The oldest three-dimensionally preserved vertebrate neurocranium.

The neurocranium is an integral part of the vertebrate head, itself a major evolutionary innovation1,2. However, its early history remains poorly understood, with great dissimilarity in form between the two living vertebrate groups: gnathostomes (jawed vertebrates) and cyclostomes (hagfishes and lampreys)2,3. The 100 Myr gap separating the Cambrian appearance of vertebrates4-6 from the earliest three-dimensionally preserved vertebrate neurocrania7 further obscures the origins of modern states. Here we use computed tomography to describe the cranial anatomy of an Ordovician stem-group gnathostome: Eriptychius americanus from the Harding Sandstone of Colorado, USA8. A fossilized head of Eriptychius preserves a symmetrical set of cartilages that we interpret as the preorbital neurocranium, enclosing the fronts of laterally placed orbits, terminally located mouth, olfactory bulbs and pineal organ. This suggests that, in the earliest gnathostomes, the neurocranium filled out the space between the dermal skeleton and brain, like in galeaspids, osteostracans and placoderms and unlike in cyclostomes2. However, these cartilages are not fused into a single neurocranial unit, suggesting that this is a derived gnathostome trait. Eriptychius fills a major temporal and phylogenetic gap in our understanding of the evolution of the gnathostome head, revealing a neurocranium with an anatomy unlike that of any previously described vertebrate.

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes.

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However, its anatomy remains poorly understood. Here, we use X-ray microtomography to characterize the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of 13 plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition.

Exploring fluctuating asymmetry in two recovering populations of the Eurasian otter.

The Eurasian otter is a wide-ranging semi-aquatic mammal that underwent a significant population decline in the last century, leading to local extinctions, reduction and fragmentation of populations. The individuals of populations exposed to both external and internal stress may present the inability to produce a specific developmental outcome, generating developmental 'noise' (developmental instability (DI)). Factors contributing to DI include inbreeding depression, population bottlenecks, habitat loss and exposure to pollution. We analysed fluctuating asymmetry (FA) as a proxy of DI in two European otter populations that experienced a major decline in the 1990s. Using three-dimensional geometric morphometrics methods on skull samples of otters from the UK and Sweden, we compared the degree of FA both between populations before, during and after the decline. We found a higher FA in the UK populations compared with Sweden. The level of asymmetry differed significantly over time only in the UK population, where it was higher during the decline phase. FA in the UK populations can be attributed to the specific impact of polychlorinated biphenyls pollution that caused a bottleneck. More generally, our study suggests that habitat loss, pollution and limited gene flow may contribute to DI in declining otter populations, highlighting the need for continued investigation to identify and quantify the specific stressors behind this trend in local populations.

Testing heterochrony: Connecting skull shape ontogeny and evolution of feeding adaptations in baleen whales.

Ontogeny plays a key role in the evolution of organisms, as changes during the complex processes of development can allow for new traits to arise. Identifying changes in ontogenetic allometry-the relationship between skull shape and size during growth-can reveal the processes underlying major evolutionary transformations. Baleen whales (Mysticeti, Cetacea) underwent major morphological changes in transitioning from their ancestral raptorial feeding mode to the three specialized filter-feeding modes observed in extant taxa. Heterochronic processes have been implicated in the evolution of these feeding modes, and their associated specialized cranial morphologies, but their role has never been tested with quantitative data. Here, we quantified skull shapes ontogeny and reconstructed ancestral allometric trajectories using 3D geometric morphometrics and phylogenetic comparative methods on sample representing modern mysticetes diversity. Our results demonstrate that Mysticeti, while having a common developmental trajectory, present distinct cranial shapes from early in their ontogeny corresponding to their different feeding ecologies. Size is the main driver of shape disparity across mysticetes. Disparate heterochronic processes are evident in the evolution of the group: skim feeders present accelerated growth relative to the ancestral nodes, while Balaenopteridae have overall slower growth, or pedomorphosis. Gray whales are the only taxon with a relatively faster rate of growth in this group, which might be connected to its unique benthic feeding strategy. Reconstructed ancestral allometries and related skull shapes indicate that extinct taxa used less specialized filter-feeding modes, a finding broadly in line with the available fossil evidence.

The ontogeny of asymmetry in echolocating whales.

Extreme asymmetry of the skull is one of the most distinctive traits that characterizes toothed whales (Odontoceti, Cetacea). The origin and function of cranial asymmetry are connected to the evolution of echolocation, the ability to use high-frequency sounds to navigate the surrounding environment. Although this novel phenotype must arise through changes in cranial development, the ontogeny of cetacean asymmetry has never been investigated. Here we use three-dimensional geometric morphometrics to quantify the changes in degree of asymmetry and skull shape during prenatal and postnatal ontogeny for five genera spanning odontocete diversity (oceanic dolphins, porpoises and beluga). Asymmetry in early ontogeny starts low and tracks phylogenetic relatedness of taxa. Distantly related taxa that share aspects of their ecology overwrite these initial differences via heterochronic shifts, ultimately converging on comparable high levels of skull asymmetry. Porpoises maintain low levels of asymmetry into maturity and present a decelerated rate of growth, probably retained from the ancestral condition. Ancestral state reconstruction of allometric trajectories demonstrates that both paedomorphism and peramorphism contribute to cranial shape diversity across odontocetes. This study provides a striking example of how divergent developmental pathways can produce convergent ecological adaptations, even for some of the most unusual phenotypes exhibited among vertebrates.

Enhancing CT imaging: A safe protocol to stain and de-stain rare fetal museum specimens using diffusible iodine-based staining (diceCT).

Computed tomography (CT) scanning is being increasingly employed in the study of natural history, particularly to investigate the internal anatomy of unique specimens in museum collections. Different techniques to enhance the contrast between tissues have been developed to improve the quality of the scans while preserving the integrity of these rare specimens. Diffusible iodine-based contrast enhanced computed tomography (diceCT) was found to be particularly effective and reversible for staining tissues in formalin preserved specimens. While it can also be effectively employed to stain ethanol-preserved specimens of small size, the reversibility of this process and the applicability to large-bodied animals has never been thoroughly tested. Here, we describe a novel diceCT protocol developed to stain and de-stain ethanol-preserved prenatal specimens of baleen whales (Mysticeti, Cetacea). These large (10-90 cm in length only considering early fetal stages) specimens present unique challenges as they are rare in collections and irreplaceable, therefore it is imperative to not damage them with the staining process. Before trying this protocol on baleen whales' specimens, we conducted a pilot study on commercially available fetal pigs using the same parameters. This allowed us to optimize the staining time to obtain the best results in CT scanning and to test first-hand the effect of de-staining on ethanol-based specimens. External coloration of the specimens is also a concern with iodine-staining, as stained specimens assume a bright red color that needs to be removed from both internal and external tissues before they can be stored. To test the reversibility of the stain in ethanol-preserved specimens with fur, we also conducted a small experiment using commercially acquired domestic mice. After these trials were successful, we applied the staining and de-staining protocol to multiple fetal specimens of mysticetes up to 90 cm in length, both ethanol- and formalin-preserved. Specimens were soaked in a solution of 1% pure iodine in 70% ethanol for 1-28 days, according to their size. After scanning, specimens are soaked in a solution of 3% sodium thiosulfate in 70% ethanol that is able to completely wash out the iodine from the tissues in a shorter time frame, between a few hours and 14 days. The same concentrations were used for formalin-preserved specimens, but DI water was used as solvent instead of ethanol. The staining technique proved particularly useful to enhance the contrast difference between cartilage, mineralized bone, teeth, and the surrounding soft tissues even when the specimens where scanned in medical-grade CT scans. The specimens did not suffer any visible damage or shrinkage due to the procedure, and in both the fetal samples and in the mice the color of the stain was completely removed by the de-staining process. We conclude therefore that this protocol can be safely applied to a variety of ethanol-preserved museum specimens to enhance the quality of the CT scanning and highlight internal morphological features without recurring to dissection or other irreversible procedures. We also provide tips to best apply this protocol, from how to mix the solutions to how to minimize the staining time.

Prenatal developmental sequence of the skull of minke whales and its implications for the evolution of mysticetes and the teeth-to-baleen transition.

Baleen whales (Mysticeti) have an extraordinary fossil record documenting the transition from toothed raptorial taxa to modern species that bear baleen plates, keratinous bristles employed in filter-feeding. Remnants of their toothed ancestry can be found in their ontogeny, as they still develop tooth germs in utero. Understanding the developmental transition from teeth to baleen and the associated skull modifications in prenatal specimens of extant species can enhance our understanding of the evolutionary history of this lineage by using ontogeny as a relative proxy of the evolutionary changes observed in the fossil record. Although at present very little information is available on prenatal development of baleen whales, especially regarding tooth resorption and baleen formation, due to a lack of specimens. Here I present the first detailed description of prenatal specimens of minke whales (Balaenoptera acutorostrata and Balaenoptera bonaerensis), focusing on the skull anatomy and tooth germ development, resorption, and baleen growth. The ontogenetic sequence described consists of 10 specimens of both minke whale species, from the earliest fetal stages to full term. The internal skull anatomy of the specimens was visualized using traditional and iodine-enhanced computed tomography scanning. These high-quality data allow detailed description of skull development both qualitatively and quantitatively using three-dimensional landmark analysis. I report distinctive external anatomical changes and the presence of a denser tissue medial to the tooth germs in specimens from the final portion of gestation, which can be interpreted as the first signs of baleen formation (baleen rudiments). Tooth germs are only completely resorbed just before the eruption of the baleen from the gums, and they are still present for a brief period with baleen rudiments. Skull shape development is characterized by progressive elongation of the rostrum relative to the braincase and by the relative anterior movement of the supraoccipital shield, contributing to a defining feature of cetaceans, telescoping. These data aid the interpretation of fossil morphologies, especially of those extinct taxa where there is no direct evidence of presence of baleen, even if caution is needed when comparing prenatal extant specimens with adult fossils. The ontogeny of other mysticete species needs to be analyzed before drawing definitive conclusions about the influence of development on the evolution of this group. Nonetheless, this work is the first step towards a deeper understanding of the most distinctive patterns in prenatal skull development of baleen whales, and of the anatomical changes that accompany the transition from tooth germs to baleen. It also presents comprehensive hypotheses to explain the influence of developmental processes on the evolution of skull morphology and feeding adaptations of mysticetes.

Inactivation of C4orf26 in toothless placental mammals.

Previous studies have reported inactivated copies of six enamel-related genes (AMBN, AMEL, AMTN, ENAM, KLK4, MMP20) and one dentin-related gene (DSPP) in one or more toothless vertebrates and/or vertebrates with enamelless teeth, thereby providing evidence that these genes are enamel or tooth-specific with respect to their critical functions that are maintained by natural selection. Here, we employ available genome sequences for edentulous and enamelless mammals to evaluate the enamel specificity of four genes (WDR72, SLC24A4, FAM83H, C4orf26) that have been implicated in amelogenesis imperfecta, a condition in which proper enamel formation is abrogated during tooth development. Coding sequences for WDR72, SCL24A4, and FAM83H are intact in four edentulous taxa (Chinese pangolin, three baleen whales) and three taxa (aardvark, nine-banded armadillo, Hoffmann's two-toed sloth) with enamelless teeth, suggesting that these genes have critical functions beyond their involvement in tooth development. By contrast, genomic data for C4orf26 reveal inactivating mutations in pangolin and bowhead whale as well as evidence for deletion of this gene in two minke whale species. Hybridization capture of exonic regions and PCR screens provide evidence for inactivation of C4orf26 in eight additional baleen whale species. However, C4orf26 is intact in all three species with enamelless teeth that were surveyed, as well as in 95 additional mammalian species with enamel-capped teeth. Estimates of selection intensity suggest that dN/dS ratios on branches leading to taxa with enamelless teeth are similar to the dN/dS ratio on branches leading to taxa with enamel-capped teeth. Based on these results, we conclude that C4orf26 is tooth-specific, but not enamel-specific, with respect to its essential functions that are maintained by natural selection. A caveat is that an alternative splice site variant, which translates exon 3 in a different reading frame, is putatively functional in Catarrhini and may have evolved an additional role in this primate clade.

Drivers of morphological evolution in the toothed whale jaw.

Toothed whales (odontocetes) emit high-frequency underwater sounds (echolocate)-an extreme and unique innovation allowing them to sense their prey and environment. Their highly specialized mandible (lower jaw) allows high-frequency sounds to be transmitted back to the inner ear. Echolocation is evident in the earliest toothed whales, but little research has focused on the evolution of mandibular form regarding this unique adaptation. Here, we use a high-density, three-dimensional geometric morphometric analysis of 100 living and extinct cetacean species spanning their ∼50-million-year evolutionary history. Our analyses demonstrate that most shape variation is found in the relative length of the jaw and the mandibular symphysis. The greatest morphological diversity was obtained during two periods of rapid evolution: the initial evolution of archaeocetes (stem whales) in the early to mid-Eocene as they adapted to an aquatic lifestyle, representing one of the most extreme adaptive transitions known, and later on in the mid-Oligocene odontocetes as they became increasingly specialized for a range of diets facilitated by increasingly refined echolocation. Low disparity in the posterior mandible suggests the shape of the acoustic window, which receives sound, has remained conservative since the advent of directional hearing in the aquatic archaeocetes, even as the earliest odontocetes began to receive sounds from echolocation. Diet, echolocation, feeding method, and dentition type strongly influence mandible shape. Unlike in the toothed whale cranium, we found no significant asymmetry in the mandible. We suggest that a combination of refined echolocation and associated dietary specializations have driven morphology and disparity in the toothed whale mandible.

Evolution: Killer whale bites and appetites.

A newly discovered fossil dolphin shows that modern killer and false-killer whales evolved from fish-eating ancestors. While today both species occasionally feed on large warm-blooded prey, including seals and other whales, this diet specialization has evolved only recently.

Prenatal Development of the Humpback Whale: Growth Rate, Tooth Loss and Skull Shape Changes in an Evolutionary Framework.

Extant baleen whales (Mysticeti) share a distinct suite of extreme and unique adaptations to perform bulk filter feeding, such as a long, arched skull, and mandible and the complete loss of adult dentition in favor of baleen plates. However, mysticetes still develop tooth germs during ontogeny. In the fossil record, multiple groups document the transition from ancestral raptorial feeding to filter feeding. Fetal specimens give us an extraordinary opportunity to observe when and how this macroevolutionary transition occurs during gestation. We used iodine-enhanced and traditional CT scanning to visualize the internal anatomy of five fetuses of humpback whale representing the first two-thirds of gestation, and we combine these data with previously published reports to provide the first comprehensive qualitative description of the sequence of developmental changes that characterize the skull and dentition. We also use quantitative methods based on 3D landmarks to investigate the shape changes in the fetuses in relation to a juvenile cranial morphology. We found similarities in the ossification patterns of the humpback and other cetaceans (dolphins), but there appear to be major differences when comparing them to terrestrial artiodactyls. As for the tooth germs, this developmental sequence confirms that the tooth-to-baleen transition occurs in the last one-third of gestation. Analysis of cranial shape development revealed a progressive elongation of the rostrum and a resulting posterior movement of the nasals relative to the braincase. Future work will involve acquisition of data from other species to complete our documentation of the teeth-to-baleen transition. Anat Rec, 2018. © 2018 American Association for Anatomy.

From Teeth to Baleen and Raptorial to Bulk Filter Feeding in Mysticete Cetaceans: The Role of Paleontological, Genetic, and Geochemical Data in Feeding Evolution and Ecology.

The origin of baleen and filter feeding in mysticete cetaceans occurred sometime between approximately 34 and 24 million years ago and represents a major macroevolutionary shift in cetacean morphology (teeth to baleen) and ecology (raptorial to filter feeding). We explore this dramatic change in feeding strategy by employing a diversity of tools and approaches: morphology, molecules, development, and stable isotopes from the geological record. Adaptations for raptorial feeding in extinct toothed mysticetes provide the phylogenetic context for evaluating morphological apomorphies preserved in the skeletons of stem and crown edentulous mysticetes. In this light, the presence of novel vascular structures on the palates of certain Oligocene toothed mysticetes is interpreted as the earliest evidence of baleen and points to an intermediate condition between an ancestral condition with teeth only and a derived condition with baleen only. Supporting this step-wise evolutionary hypothesis, evidence from stable isotopes show how changes in dental chemistry in early toothed mysticetes tracked the changes in diet and environment. Recent discoveries also demonstrate how this transition was made possible by radical changes in cranial ontogeny. In addition, genetic mutations and the possession of dental pseudogenes in extant baleen whales support a toothed ancestry for mysticetes. Molecular and morphological data also document the dramatic developmental shifts that take place in extant fetal baleen whales, in skull development, resorption of a fetal dentition and growth of baleen. The mechanisms involved in this complex evolutionary transition that entails multiple, integrated aspects of anatomy and ecology are only beginning to be understood, and future work will further clarify the processes underlying this macroevolutionary pattern.