Locomotion in water and on land impose dramatically different demands, yet many animals successfully move in both environments. Most turtle species perform both aquatic and terrestrial locomotion but vary in how they use their limbs. Freshwater turtles use anteroposterior movements of the limbs during walking and swimming with contralateral fore- and hindlimbs moving in synchrony. In contrast, sea turtles swim primarily with "powerstroke" movements, characterized by synchronous forelimb motions while the hindlimbs act as rudders. High-speed video has been used to study powerstroking, but pectoral girdle movements and long-axis rotation (LAR) of the humerus are likely both key components to turtle locomotor function and cannot be quantified from external video. Here, we used XROMM to measure pectoral girdle and humeral movements in a sea turtle (loggerhead, Caretta caretta) compared to the freshwater river cooter (Pseudemys concinna) during terrestrial and aquatic locomotion. The largest difference among species was in yaw of the pectoral girdle during swimming, with loggerheads showing almost no yaw during powerstroking whereas pectoral girdle yaw in the cooter during rowing was over 30°. The magnitude of humeral LAR was greatest during loggerhead powerstroking and the temporal pattern of supination and pronation was opposite from that of cooters. We hypothesize that these kinematic differences are driven by differences in how the limbs are used to power propulsion. Rotations at the glenoid drive the overall patterns of movement in freshwater turtles, whereas glenohumeral LAR in loggerheads is used to direct the position and orientation of the elbow, which is the joint that determines the orientation of the thrust-generating structure (the flipper) in loggerheads.

Currently, there is limited histological data for spermatid morphologies within the testes of squamates. There are only 10 species of lizard that have complete ultrastructural data across the entire process of spermiogenesis, including several species of Sceloporus. These studies have shown that differences can be seen between spermatids of saurians within the same family or genus. Thus, the present study continues to test the hypothesis that differences exist in spermatid morphology between species within the same family. We collected five Phrynosoma cornutum males from Arizona. Their testes were extracted and processed with standard TEM techniques. Many of the characteristics of spermiogenesis within P. cornutum are conserved and similar in morphology to other phrynosomatid lizards. These similarities include the development of the acrosome, perforatorium, subacrosomal cone, nuclear rostrum, and epinuclear lucent zone. However, there were also differences observed in P. cornutum spermatids that are distinct compared to other phyrnosomatids. For example, P. cornutum spermatids include a wider and more robust perforatorium and less spiraling of the chromatin during condensation than that of other phrynosomatid lizards. The present results corroborate previous studies and indicate that even with morphological conservation within saurian spermatids, character differences between species can be recognized. Further studies on spermiogenesis are required to judge the relevance of these ontogenetic changes in terms of using them in amniotic or squamate spermatid/spermatozoa phylogenic analysis.

New Zealand scincid lizards, genus Oligosoma, represent a monophyletic radiation of a clade, Eugongylini, of species distributed geographically throughout the South Pacific with major radiations in Australia and New Caledonia. Viviparity has evolved independently on multiple occasions within these lineages. Studies of Australian species have revealed that placental specializations resulting in substantial placentotrophy have evolved in two lineages. The pattern of extraembryonic membrane development of oviparous species differs from viviparous species and identical placental architecture has evolved in both placentotrophic lineages. We analyzed extraembryonic membrane development in two New Zealand species, the sole oviparous species, Oligosoma suteri, and placental development of a representative viviparous species, Oligosoma polychroma, using histological techniques. We conclude that these two species share a basic pattern of extraembryonic membrane development with other squamates. Comparisons with Australian species indicate that morphogenesis of the yolk sac of O. suteri results in an elaborate structure previously known only in Oligosoma lichenigerum with a geographic distribution on Lord Howe Island and Norfolk Island. This finding supports a close relationship between these two taxa. We conclude also that the pattern of placental development of O. polychroma is identical to that of viviparous species of Australia. The terminal placental stage for each of these lineages includes a chorioallantoic placenta and an elaborate omphaloplacenta. This level of homoplasy in placental evolution is consistent with a hypothesis that selection favors regional differentiation of the maternal-embryonic interface and that the omphaloplacenta is an adaptation for histotrophic transport.

Literature reports of hyperostosis are often misleading and have been confused with osteomas, a pathological condition. Hyperostotic bones are known to occur only in bony fishes of the class Actinopterygii, within at least 16 orders, 35 families, 89 genera, and 153 species. They are present almost exclusively in marine fishes and exceptionally in a few extinct freshwater species known from hypersaline environments and one extant cichlid. Hyperostosis is best represented in the family Carangidae where it is known to occur in 53 of approximately 181 valid species. We also provide a synthetic report on what we know and what misconceptions exist regarding hyperostosis. Patterns of hyperostosis are often species-specific but provide no useful phylogenetic information. In species known to develop hyperostosis, it is usually not apparent (non-histologically) in juveniles and typically only becomes fully developed in the largest individuals. The timing of hyperostosis on-set in different bones is often sequential rather than simultaneous across different bones. Most marine Neoteleostei have acellular skeletons but histological observations have shown that in species exhibiting hyperostosis, areas of active remodeling are composed primarily of cellular bone characterized by a rich vascular network and bone-resorbing osteoclasts.

The ultrastructure of the nervous system has been studied in sexually mature Nybelinia surmenicola (Cestoda: Trypanorhyncha) from the intestine of a shark Lamna ditropis. The central nervous system (CNS) reveals a complex organization within cestodes and corresponds to the trypanorhynch pattern of brain architecture. The brain of N. surmenicola is differentiated into nine clearly defined lobes and semicircular, median, and X-shaped cruciate commissures. A specific feature is the presence of a powerful extracellular capsule that surrounds the brain lobes with the cortical glial cells. Moreover, the architecture of the anterior lobes clearly distinguishes the species of Tentacularioidea. The neurons of the anterior lobes form compact groups looking like frontal horns. There are approximately 120 neurons in the anterior lobes and a preliminary estimate of more than 300 perikarya in the brain. Several ultrastructural types of neurons have been identified, differing in the size and shape of the soma, the density of the cytoplasm, and the ultrastructure of synaptic vesicles. Numerous synapses involving clear and electron-dense vesicles have been observed in neuropils. Two types of glial cells have been found in the brain that participate in neuronal metabolism and wrap around the giant axons, brain lobes, neuropil compartments, and the main nerve cords. Such a powerful extracellular fibrillar brain capsule has not been observed in the brain of other studied cestodes and has been demonstrated in this study for the first time. The differentiation of the brain lobes reveals the important role of the rhyncheal system in the evolution of cestodes and correlates with their behavior. The anterior nerves arising from the anterior lobes innervate the radial muscles stabilizing the position of the tentacle sheaths and movements of the attachment organs. The nervous system anatomy and the brain architecture may reflect the morphofunctional aspects of the tapeworm evolution.

Muscle loading is known to influence skeletal morphology. Therefore, modification of the biomechanical environment is expected to cause coordinated morphological changes to the bony and cartilaginous tissues. Understanding how this musculoskeletal coordination contributes to morphological variation has relevance to health sciences, developmental biology, and evolutionary biology. To investigate how muscle loading influences skeletal morphology, we replicate a classic in ovo embryology experiment in the domestic chick (Gallus gallus domesticus) while harnessing modern methodologies that allow us to quantify skeletal anatomy more precisely and in situ. We induced rigid muscle paralysis in developing chicks mid-incubation, then compared the morphology of the cranium and mandible between immobilized and untreated embryos using microcomputed tomography and landmark-based geometric morphometric methods. Like earlier studies, we found predictable differences in the size and shape of the cranium and mandible in paralyzed chicks. These differences were concentrated in areas known to experience high strains during feeding, including the jaw joint and jaw muscle attachment sites. These results highlight specific areas of the skull that appear to be mechanosensitive and suggest muscles that could produce the biomechanical stimuli necessary for normal hatchling morphology. Interestingly, these same areas correspond to areas that show the greatest disparity and fastest evolutionary rates across the avian diversity, which suggests that the musculoskeletal integration observed during development extends to macroevolutionary scales. Thus, selection and evolutionary changes to muscle physiology and architecture could generate large and predictable changes to skull morphology. Building upon previous work, the adoption of modern imaging and morphometric techniques allows richer characterization of musculoskeletal integration that empowers researchers to understand how tissue-to-tissue interactions contribute to overall phenotypic variation.

Woodpeckers (Order Piciformes) belong to a group of birds characterized by their hammering capabilities in which the bill is utilized as a tool to probe for food and to excavate nest cavities. They have numerous specializations for this behavior, including their bill and tongue, feet for gripping vertical tree trunks, and tail feathers with thickened shafts to provide stability as a postural appendage. We hypothesized that (1) woodpecker tail musculature is also modified for clinging behaviors with a heterogeneous distribution of fast and slow muscle fibers, and that (2) the tree-trunk foraging Hairy Woodpeckers would have more slow muscle fibers in their M. depressor caudae than Northern Flickers, which forage on the ground where they probe the substrate for insects. We performed immunohistochemistry to identify the fiber type distributions for tail muscles Mm. caudofemoralis pars caudalis, lateralis caudae, levator caudae, and depressor caudae in four Hairy Woodpeckers and five Northern Flickers. Our results show that these tail muscles in the two woodpecker species are comprised of a majority of fast muscle fibers common among dynamic locomotor muscles. Interestingly, we report a functionally-significant distribution of slow muscle fibers in M. depressor caudae predicted to be utilized in propping of the tail during tree climbing and support. Further, we found more slow fibers (13.80% ± 4.49%) in the trunk-foraging Hairy Woodpeckers compared with the ground-foraging Northern Flicker (7.40% ± 4.95%), which we interpret to be related to the trunk-foraging habits of Hairy Woodpeckers.

The craniocervical junction is the transition between the skull and the vertebral column that provides mobility while maintaining sufficient stability (i.e., protection of the brainstem and the spinal cord). The key elements involved are the occiput, the first cervical vertebra (CV1, atlas) and the second cervical vertebra (CV2, axis). The two vertebrae forming the atlas-axis complex are distinct in their morphology and differences in form have been linked to differences in ecological function in mammals. Here, we quantified the morphological diversity of the cranium, CV1 and CV2 in a sample of Carnivora using 3D geometric morphometrics to reveal phylogenetic and ecological patterns. Our results indicate that the observed variation in CV2 is related to the taxonomic diversity (i.e., strong phylogenetic signal), whereas variation in CV1 appears to be decoupled from species diversity in Carnivora and, thus, is likely to reflect a functional signal. The phylogenetically informed correlation analyses showed an association between the CV1 morphology and diet. Taxa that primarily feed on large prey tend to have larger transverse processes on CV1 which provides larger muscle attachment areas and may correlate with stronger muscles. The latter needs to be verified by future quantitative covariation analyses between bone and muscle data. Morphological peculiarities within Pinnipedia and Mustelidae could be explained by differences in terrestrial locomotion between Phocidae and Otariidae and the exceptional defensive behavior (i.e., handstanding) in Mephitidae. Despite differences in the degree of morphological diversity, covariation between cranium, CV1 and CV2 morphology is consistently high (≥ 0.82) highlighting that overall, the craniocervical junction is an integrated structure, but there are traits that are not constrained.

The external ear in eutherian mammals is composed of the annular, auricular (pinna), and scutellar cartilages. The latter extends between the pinnae, across the top of the head, and lies at the intersection of numerous auricular muscles and is thought to be a sesamoid element. In bats, this scutulum consists of two distinct regions, (1) a thin squama that is in contact with the underlying temporalis fascia and (2) a lateral bossed portion that is lightly tethered to the medial surface of the pinna. The planar size, shape, and proportions of the squama vary by taxa, as does the relative size and thickness of the boss. The origins, insertions, and relative functions of the auricular muscles are complicated. Here, 30 muscles were tallied as to their primary attachment to the pinnae, scutula, or a pre-auricular musculo-aponeurotic plate that is derived from the epicranius. In contrast to Yangochiroptera, the origins and insertions of many auricular muscles have shifted from the scutulum to this aponeurotic plate, in both the Rhinolophidae and Hipposideridae. We propose that this functional shift is a derived character related primarily to the rapid translations and rotations of the pinna in high-duty-cycle rhinolophid and hipposiderid bats.

Many planktonic rotifers carry their oviposited eggs until hatching. In some species, the eggs are attached to the mother via secretions from her style gland, which forms a thread that extends from her cloaca. In species of Pompholyx, the mother possesses the rare ability to change the tension on the secreted thread, which alters the proximity of the egg with respect to her body. In this study, we used behavioral observations, confocal microscopy, and transmission electron microscopy to study the functional morphology of the stalk gland, which secretes a similar thread to the style gland. Our observations reveal that six longitudinal muscles insert on a stalk-gland complex, which is a combination of a two-headed gland and an epithelial duct that connects to the posterior cloaca. The gland secretes a single, long, electron-dense thread that traverses the duct and attaches to the egg surface through the cloaca. Three retractor muscles insert on the stalk gland and function to pull the entire complex anteriorly, thereby increasing tension on the thread and moving the egg close to the mother's body. A set of three (two pairs and a single dorsal) protractor muscles antagonize these actions, and their contraction pulls the gland complex close to the cloaca, thereby releasing tension on the thread and allowing the egg to distance itself from the mother. The stalk gland complex does not appear to be homologous to the style glands of other rotifers, but we hypothesize that it functions as a form of maternal protection as is the case with style glands.

Chitons possess the most elaborate system of shell pores found in any hard-shelled invertebrate. Although chitons possess some anteriorly located sense organs, they lack true cephalization, as their major sensory systems are not concentrated in a distinct head region. Instead, the aesthete system within their shells forms a dense sensory network that overcomes the barrier of their hard dorsal armour. The basic arrangement of neural structures embedded within a solid, opaque matrix, has confounded understanding of the overall network. In this study, we use synchrotron X-ray μCT to visualise the aesthete canal networks inside chiton valves. We selected representatives from all three major chiton clades: Lepidopleurida, the basal branching clade, and Callochitonida and Chitonida, which both have more complex shell morphology, to compare internal structure. Lepidopleurida aesthete canals are oriented vertically and pass directly through the shell to connect with the body. By contrast, aesthetes canals in Callochitonida and Chitonida have complex internal structures with extended horizontal passages, coalescing at the shell diagonal that corresponds to the valve insertion slits. This represents a stepwise evolution of chiton shell form, where thicker and more complex valves require a diverting and rewiring of the entire sensory network. Aspects of the aesthete system, such as the microscopic arrangement of surface pores, have long been used in chiton taxonomy for species diagnoses; insertion slits should also be understood as a secondary feature of the aesthete system. Chiton shell structures that are used for morphological systematics are driven by sensory adaptations.

New World porcupines (Erethizontidae) exhibit behaviors and possess integumentary structures, including the quills, that are used for self-defense. The North American porcupine (Erethizon dorsatum) has been well studied regarding these features; however, information is lacking for the South American Coendou species. We describe the defensive behavior and integumentary morphology of Coendou spinosus to understand the defensive strategies of this species and to compare with those reported for other species. We assessed the behaviors related to warning, defense, and escape of eight porcupines, as well as the characteristics of their pelage and quills. Furthermore, we microscopically analyzed skin samples of a roadkill adult male specimen. Similar to E. dorsatum, C. spinosus exhibited omnidirectional quill erection, revealing an aposematic color and, with their backs toward the perceived human threat, they performed quick tail and body movements to strike the hands of the human trying to capture them by the tail. Furthermore, C. spinosus presented an integumentary structure similar to that of E. dorsatum, and mechanisms to facilitate quill release when touched, penetration, and fixation in the opponent. The most distinct warning behavior noted was the vibration of the quills, which has not been reported for Erethizon. Our study confirms that, like other erethizontids, C. spinosus does not attack but exhibits warning, defense, and escape mechanisms and behaviors when threatened or touched. The dissemination of such information helps to counter the negative stigma associated with porcupines, as they can be the victims of attacks by dogs and humans, and to promote their conservation.

Species of mites (Chelicerata: Arachnida) show a great variety of structures of the female gonads. In both evolutionary lines, Acariformes and Parasitiformes, the panoistic ovary, in which all germline cysts differentiate into oocytes, and the meroistic ovary, in which the oocytes grow supported by the nurse cells, have been documented. A less pronounced variation in the gonad structure could be expected at lower systematic levels, hence, we ask about the degree of differences within the family that is subordinate to Acariformes and represents the cohort Parasitengona. Based on the members of Trombidiidae (Acariformes: Trombidiformes, Parasitengona, Trombidioidea), we test the hypothesis that the general ovary type is constant at the family level. Our previous research on the female gonad in Allothrombium fuliginosum revealed that the meroistic ovary occurs in these mites. Here, we proceed with a detailed insight into the ovary structure in A. fuliginosum and examine the structure of the female gonad in other members of Trombidiidae, focusing on the following representatives of its nominotypical genus Trombidium: Trombidium brevimanum, Trombidium holosericeum, Trombidium heterotrichum, and Trombidium latum. For all species, studied with light, fluorescence, and transmission electron microscopy, we could confirm the presence of the meroistic ovary that is highly similar with respect to general architecture. The germline cysts show similarities in general morphology and the mode of germline cell differentiation; they consist of a few nurse cells and one oocyte. The connection between the nurse cells and oocytes is maintained by trophic cords that serve for the transport of organelles and macromolecules. Our results confirm the constancy of the structure of the female gonad at the intrageneric level and provide further support for the hypothesis on the lack of differences at the intrafamily level.

Fish vertebrae are primarily morphologically classified into precaudal vertebrae jointed to the ribs and caudal vertebrae with hemal spines, through which the caudal artery and veins pass. Moray eels (family Muraenidae) capture prey by directly biting, combining oral and pharyngeal jaw. During feeding motions, they exhibit various head manipulations, such as neurocranial elevation, ventral flexion, and horizontal shaking, with their postcranial region acting like the neck of amniotes. However, the bone morphology supporting these movements remains unclear. In this study, the vertebral morphologies of the Kidako moray (Gymnothorax kidako), starry moray (Echidna nebulosa), pink-lipped moray (Echidna rhodochilus), tidepool snake moray (Uropterygius micropterus), and Seychelles moray (Anarchias seychellensis) were investigated using X-ray computed tomography. These five species exhibited longitudinal ventral processes in the second to approximately 12th precaudal vertebrae with canals for blood vessels, structurally similar to hemal spines. In addition, the morphology of the precaudal vertebrae in three Anguilliformes species closely related to moray eels and two Gasterosteiformes species, including a seahorse that flexes its head ventrally as a feeding motion, was compared with that of moray eels. However, no remarkable ventral processes were observed in their precaudal vertebrae in the postcranial region, suggesting that these structural features in the postcranial vertebrae were preserved in Muraenidae but not necessarily required for the fish to bend its head ventrally. Although the functional significance of the ventral process has yet to be determined, our findings highlight a novel aspect of fish vertebral morphology.

Evolutionary body size decrease has profound consequences for the morphology of an organism. In the evolution of the Characidae, the most species-rich family of Neotropical fishes, a prominent trend is the reduction of body size. The most typical effect is the simplification and reduction of morphological features through terminal deletion processes, resulting in the loss of skeletal elements and structures. To provide further information on the matter, we present a detailed description of the skeleton of Hyphessobrycon piabinhas, a poorly known, small representative of the largest genus of Characidae. We further discuss the identity and phylogenetic relationships of H. piabinhas. It belongs to the subfamily Stethaprioninae and exhibits considerable morphological similarity to other congeners from neighboring drainage systems. We identify several morphological simplifications in H. piabinhas and discuss them based on ontogenetic data available for Characiformes. These developmentally truncated elements are also present in many other small representatives of the family and seem to be among the first morphological modifications to occur in the context of body size reduction of Characidae. We argue that structural losses are not strictly correlated with sizes below 26 mm SL, although the most notable simplifications are typically observed in the miniatures.

The mammalian order Primates is known for widespread sexual dimorphism in size and phenotype. Despite repeated speculation that primate sexual size dimorphism either facilitates or is in part driven by functional differences in how males and females interact with their environments, few studies have directly assessed the influence of sexual dimorphism on performance traits. Here, we use a theoretical morphology framework to show that sexual dimorphism in primate crania is associated with divergent biomechanical performance traits. The degree of dimorphism is a significant covariate in biomechanical trait divergence between sexes. Males exhibit less efficient but stiffer cranial shapes and significant evolutionary allometry in biomechanical performance, whereas females maintain performance stability across their size spectrum. Evolutionary rates are elevated for efficiency in females whereas males emphasize size-dependent cranial stiffness. These findings support a hypothesis of sex-linked bifurcation in masticatory system performance: larger male crania and faster size evolution partially compensate for low efficiency and reflect a de-emphasis of mechanical leverage, whereas female crania maintain higher mechanical efficiency overall and evolve more rapidly in molar-based masticatory performance. The evolutionary checks-and-balances between size dimorphism and cranial mechanical performance may be a more important driver of primate phenotypic evolution than has been hitherto appreciated.

Carnivorous polychaetes are known to bear diversified and often unique anatomical and behavioural adaptations for predation and defence. Halla parthenopeia, a species known to be a specialized predator of clams, thrives in the soft bottoms of the Mediterranean Sea, holding potential for polyculture and biotechnology due to the secretion of bioactive compounds. Our objective was to provide a comprehensive description of H. parthenopeia's anatomy and microanatomy, shedding light on the relation between morphology and habitat, chemical defences, and feeding behaviour. The pharynx, housing maxillae and mandibles connected to an extensive mucus gland, occupies a considerable portion of the worm's length, reaching beyond the oesophagus. This unique gland is responsible for secreting the feeding mucus, which immobilizes and aids in the digestion of clams probably acting as a vehicle of bioactive compounds synthesized by specialized serous cells in the mouth. Moreover, H. parthenopeia combines behavioural tactics, such as burrowing, and anatomical defences to evade predators. Examination of its epidermis revealed a thick cuticle layer and abundant mucocytes secreting locomotion mucus, both of which save the worm from mechanical harm during movement. When it is preyed upon, the worm can release a substantial amount of Hallachrome, a toxic anthraquinone produced by specific cells in its distal region. This pigment, with its known antimicrobial properties, likely acts as a chemical shield in case of injury. The results suggest that the ability of H. parthenopeia to prey on bivalves and to provide mechanical protection plus defence against pathogens rely on its ability to secrete distinct types of mucus. The interplay between highly specialized microanatomical features and complex behaviours underscores its adaptation as a predator in marine benthic environments.

Extraembryonic membranes provide protection, oxygen, water, and nutrients to developing embryos, and their study generates information on the origin of the terrestrial egg and the evolution of viviparity. In this research, the morphology of the extraembryonic membranes and the types of placentation in the viviparous snake Conopsis lineata are described through optical microscopy during early and late gestation. When embryos develop inside the uterus, they become surrounded by a thin eggshell membrane. In early gestation, during stages 16 and 18, the embryo is already surrounded by the amnion and the chorion, and in a small region by the chorioallantois, which is product of the contact between the chorion and the growing allantois. A trilaminar omphalopleure covers the yolk sac from the embryonic hemisphere to the level of the equator where the sinus terminalis is located, and from there a bilaminar omphalopleure extends into the abembryonic hemisphere. Thus, according to the relationship of these membranes with the uterine wall, the chorioplacenta, the choriovitelline placenta, and the chorioallantoic placenta are structured at the embryonic pole, while the omphaloplacenta is formed at the abembryonic pole. During late gestation (stages 35, 36, and 37), the uterus and allantois are highly vascularized. The allantois occupies most of the extraembryonic coelom and at the abembryonic pole, it contacts the omphaloplacenta and form the omphalallantoic placenta. This is the first description of all known placenta types in Squamata for a snake species member of the subfamily Colubrinae; where an eggshell membrane with 2.9 μm in width present throughout development is also evident. The structure of extraembryonic membranes in C. lineata is similar to that of other oviparous and viviparous squamate species. The above indicates not only homology, but also that the functional characteristics have been maintained throughout the evolution of the reproductive type.

Nudibranchs, with their mesmerizing diversity and ecological significance, play crucial roles in marine ecosystems. Central to their feeding prowess is the radula, a chitinous structure with diverse morphologies adapted to prey preferences and feeding strategies. This study focuses on elucidating wear coping mechanisms in radular teeth of carnivorous molluscs, employing Dendronotus lacteus (Dendronotidae) and Flabellina affinis (Flabellinidae) as model species. Both species forage on hydrozoans. Through scanning electron microscopy, confocal laser scanning microscopy, nanoindentation, and energy-dispersive X-ray spectroscopy, the biomechanical and compositional properties of their teeth were analyzed. Notably, tooth coatings, composed of calcium (Ca) or silicon (Si) and high hardness and stiffness compared to the internal tooth structure, with varying mineral contents across tooth regions and ontogenetic zones, were found. The presence of the hard and stiff tooth coatings highlight their role in enhancing wear resistance. The heterogeneities in the autofluorescence patterns related to the distribution of Ca and Si of the coatings. Overall, this study provides into the biomechanical adaptations of nudibranch radular teeth, shedding light on the intricate interplay between tooth structure, elemental composition, and ecological function in marine molluscs.

The plains vizcacha, Lagostomus maximus, is the only living species in the genus, being notably larger than fossil congeneric species, such as Lagostomus incisus, from the Pliocene of Argentina and Uruguay. Here, we compare the skull growth allometric pattern and sexual dimorphism of L. maximus and L. incisus, relating shape and size changes with skull function. We also test whether the ontogenetic trajectories and allometric trends between both sexes of L. maximus follow the same pattern. A common allometric pattern between both species was the elongation of the skull, a product of the lengthening of rostrum, and chondrogenesis on the spheno-occipitalis synchondrosis and coronalis suture. We also detected a low proportion of skull suture fusion. In some variables, older male specimens did not represent a simple linear extension of female trajectory, and all dimorphic traits were related to the development of the masticatory muscles. Sexual dimorphism previously attributed to L. incisus would indicate that this phenomenon was present in the genus since the early Pliocene and suggests social behaviors such as polygyny and male-male competition. Ontogenetic changes in L. incisus were similar to L. maximus, showing a conservative condition of the genus. Only two changes were different in the ontogeny of both species, which appeared earlier in L. incisus compared to L. maximus: the development of the frontal process of the nasals in a square shape, and the straight shape of the occipital bone in lateral view. Juveniles of L. maximus were close to adult L. incisus in the morphospace, suggesting a peramorphic process. The sequence of suture and synchondroses fusion showed minor differences in temporozygomatica and frontonasalis sutures, indicating major mechanical stress in L. maximus related to size. We suggest a generalized growth path in Chinchillidae, but further analyses are necessary at an evolutionary level, including Lagidium and Chinchilla.