Braincase Region Research Articles

Sexual dimorphism and ontogenetic variation in the head shape of two neighbouring populations of the common lancehead Bothrops jararaca (Serpentes: Viperidae)

Abstract The head is the most important element in trophic ecology among snakes. Head shape is affected by several factors, both intrinsic, such as size and sex, and extrinsic, such as geographical variation. Often, different populations are subject to different environmental conditions that are reflected in different phenotypes adapted to local conditions. Here, we investigate sexual dimorphism, ontogenetic allometry and environmental effects on head shape in two populations of Bothrops jararaca. We have found a significant effect of sex and population, with females from the highland population having a large braincase region and a more arrow-shaped head. Size increase accounted for a considerable amount of variation in shape, but the coefficient of correlation diverged between populations. Sexual dimorphism in head shape was not found in juveniles, but the ontogenetic trajectory was non-parallel between populations. Rainfall was most associated with a larger snout and reduced braincase. In the genus Bothrops, except in early stages, females often grow faster and attain larger sizes than males, which might affect head shape. Also, possible differences in diet, behaviour and ontogenetic variations between populations might be a reasonable cause of specific allometric trajectories.

First evidence of Proganochelys quenstedtii (Testudinata) from the Plateosaurus bonebeds (Norian, Late Triassic) of Frick, Canton Aargau, Switzerland

Proganochelys quenstedtii represents the best-known stem turtle from the Late Triassic, with gross anatomical and internal descriptions of the shell, postcranial bones and skull based on several well-preserved specimens from Central European fossil locations. We here report on the first specimen of P. quenstedtii from the Late Triassic (Klettgau Formation) Frickberg near the town of Frick, Canton Aargau, Switzerland. Similar to other Late Triassic ‘Plateosaurus-bearing bonebeds’, Proganochelys is considered to be a rare faunal element in the Swiss locality of Frick as well. The specimen, which is largely complete but was found only partially articulated and mixed with large Plateosaurus bones, overall resembles the morphology of the classical specimens from Germany. Despite being disarticulated, most skull bones could be identified and micro-computed tomography (CT) scanning of the posterior skull region reveals new insights into the braincase and neurovascular anatomy, as well as the inner ear region. These include the presence of a fenestra perilymphatica, potentially elongated cochlear ducts, and intense vascularization of small tubercles on the posterior end of the skull roof, which we interpret as horn cores. Other aspects of the skull in the braincase region, such as the presence or absence of a supratemporal remain ambiguous due to the fusion of individual bones and thus lack of visible sutures (externally and internally). Based on the size of the shell and fusion of individual elements, the specimen is interpreted as a skeletally mature animal.

Convergence is Only Skin Deep: Craniofacial Evolution in Electric Fishes from South America and Africa (Apteronotidae and Mormyridae).

SynopsisApteronotidae and Mormyridae are species-rich clades of weakly electric fishes from Neotropical and Afrotropical freshwaters, respectively, known for their high morphological disparity and often regarded as a classic example of convergent evolution. Here, we use CT-imaging and 3D geometric morphometrics to quantify disparity in craniofacial morphologies, and to test the hypothesis of convergent skull-shape evolution in a phylogenetic context. For this study, we examined 391 specimens representing 78 species of Apteronotidae and Mormyridae including 30 of 37 (81%) of all valid genera with the goal to sample most of the craniofacial disparity known in these clades. We found no overlap between Apteronotidae and Mormyridae in the skull-shape morphospace using PCA and a common landmark scheme, and therefore no instances of complete phenotypic convergence. Instead, we found multiple potential instances of incomplete convergence, and at least one parallel shift among electric fish clades. The greatest components of shape variance in both families are the same as observed for most vertebrate clades: heterocephaly (i.e., opposite changes in relative sizes of the snout and braincase regions of the skull), and heterorhynchy (i.e., dorsoventral changes in relative snout flexion and mouth position). Mormyrid species examined here exhibit less craniofacial disparity than do apteronotids, potentially due to constraints associated with a larger brain size, ecological constraints related to food-type availability. Patterns of craniofacial evolution in these two clades depict a complex story of phenotypic divergence and convergence in which certain superficial similarities of external morphology obscure deeper osteological and presumably developmental differences of skull form and function. Among apteronotid and mormyrid electric fishes, craniofacial convergence is only skin deep.

Inner ear morphology of diadectomorphs and seymouriamorphs (Tetrapoda) uncovered by high‐resolution x‐ray microcomputed tomography, and the origin of the amniote crown group

AbstractThe origin of amniotes was a key event in vertebrate evolution, enabling tetrapods to break their ties with water and invade terrestrial environments. Two pivotal clades of early tetrapods, the diadectomorphs and the seymouriamorphs, have played an unsurpassed role in debates about the ancestry of amniotes for over a century, but their skeletal morphology has provided conflicting evidence for their affinities. Using high‐resolution X‐ray microcomputed tomography, we reveal the three‐dimensional architecture of the well preserved endosseous labyrinth of the inner ear in representative species belonging to both groups. Data from the inner ear are coded in a new cladistic matrix of stem and primitive crown amniotes. Both maximum parsimony and Bayesian inference analyses retrieve seymouriamorphs as derived non‐crown amniotes and diadectomorphs as sister group to synapsids. If confirmed, this sister group relationship invites re‐examination of character polarity near the roots of the crown amniote radiation. Major changes in the endosseous labyrinth and adjacent braincase regions are mapped across the transition from non‐amniote to amniote tetrapods and include: a ventral shift of the cochlear recess relative to the vestibule and the semicircular canals; cochlear recess (primitively housed exclusively within the opisthotic) accommodated within both the prootic and the opisthotic; development of a distinct fossa subarcuata. The inner ear of seymouriamorphs foreshadows conditions of more derived groups, whereas that of diadectomorphs shows a mosaic of plesiomorphic and apomorphic traits, some of which are unambiguously amniote‐like, including a distinct and pyramid‐like cochlear recess.

The many faces of synapsid cranial allometry

AbstractPrevious studies of cranial shape have established a consistent interspecific allometric pattern relating the relative lengths of the face and braincase regions of the skull within multiple families of mammals. In this interspecific allometry, the facial region of the skull is proportionally longer than the braincase in larger species. The regularity and broad taxonomic occurrence of this allometric pattern suggests that it may have an origin near the base of crown Mammalia, or even deeper in the synapsid or amniote forerunners of mammals. To investigate the possible origins of this allometric pattern, we used geometric morphometric techniques to analyze cranial shape in 194 species of nonmammalian synapsids, which constitute a set of successive outgroups to Mammalia. We recovered a much greater diversity of allometric patterns within nonmammalian synapsids than has been observed in mammals, including several instances similar to the mammalian pattern. However, we found no evidence of the mammalian pattern within Therocephalia and nonmammalian Cynodontia, the synapsids most closely related to mammals. This suggests that the mammalian allometric pattern arose somewhere within Mammaliaformes, rather than within nonmammalian synapsids. Further investigation using an ontogenetic series of the anomodontDiictodon felicepsshows that the pattern of interspecific allometry within anomodonts parallels the ontogenetic trajectory ofDiictodon.This indicates that in at least some synapsids, allometric patterns associated with ontogeny may provide a “path of least resistance” for interspecific variation, a mechanism that we suggest produces the interspecific allometric pattern observed in mammals.

Why the short face? Developmental disintegration of the neurocranium drives convergent evolution in neotropical electric fishes.

Convergent evolution is widely viewed as strong evidence for the influence of natural selection on the origin of phenotypic design. However, the emerging evo‐devo synthesis has highlighted other processes that may bias and direct phenotypic evolution in the presence of environmental and genetic variation. Developmental biases on the production of phenotypic variation may channel the evolution of convergent forms by limiting the range of phenotypes produced during ontogeny. Here, we study the evolution and convergence of brachycephalic and dolichocephalic skull shapes among 133 species of Neotropical electric fishes (Gymnotiformes: Teleostei) and identify potential developmental biases on phenotypic evolution. We plot the ontogenetic trajectories of neurocranial phenotypes in 17 species and document developmental modularity between the face and braincase regions of the skull. We recover a significant relationship between developmental covariation and relative skull length and a significant relationship between developmental covariation and ontogenetic disparity. We demonstrate that modularity and integration bias the production of phenotypes along the brachycephalic and dolichocephalic skull axis and contribute to multiple, independent evolutionary transformations to highly brachycephalic and dolichocephalic skull morphologies.

Fluctuations in Evolutionary Integration Allow for Big Brains and Disparate Faces

In theory, evolutionary modularity allows anatomical structures to respond differently to selective regimes, thus promoting morphological diversification. These differences can then influence the rate and direction of phenotypic evolution among structures. Here we use geometric morphometrics and phenotypic matrix statistics to compare rates of craniofacial evolution and estimate evolvability in the face and braincase modules of a clade of teleost fishes (Gymnotiformes) and a clade of mammals (Carnivora), both of which exhibit substantial craniofacial diversity. We find that the face and braincase regions of both clades display different degrees of integration. We find that the face and braincase evolve at similar rates in Gymnotiformes and the reverse in Carnivora with the braincase evolving twice as fast as the face. Estimates of evolvability and constraints in these modules suggest differential responses to selection arising from fluctuations in phylogenetic integration, thus influencing differential rates of skull-shape evolution in these two clades.

Cranial Morphology of the Brachystelechid 'Microsaur' Quasicaecilia texana Carroll Provides New Insights into the Diversity and Evolution of Braincase Morphology in Recumbirostran 'Microsaurs'.

Recumbirostran ‘microsaurs,’ a group of early tetrapods from the Late Carboniferous and Early Permian, are the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. However, understanding of the diversity of fossorial adaptation within the Recumbirostra has been hindered by poor anatomical knowledge of the more divergent forms within the group. Here we report the results of μCT study of Quasicaecilia texana, a poorly-known recumbirostran with a unique, broad, shovel-like snout. The organization of the skull roof and braincase of Quasicaecilia is found to be more in line with that of other recumbirostrans than previously described, despite differences in overall shape. The braincase is found to be broadly comparable to Carrolla craddocki, with a large presphenoid that encompasses much of the interorbital septum and the columella ethmoidalis, and a single compound ossification encompassing the sphenoid, otic, and occipital regions. The recumbirostran braincase conserves general structure and topology of braincase regions and cranial nerve foramina, but it is highly variable in the number of ossifications and their extent, likely associated with the reliance on braincase ossifications to resist compression during sediment compaction and mechanical manipulation by epaxial and hypaxial musculature. Expansion of the deep ventral neck musculature in Quasicaecilia, autapomorphic among recumbirostrans, may reflect unique biomechanical function, and underscores the importance of future attention to the role of the cervical musculature in contextualizing the origin and evolution of fossoriality in recumbirostrans.

The skull and endocranium of a Lower Jurassic ichthyosaur based on digital reconstructions

AbstractEven after 200 years of study, some details of the cranial anatomy of ichthyosaurs, one of the most successful groups of marine vertebrates in the Mesozoic, are still unclear. New information on the braincase, palate and occiput are provided from three‐dimensional scans of an exceptionally preserved ichthyosaur (‘Hauffiopteryx’ typicus) skull from the Toarcian (183–174 Ma, Lower Jurassic) of Strawberry Bank, England. This ichthyosaur has unusual, hollow, tubular hyoid bars. The occipital and braincase region is fully reconstructed, creating the first digital cranial endocast of an ichthyosaur. Enlarged optic lobes and an enlarged cerebellum suggest neuroanatomical adaptations that allowed it to be a highly mobile, visual predator. The olfactory region also appears to be enlarged, suggesting that olfaction was more important for ichthyosaurs than has been assumed. Phylogenetic analysis suggests this ichthyosaur is closely related to, but distinct from, Hauffiopteryx, and positioned within Thunnosauria, a more derived position than previously recovered. These results further our knowledge of ichthyosaur cranial anatomy in three dimensions and provide a platform in which to study the anatomical adaptations that allowed ichthyosaurs to dominate the marine realm during the Mesozoic.

New morphological information on the ptyctodontid fishes (Placodermi, Ptyctodontida) from Western Australia

ABSTRACT A full description of a complete and articulated, three-dimensionally preserved, placoderm fish, Materpiscis attenboroughi (Ptyctodontida), from the Late Devonian Gogo Formation is presented. The jaw articulation is unique in being preserved in situ, providing new morphological information on the articulation of the quadrate, metapterygoid, and labial skeleton. In addition, separate hyoid and opercular cartilages are identified, thus confirming their presence for the first time within placoderms. Preserved orbital ossifications indicate an anteriomesial orientation of this braincase region. The complete scapulocoracoid provides additional morphological information on the pectoral fin articulation. A new specimen of Austroptyctodus gardineri preserving the anterior portion of the head and jaws allows for revised descriptions of the endocranium and visceral skeleton, including the most complete endocranial ossifications recovered to date. As well, elements of the labial skeleton not known previously in ptyctodonts are preserved. Small amounts of soft tissues are recovered from Materpiscis, and extensive postcranial body muscle from Austroptyctodus. This new material resolves several questions regarding cranial and postcranial morphology of the Ptyctodontida. Moreover, similarities in placoderm and chondrichthyan pelvic fins and reproductive structures (claspers) suggest early parallel expression of HoxD and sonic hedgehog genes (shh), potentially representing the first evidence for expression of these genes in placoderms.

Morphometric evaluation of the two mink subspecies in Illinois

The 2 subspecies of mink (Neovison vison letifera and N. v. mink) in Illinois were characterized early in the 20th century; however, quantifiable morphological distinctions between the 2 subspecies remain unclear. Illinois biogeography, and the influence of glaciations on animal populations, might explain the distinctions between the 2 mink subspecies. We evaluated sexual dimorphism within each subspecies and morphologic variation between subspecific populations. Oral, facial, and braincase regions were defined by 22 dimensions. Sexual dimorphism was present in both subspecies of mink. Data were adjusted for sex differences and tested for differences between subspecies using analysis of variance and multivariate analysis of variance. Cranial dimensions differed significantly between N. v. letifera and N. v. mink. Cranial dimensions distinguish individuals of the 2 geographically distinct subspecies; however, an intergradation zone might be present. The results are consistent with population response to glaciations and subsequent expansion, however, further genetic testing is necessary to test this historical biogeography of N. vison.

New Insights Into the Brain, Braincase, and Ear Region of Tyrannosaurs (Dinosauria, Theropoda), with Implications for Sensory Organization and Behavior

The braincase region of tyrannosaurs was investigated to provide insights on anatomical attributes relevant to inferences of sensory biology and behavior. CT scanning focused on three specimens of Tyrannosaurus rex, a juvenile Gorgosaurus, and the controversial Cleveland skull (CMNH 7541). Analysis shows that the cerebral hemispheres were enlarged, but conflicting information on the optic lobes suggests that brain conformation was not fully avian. Previous estimates of olfactory bulb size for T. rex were much too large, but even the corrected sizes are relatively larger than other theropods, suggesting that odor detection was indeed of particular importance to tyrannosaurs. The inner ears show a number of coelurosaurian traits, such as elongate and rounded and rostral, lateral semicircular canals, and incipient twisting of the common crus, which we interpret to be related to enhanced reflexes coordinating rapid eye and head movements. The cochlea is elongate, which, coupled with the finding of extensive tympanic pneumaticity, supports the inference of behavioral emphasis of low-frequency sounds. Three main groups of sinuses pneumatized the braincase, and there are a number of perhaps systematically relevant differences. Orientation of the endosseous labyrinth reveals that alert head postures of T. rex and Gorgosaurus were somewhat depressed below the horizontal, but the Cleveland skull had a very strongly down-turned posture. It is concluded that tyrannosaur sensory biology is consistent with their predatory coelurosaurian heritage, with emphasis on relatively quick, coordinated eye and head movements, and probably sensitive low-frequency hearing; tyrannosaurs apomorphically enhanced their olfactory apparatus. The taxonomic status of the Cleveland skull remains unresolved.