Matthew W. Colbert

Three-dimensional analysis of plant structure using high-resolution X-ray computed tomography

High-resolution X-ray computed tomography (HRCT) is a non-invasive approach to 3D visualization and quantification of biological structure. The data, based on differential X-ray attenuation, are analogous to those otherwise obtainable only by serial sectioning. Requiring no fixing, sectioning or staining, HRCT produces a 3D digital map of the specimen that allows measurements and visualizations, including arbitrarily oriented sections. In spite of its application throughout the natural sciences, HRCT has yet to be applied in extant plant structural research.

CALSOYASUCHUS VALLICEPS, A NEW CROCODYLIFORM FROM THE EARLY JURASSIC KAYENTA FORMATION OF ARIZONA

Abstract We describe a new fossil crocodyliform archosaur from the Early Jurassic Kayenta Formation of the Navajo Nation that is surprisingly derived for so ancient a specimen. High-resolution X-ray CT analysis reveals that its long snout houses an extensive system of pneumatic paranasal cavities. These are among the most distinctive features of modern crocodylians, yet the evolutionary history of this unique system has been obscured by the inaccessibility of internal structures in most fossil crania. Preliminary phylogenetic analysis indicates that the new species is the oldest known member of a monophyletic Goniopholididae, and within this lineage to be the sister taxon of Eutretauranosuchus, from the Late Jurassic Morrison formation of Colorado. Goniopholididae became extinct at the end of the Cretaceous, but it is more closely related to living crocodylians than are several lineages known only from Cretaceous and younger fossils. The new taxon nearly doubles the known length of goniopholid history and implies a deep, as yet undiscovered, Mesozoic history for several crocodyliform lineages that were once thought to have relatively complete fossil records.

Forelimb-hindlimb developmental timing changes across tetrapod phylogeny

BackgroundTetrapods exhibit great diversity in limb structures among species and also between forelimbs and hindlimbs within species, diversity which frequently correlates with locomotor modes and life history. We aim to examine the potential relation of changes in developmental timing (heterochrony) to the origin of limb morphological diversity in an explicit comparative and quantitative framework. In particular, we studied the relative time sequence of development of the forelimbs versus the hindlimbs in 138 embryos of 14 tetrapod species spanning a diverse taxonomic, ecomorphological and life-history breadth. Whole-mounts and histological sections were used to code the appearance of 10 developmental events comprising landmarks of development from the early bud stage to late chondrogenesis in the forelimb and the corresponding serial homologues in the hindlimb.ResultsAn overall pattern of change across tetrapods can be discerned and appears to be relatively clade-specific. In the primitive condition, as seen in Chondrichthyes and Osteichthyes, the forelimb/pectoral fin develops earlier than the hindlimb/pelvic fin. This pattern is either retained or re-evolved in eulipotyphlan insectivores (= shrews, moles, hedgehogs, and solenodons) and taken to its extreme in marsupials. Although exceptions are known, the two anurans we examined reversed the pattern and displayed a significant advance in hindlimb development. All other species examined, including a bat with its greatly enlarged forelimbs modified as wings in the adult, showed near synchrony in the development of the fore and hindlimbs.ConclusionMajor heterochronic changes in early limb development and chondrogenesis were absent within major clades except Lissamphibia, and their presence across vertebrate phylogeny are not easily correlated with adaptive phenomena related to morphological differences in the adult fore- and hindlimbs. The apparently conservative nature of this trait means that changes in chondrogenetic patterns may serve as useful phylogenetic characters at higher taxonomic levels in tetrapods. Our results highlight the more important role generally played by allometric heterochrony in this instance to shape adult morphology.

Diffusible iodine-based contrast-enhanced computed tomography (diceCT) : an emerging tool for rapid, high-resolution, 3-D imaging of metazoan soft tissues.

Morphologists have historically had to rely on destructive procedures to visualize the three‐dimensional (3‐D) anatomy of animals. More recently, however, non‐destructive techniques have come to the forefront. These include X‐ray computed tomography (CT), which has been used most commonly to examine the mineralized, hard‐tissue anatomy of living and fossil metazoans. One relatively new and potentially transformative aspect of current CT‐based research is the use of chemical agents to render visible, and differentiate between, soft‐tissue structures in X‐ray images. Specifically, iodine has emerged as one of the most widely used of these contrast agents among animal morphologists due to its ease of handling, cost effectiveness, and differential affinities for major types of soft tissues. The rapid adoption of iodine‐based contrast agents has resulted in a proliferation of distinct specimen preparations and scanning parameter choices, as well as an increasing variety of imaging hardware and software preferences. Here we provide a critical review of the recent contributions to iodine‐based, contrast‐enhanced CT research to enable researchers just beginning to employ contrast enhancement to make sense of this complex new landscape of methodologies. We provide a detailed summary of recent case studies, assess factors that govern success at each step of the specimen storage, preparation, and imaging processes, and make recommendations for standardizing both techniques and reporting practices. Finally, we discuss potential cutting‐edge applications of diffusible iodine‐based contrast‐enhanced computed tomography (diceCT) and the issues that must still be overcome to facilitate the broader adoption of diceCT going forward.

Nonhuman anthropoid primate femoral neck trabecular architecture and its relationship to locomotor mode

Functional analyses of human and nonhuman anthropoid primate femoral neck structure have largely ignored the trabecular bone. We tested hypotheses regarding differences in the relative distribution and structural anisotropy of trabecular bone in the femoral neck of quadrupedal and climbing/suspensory anthropoids. We used high‐resolution X‐ray computed tomography to analyze quantitatively the femoral neck trabecular structure of Ateles geoffroyi, Symphalangus syndactylus, Alouatta seniculus, Colobus guereza, Macaca fascicularis, and Papio cynocephalus (n = 46). We analyzed a size‐scaled superior and inferior volume of interest (VOI) in the femoral neck. The ratio of the superior to inferior VOI bone volume fraction indicated that the distribution of trabecular bone was inferiorly skewed in most (but not all) quadrupeds and evenly distributed the climbing/suspensory species, but interspecific comparisons indicated that all taxa overlapped in these measurements. Degree of anisotropy values were generally higher in the inferior VOI of all species and the results for the two climbing/suspensory taxa, A. geoffroyi (1.71 ± 0.30) and S. syndactylus (1.55 ± 0.04), were similar to the results for the quadrupedal anthropoids, C. guereza (male = 1.64 ± 0.13; female = 1.68 ± 0.07) and P. cynocephalus (1.47 ± 0.13). These results suggest strong trabecular architecture similarity across body sizes, anthropoid phylogenetic backgrounds, and locomotor mode. This structural similarity might be explained by greater similarity in anthropoid hip joint loading mechanics than previously considered. It is likely that our current models of anthropoid hip joint mechanics are overly simplistic. Anat Rec, 2007.

A new fossil species supports an early origin for toothed whale echolocation

Odontocetes (toothed whales, dolphins and porpoises) hunt and navigate through dark and turbid aquatic environments using echolocation; a key adaptation that relies on the same principles as sonar. Among echolocating vertebrates, odontocetes are unique in producing high-frequency vocalizations at the phonic lips, a constriction in the nasal passages just beneath the blowhole, and then using air sinuses and the melon to modulate their transmission. All extant odontocetes seem to echolocate; however, exactly when and how this complex behaviour—and its underlying anatomy—evolved is largely unknown. Here we report an odontocete fossil, Oligocene in age (approximately 28 Myr ago), from South Carolina (Cotylocara macei, gen. et sp. nov.) that has several features suggestive of echolocation: a dense, thick and downturned rostrum; air sac fossae; cranial asymmetry; and exceptionally broad maxillae. Our phylogenetic analysis places Cotylocara in a basal clade of odontocetes, leading us to infer that a rudimentary form of echolocation evolved in the early Oligocene, shortly after odontocetes diverged from the ancestors of filter-feeding whales (mysticetes). This was followed by enlargement of the facial muscles that modulate echolocation calls, which in turn led to marked, convergent changes in skull shape in the ancestors of Cotylocara, and in the lineage leading to extant odontocetes.

Correlations between auditory structures and hearing sensitivity in non-human primates

Primates show distinctions in hearing sensitivity and auditory morphology that generally follow phylogenetic patterns. However, few previous studies have attempted to investigate how differences in primate hearing are directly related to differences in ear morphology. This research helps fill this void by exploring the form‐to‐function relationships of the auditory system in a phylogenetically broad sample of non‐human primates. Numerous structures from the outer, middle, and inner ears were measured in taxa with known hearing capabilities. The structures investigated include the overall size and shape of the pinna, the areas of the tympanic membrane and stapedial footplate, the masses and lever arm lengths of the ossicles, the volumes of the middle ear cavities, and the length of the cochlea. The results demonstrate that a variety of auditory structures show significant correlations with certain aspects of hearing (particularly low‐frequency sensitivity). Although the majority of these relationships agree with expectations from auditory theory, some traditional (and possibly outdated) ideas were not supported. For example, the common misconception that higher middle ear transformer ratios (e.g., impedance transformer ratio) result in increased hearing sensitivity was not supported. Although simple correlations between form and function do not necessarily imply causality, the relationships defined in this study not only increase our understanding of auditory patterns in extant taxa but also lay the foundation to begin investigating the hearing in fossil primates. J. Morphol., 2010.

Applications of high-resolution X-ray computed tomography in petrology, meteoritics and palaeontology

Abstract High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.

A NEW TURTLE FROM THE ARUNDEL CLAY FACIES (POTOMAC FORMATION, EARLY CRETACEOUS) OF MARYLAND, U.S.A.

Abstract A new paracryptodiran turtle, Arundelemys dardeni, gen. et sp. nov., is described on the basis of an isolated, nearly complete skull from the Early Cretaceous Arundel Clay facies of Maryland, USA. The basicranial region exhibits the paracryptodiran condition of a single foramen for the canalis caroticus internus located midway along the basisphenoid. As revealed by CT scans, the basicranial region of Arundelemys is unusual in that the right and left canales carotici interni merge just before reaching the sella turcica and the canalis caroticus lateralis is very small or absent. A phylogenetic analysis places Arundelemys dardeni as the basal-most member of the Paracryptodira. Within the Paracryptodira, Arundelemys dardeni is most similar to Compsemys victa in general proportions.

Anatomy of the Cranial Endocast of the Bottlenose Dolphin, Tursiops truncatus, Based on HRXCT

Endocranial surfaces, volumes, and interconnectivities of extant and fossil odontocetes potentially offer information on the general architecture of the brain and on the structure of the specialized cetacean circulatory system. Although conventional methods for acquiring such data have generally involved invasive preparation of the specimen, particularly in the case of fossils, new tomographic technologies afford nondestructive access to these internal morphologies. In this study we used high-resolution X-ray computed tomography (HRXCT) to scan a skull of the extant Tursiops truncatus (Cetacea: Odontoceti). We processed the data to reveal the cranial endocast and details of internal skeletal architecture (data at www.digimorph.org). Major features that can be discerned include aspects of the specimens hypertrophied retia mirabilia, the major canals and openings of the cranial cavity, and the relationship of the brain and endocranial circulatory structures to the surrounding skeleton. CT data also provide information on the shape of the brain that may be lost in conventional anatomical preparations, and readily provide volumetric and linear measurements of the endocast and its individual segments. These results demonstrate the utility of HRXCT for interpreting the internal cranial anatomy of both extant and fossil cetaceans.