorts of redundancy are identified: (i) redundancy arising through duplication (amplification); (ii) redundancy arising through regionalisation (parcellation); (iii) redundancy arising through functional convergence; and (iv) redundancy arising from shared function (functional degeneracy). Although organisms are here recognised to be functionally constrained (“burdened”, in Riedl's terminology), these constraints can be overcome through the combination of the four principles given above. Contrary to its common treatment, functional constraint is neither an ever-increasing restriction on the scope of evolution, nor does it require drastic events to overcome or “break” it. Rather, it is an evolutionary quantity, subject to selection at some level. The rules that govern morphological evolution are the primary controls on what is allowed to happen in the evolution of the overall genotype-phenotype system, suggesting strong controls on the sorts of developmental changes that might be associated with macroevolution. This model, then, sees organism functionality as the primary control on evolvability, with exact genetic make-up being of secondary importance. It should prove possible to recast traditional notions of body-plan evolution into the formulations of complex system analysis, which in the future may prove a unifying discipline for fields as disparate as palaeontology and gene regulatory networks. In particular, understanding how morphology can evolve may provide the critical link between the ecological and morphological networks that are currently the intense focus of evolutionary investigations.

Casts of forelimb elements of the Cretaceous theropod dinosaur Acrocanthosaurus atokensis were manually manipulated to determine range of motion and infer function. It was found that the humerus can swing posteriorly into a horizontal position but can neither swing laterally to glenoid height nor anteriorly much beyond the glenoid. The forearm can approach but not achieve full extension and right-angle flexion. Pronation and supination are precluded by immobility of the radius relative to the ulna. Motion also seems to be restricted at the wrist. The palm faces medially, and digital movement is subtransverse. All three digits are capable of extreme hyper-extension. Digits I and II converge during flexion. Only digit III can be abducted or adducted. The limited anterior range of brachial motion infers that Acrocanthosaurus first apprehended prey orally, using the forelimb afterwards to secure its grip or deliver fatal blows. Acrocanthosaurus could only manually grasp prey that was beneath its chest, towards which it may have used its mouth to move prey. Struggling prey would have impaled itself further upon the permanently and strongly flexed first ungual. The range of motion in the forelimb of Acrocanthosaurus resembles that of Herrerasaurus and Dilophosaurus, and exceeds that of Tyrannosaurus. Acrocanthosaurus exhibits a greater manual range of motion than ornithomimid and deinonychosaurian coelurosaurs, but less at the shoulder and elbow. Coelurosaurian theropods exhibit reduced digital flexion and hyper-extension, which suggests a change in the use of the manus in coelurosaurs.

The relationship between masticatory morphology and chewing modes in all living genera of South American rodents in the family Octodontidae was analysed. Chewing directions and the patterns of molar occlusion were assessed. Factor and regression analyses of skull and jaw characters, and attributes of the adductor musculature, especially the line of action of masseters and pterygoids, were performed to check their relations with the chewing modes. Two basic chewing strategies are present in octodontids: oblique unilateral (associated with anterolingual jaw displacement, and alternate occlusion of left and right molar series), and propalinal bilateral (associated with mostly posteroanterior jaw displacement, and simultaneous occlusion). The skull and jaw characters examined are related only partly to these chewing strategies. The temporal pattern of muscle contraction provides a possible explanation for such a functional versatility. Propalinal grinding in octodontids could be achieved through simultaneous muscle contraction, with limited reliance on the lines of action of the involved muscles. Therefore, simultaneous contraction explains a similar propalinal masticatory mode in morphologically disparate genera. In accordance with phylogenetic information, oblique unilateral chewing is primitive in octodontids, and the derived propalinal mode has been developed independently at least twice in the evolution of the family.

Sexual dimorphism is a common phenomenon among animals. The usual cause cited for sexual dimorphism in animals is sexual selection acting through female choice or male–male combat. Natural selection acting to reduce resource competition between the sexes, however, is an important alternative evolutionary scenario, but this possibility has received little empirical study. Here this issue is addressed by examining the relationships among body size, head shape and the functional aspects of diet in the adult male and female cottonmouth snake Agkistrodon piscivorus. In this species, males are larger in overall body and head size. Whereas an analysis of gross head measurements (simple linear head dimensions) shows little dimorphism in head shape, a more detailed analysis of head shape (using digital images of the snakes' heads) revealed some subtle, yet functionally significant, differences in head shape between adult male and female cottonmouths. Specifically, male cottonmouths have longer quadrate bones, and have greater lateral surface areas than females. Male cottonmouths also consumed relatively taller prey (prey size relative to snake body size) than conspecific females, and the sexes consumed significantly different proportions of prey. Because the size of the quadrate bone is a strong determinant of maximum gape in snakes, we suggest that the observed shape differences may reflect functional differences in maximum gape between similarly sized male and female cottonmouths. In turn, such differences in maximum gape width may explain why males consume taller prey than similarly sized females.

The burrowing habits of thalassinidean shrimps presuppose their high susceptibility to the deleterious effects of gill fouling by particulate debris and epibionts. The structures and strategies used to prevent gill fouling were examined in Nihonotrypaea japonica and Upogebia major by morphological inference and aquarium observations. The gill complement of both species consists of 10 arthrobranchs, with flattened filaments of the phylloid trichobranch type. Nihonotrypaea japonica and U. major lack passive gill-cleaning setal systems in the branchial chamber, but were found to actively groom gills by inserting and brushing the gill filaments with a pad of dense setae on the distal segments of their fifth pereiopods (P5 brush). Grooming setae of the P5 brush in N. japonica are of serrate and plumose types, whereas they are serrate and pappose in U. major. Serrate setae with distally curved spiniform denticules were morphologically similar in both species. Phylogenetic mapping of the gill-cleaning mechanisms suggests that gill-cleaning characters are homoplasic and systematically conservative among the families of the Thalassinidea.

Five populations of habitat-specific, morphologically distinct Algophagus mites (Astigmata: Algophagidae) were found to occur on the sub-Antarctic Prince Edward islands. Although these mites did not differ among themselves with respect to idiosomal setal characteristics, which were also similar to those of known sub-Antarctic Algophagus taxa, morphometric analyses revealed leg and idiosomal size differences. In addition to morphometric differences, leg setal positions and porose axillary organ structure, indicated that the morphotypes were taxonomically discrete. The taxa were found to include the known A. antarcticus Hughes, 1955, A. laticollaris Fain, 1974 (new status), a sub-species of A. semicollaris Fain, 1974, and two new species, A. brachytarsus sp. nov. and A. macrolithus sp. nov. With the exception of A. semicollaris, the taxa are described or redescribed in this paper. Morphological features of the females in particular, and notably leg proportions, claw dimensions, and apodeme positions, corroborate discrete terrestrial (A. brachytarsus and A. macrolithus) and aquatic forms (A. antarcticus, A. laticollaris, and A. semicollaris), and vary in a manner suggesting a functional relationship to habitat occupation by the various taxa.

The Rhizodontida (Pisces: Sarcopterygii) is a clade of predatory fishes from the Upper Devonian (Aztecia; ?Givetian of Antarctica) through to the Upper Carboniferous (Strepsodus; Moscovian of northern Europe and North America). They form the most basal plesion within the tetrapod stem-lineage. The mandibles were dominated by large symphysial tusks on the dentary. Not much else is known of the mandibles in primitive rhizodontids. However, later forms show several derived characters: the mandible is very deep dorsoventrally and narrow linguolabially; the coronoid fangs bear only a single fang and no other dentition; the Meckelian element was unossified, leaving the adductor fossa unfloored by bone; the prearticular produced a large dorsal process lingual to the adductor fossa, presumably for muscle attachment. These and other characters are discussed in the context of the evolution of the tetrapod stem-group.

The mandible appears to have been split into two functional units, one comprising the firmly sutured prearticular, coronoids and dentary, the other comprising the firmly sutured infradentaries. The connection between the two units was weak, suggesting a longitudinal intramandibular hinge. The possibility that this acted as a ‘torsion grip’ during feeding is discussed.

The aim of this report is the synthetic presentation of observations related to ultrastructural aspects of the lipid metabolism during the ontogenesis of trophic mechanisms in three teleost species, the sea bass, Dicentrarchus labrax, the sea bream, Sparus aurata and the pike-perch, Stizostedion lucioperca. The results have shown the respective roles of the vitelline syncytium, the intestine, the liver and the exocrine pancreas during the change from an endogenous lipid source, the yolk vesicle, to an exogenous lipid one, food. They make it possible to stress the close correlation between the development of the biliary function, that of the exocrine function of the pancreas and that of the intestinal absorption of lipids. These activities established during the endotrophic stage are slight at the beginning of trophic life and then increase at the end of the endo-exotrophic stage. Analysis of the results suggests the occurrence of four physiological phases characteristic in early lipid metabolism establishing the main heads in acquisition of the trophic autonomy according to a pattern similar to that of mammals.