Hugo Dutel

The Giant Cretaceous Coelacanth (Actinistia, Sarcopterygii) Megalocoelacanthus dobiei Schwimmer, Stewart & Williams, 1994, and Its Bearing on Latimerioidei Interrelationships

We present a redescription of Megalocoelacanthus dobiei, a giant fossil coelacanth from Upper Cretaceous strata of North America. Megalocoelacanthus has been previously described on the basis of composite material that consisted of isolated elements. Consequently, many aspects of its anatomy have remained unknown as well as its phylogenetic relationships with other coelacanths. Previous studies have suggested that Megalocoelacanthus is closer to Latimeria and Macropoma than to Mawsonia. However, this assumption was based only on the overall similarity of few anatomical features, rather than on a phylogenetic character analysis. A new, and outstandingly preserved specimen from the Niobrara Formation in Kansas allows the detailed description of the skull of Megalocoelacanthus and elucidation of its phylogenetic relationships with other coelacanths. Although strongly flattened, the skull and jaws are well preserved and show many derived features that are shared with Latimeriidae such as Latimeria, Macropoma and Libys. Notably, the parietonasal shield is narrow and flanked by very large, continuous vacuities forming the supraorbital sensory line canal. Such an unusual morphology is also known in Libys. Some other features of Megalocoelacanthus, such as its large size and the absence of teeth are shared with the mawsoniid genera Mawsonia and Axelrodichthys. Our cladistic analysis supports the sister-group relationship of Megalocoelacanthus and Libys within Latimeriidae. This topology suggests that toothless, large-sized coelacanths evolved independently in both Latimeriidae and Mawsoniidae during the Mesozoic. Based on previous topologies and on ours, we then review the high-level taxonomy of Latimerioidei and propose new systematic phylogenetic definitions.

The buccohypophyseal canal is an ancestral vertebrate trait maintained by modulation in sonic hedgehog signaling

BackgroundThe pituitary gland is formed by the juxtaposition of two tissues: neuroectoderm arising from the basal diencephalon, and oral epithelium, which invaginates towards the central nervous system from the roof of the mouth. The oral invagination that reaches the brain from the mouth is referred to as Rathke’s pouch, with the tip forming the adenohypophysis and the stalk disappearing after the earliest stages of development. In tetrapods, formation of the cranial base establishes a definitive barrier between the pituitary and oral cavity; however, numerous extinct and extant vertebrate species retain an open buccohypophyseal canal in adulthood, a vestige of the stalk of Rathke’s pouch. Little is currently known about the formation and function of this structure. Here we have investigated molecular mechanisms driving the formation of the buccohypophyseal canal and their evolutionary significance.ResultsWe show that Rathke’s pouch is located at a boundary region delineated by endoderm, neural crest-derived oral mesenchyme and the anterior limit of the notochord, using CD1, R26R-Sox17-Cre and R26R-Wnt1-Cre mouse lines. As revealed by synchrotron X-ray microtomography after iodine staining in mouse embryos, the pouch has a lobulated three-dimensional structure that embraces the descending diencephalon during pituitary formation. Polarisfl/fl; Wnt1-Cre, Ofd1-/- and Kif3a-/- primary cilia mouse mutants have abnormal sonic hedgehog (Shh) signaling and all present with malformations of the anterior pituitary gland and midline structures of the anterior cranial base. Changes in the expressions of Shh downstream genes are confirmed in Gas1-/- mice. From an evolutionary perspective, persistence of the buccohypophyseal canal is a basal character for all vertebrates and its maintenance in several groups is related to a specific morphology of the midline that can be related to modulation in Shh signaling.ConclusionThese results provide insight into a poorly understood ancestral vertebrate structure. It appears that the opening of the buccohypophyseal canal depends upon Shh signaling and that modulation in this pathway most probably accounts for its persistence in phylogeny.

A reevaluation of the anatomy of the jaw-closing system in the extant coelacanth Latimeria chalumnae

The coelacanth Latimeria is the only extant representative of the Actinistia, a group of sarcopterygian fishes that originated in the Devonian. Moreover, it is the only extant vertebrate in which the neurocranium is divided into an anterior and a posterior portion that articulate by means of an intracranial joint. This joint is thought to be highly mobile, allowing an elevation of the anterior portion of the skull during prey capture. Here we provide a new description of the skull and jaw-closing system in Latimeria chalumnae in order to better understand its skull mechanics during prey capture. Based on a dissection and the CT scanning of an adult coelacanth, we provide a detailed description of the musculature and ligaments of the jaw system. We show that the m. adductor mandibulae is more complex than previously reported. We demonstrate that the basicranial muscle inserts more anteriorly than has been described previously, which has implications for its function. Moreover, the anterior insertion of the basicranial muscle does not correspond to the posterior tip of the tooth plate covering the parasphenoid, questioning previous inferences made on fossil coelacanths and other sarcopterygian fishes. Strong ligaments connect the anterior and the posterior portions of the skull at the level of the intracranial joint, as well as the notochord and the catazygals. These observations suggest that the intracranial joint is likely to be less mobile than previously thought and that its role during feeding merits to be reexamined.

Bite force in the extant coelacanth Latimeria: the role of the intracranial joint and the basicranial muscle

The terrestrialization process involved dramatic changes in the cranial anatomy of vertebrates. The braincase, which was initially divided into two portions by the intracranial joint in sarcopterygian fishes, became consolidated into a single unit in tetrapods and lungfishes [1-3]. The coelacanth Latimeria is the only extant vertebrate that retains an intracranial joint, which is associated with a unique paired muscle: the basicranial muscle. The intracranial joint has long been thought to be involved in suction feeding by allowing an extensive elevation of the anterior portion of the skull, followed by its rapid depression driven by the basicranial muscle [4-7]. However, we recently challenged this hypothesis [8, 9], and the role of the basicranial muscle with respect to the intracranial joint thus remains unclear. Using 3D biomechanical modeling, we show here that the basicranial muscle and the intracranial joint are involved in biting force generation. By flexing the anterior portion of the skull at the level of the intracranial joint, the basicranial muscle increases the overall bite force. This likely allows Latimeria to feed on a broad range of preys [10, 11] and coelacanths to colonize a wide range of environments during their evolution [4]. The variation in the morphology of the intracranial joint observed in Devonian lobe-finned fishes would have impacted to various degrees their biting performance and might have permitted feeding specializations despite the stability in their lower jaw morphology [12]. VIDEO ABSTRACT.

Redescription of the hyoid apparatus and associated musculature in the extant coelacanth Latimeria chalumnae: functional implications for feeding

The coelacanth Latimeria is the only extant vertebrate in which the neurocranium is divided into an anterior and a posterior portion which articulate by means of an intracranial joint. This articulation is thought to allow an elevation of the snout up to 20‐degree angle, which is supposed to enhance mouth gape and velocity, in turn allowing for a powerful suction. Several functional models have been proposed to explain the skull movement in Latimeria, but they disagree on the mechanisms responsible for mandibular depression and intracranial elevation, and more precisely on the role and mobility of the hyoid apparatus during these processes. We here show that the m. coracomandibularis spans ventrally to the palate‐mandible joint, and is likely involved in mandibular depression. The hyoid apparatus is sheathed by several layers of ligaments, rendering extensive movements of the hyoid bones in the anteroposterior direction unlikely. Together with the manipulation of the 3D virtual model of the skull, these observations suggest that the hyoid arch is less mobile than previously proposed, and that the movements proposed in previous models are unlikely. In the light of our new observations, we suggest that the mechanisms proposed for explaining the intracranial elevation are incomplete. Moreover, we suggest that the extensive movements of the hyoid arch elements, which were thought to accompany intracranial elevation, are unlikely. In the absence of intracranial elevation, we propose that the movements of the hyoid mainly take place in the transverse plane, allowing the lateral expansion of the orobranchial chamber. Anat Rec, 298:579–601, 2015.

Form–function relationships in dragonfly mandibles under an evolutionary perspective

Functional requirements may constrain phenotypic diversification or foster it. For insect mouthparts, the quantification of the relationship between shape and function in an evolutionary framework remained largely unexplored. Here, the question of a functional influence on phenotypic diversification for dragonfly mandibles is assessed with a large-scale biomechanical analysis covering nearly all anisopteran families, using finite element analysis in combination with geometric morphometrics. A constraining effect of phylogeny could be found for shape, the mandibular mechanical advantage (MA), and certain mechanical joint parameters, while stresses and strains, the majority of joint parameters and size are influenced by shared ancestry. Furthermore, joint mechanics are correlated with neither strain nor mandibular MA and size effects have virtually play no role for shape or mechanical variation. The presence of mandibular strengthening ridges shows no phylogenetic signal except for one ridge peculiar to Libelluloidea, and ridge presence is also not correlated with each other. The results suggest that functional traits are more variable at this taxonomic level and that they are not influenced by shared ancestry. At the same time, the results contradict the widespread idea that mandibular morphology mainly reflects functional demands at least at this taxonomic level. The varying functional factors rather lead to the same mandibular performance as expressed by the MA, which suggests a many-to-one mapping of the investigated parameters onto the same narrow mandibular performance space.

A Giant Marine Coelacanth from the Jurassic of Normandy, France

Actinistia (coelacanths) is a clade of sarcopterygian fishes (lobe-finned vertebrates) that are first known from the Early Devonian and are now represented by a single living genus, Latimeria. Although the diversity of the group is considerably reduced today, the fossil record of coelacanths displays a remarkable diversity in terms of number of species, morphology, and ecology (Forey, 1998). Although the total length of the two Latimeria species can reach about 2 m, most of the fossil coelacanths are much smaller in size (i.e., less than 50 cm) (Forey, 1998). Nevertheless, occurrences of giant coelacanths have been reported (Wenz, 1980, 1981; Schwimmer et al., 1994; Carvalho and Maisey, 2008; Dutel et al., 2012) in the Late Jurassic–Cretaceous of North America and Gondwana. These giant coelacanths are represented by two genera, Megalocoelacanthus (Schwimmer et al., 1994; Dutel et al., 2012), which is marine and estimated to have reached 4.5 m in length, and Mawsonia, which is fresh-brackish water and estimated to range between 3.5 and 6.3 m (Wenz, 1981; Carvalho and Maisey, 2008; Medeiros et al., 2011). Interestingly, Megalocoelacanthus and Mawsonia belong to two distinct coelacanth lineages, respectively the latimeriids and the mawsoniids, showing that gigantism has evolved convergently in these Mesozoic coelacanths (Dutel et al., 2012). Moreover, very fragmentary remains also suggest the presence of large marine coelacanths in the Triassic from Spitsbergen (Stensiö, 1921), taxonomical identification of which remains uncertain (Forey, 1998). Here, we report the first occurrence of a giant marine coelacanth from the Jurassic of Europe. Anatomical Abbreviations—f.post.cart.Ptq, fossa for the insertion of the posterior cartilage plate of the palatoquadrate; i.ant.cart.Ptq, ridge for the insertion of the anterior cartilage plate of the pterygoid; i.lig.spt, insertion point for the suprapterygoid ligament; Pt, pterygoid; Q, quadrate.

Submicron Imaging of Soft-Tissues Using Low-Dose Phase-Contrast X-Ray Synchrotron Microtomography with an Iodine Contrast Agent

3-D visualization of forming organs and tissues in early embryos helps understanding their developmental dynamics. 3-D reconstruction of an organ from an image stack requires: (1) a sufficient number of slices in order to obtain smooth contours, and (2) a satisfactory contrast that allows differentiating between tissue layers during segmentation. Based on these principles, satisfactory but very time-consuming techniques are available for manual segmentation and step-by-step 3-D reconstructions of small embryonic structures using histology (Viriot et al., 1997, 2000). Usual micro-CT devices available in developmental biology units do not provide the sufficient resolution required to visualize the smaller developing structures at early embryonic stages, such as tooth germs at E11.5. Furthermore, the contrast obtained from soft-tissues is low if high doses of radiation—not supported by small samples—are not used. A recent study (Raj et al., 2014) has shown that synchrotron imaging with a sliver-based contrast agent provides images of embryonic soft-tissues with a resolution of 4–10 lm. Here we show that the combination of propagation phase contrast, rapid imaging, phase retrieval and iodine contrast agent allows soft-tissue imaging with a voxel size of 0.695 lm and a relatively low radiation dose. This method provides submicronic images where single cells can be individualized.

The third dimension: a novel set‐up for filming coelacanths in their natural environment

1: Here we describe a novel design to obtain three dimensional data on the movements of aquatic organisms at depths of up to 140 meters. 2: The setup consists of two synchronized high-speed cameras fixed to two articulated arms. 3: The setup was successfully used to film and quantify the locomotion of coelacanths (Latimeria chalumnae) living at a depth of about 120 meters in Sodwana Bay, South Africa. As an example, the detailed motion of the dorsal fin is presented here. 4: This setup can be used for any underwater applications that require synchronized video recordings of medium to large sized animals. This article is protected by copyright. All rights reserved.

The biomechanical role of the chondrocranium and sutures in a lizard cranium

The role of soft tissues in skull biomechanics remains poorly understood. Not least, the chondrocranium, the portion of the braincase which persists as cartilage with varying degrees of mineralization. It also remains commonplace to overlook the biomechanical role of sutures despite evidence that they alter strain distribution. Here, we examine the role of both the sutures and the chondrocranium in the South American tegu lizard Salvator merianae. We use multi-body dynamics analysis (MDA) to provide realistic loading conditions for anterior and posterior unilateral biting and a detailed finite element model to examine strain magnitude and distribution. We find that strains within the chondrocranium are greatest during anterior biting and are primarily tensile; also that strain within the cranium is not greatly reduced by the presence of the chondrocranium unless it is given the same material properties as bone. This result contradicts previous suggestions that the anterior portion (the nasal septum) acts as a supporting structure. Inclusion of sutures to the cranium model not only increases overall strain magnitudes but also leads to a more complex distribution of tension and compression rather than that of a beam under sagittal bending.