EURYPTERID INFLUENCE ON VERTEBRATE HISTORY

How to Survive a (Juvenile) Piranha Attack: An Integrative Approach to Evaluating Predator Performance

Figures

Cory cat panel figureDrawing of bite force measuring equipment and indentation rig Pygocentrus nattereri jaw muscle morphology and skull anatomyBox plot grid of number of Pygocentrus nattereri bites before puncture along different body regions of Corydoras trilineatus during feeding trials resultsDrawing of color-coded Corydoras trilineatus with attack frequencies and average bites until puncture by Pygocentrus nattereriBox plot of average voluntary juvenile Pygocentrus nattereri bite forces to standard lengthPanel of linear ordinary least-squares regressions of Pygocentrus nattereri bite force to adductor mandibulae mass, standard length, and body massOrdinary least-squares regressions of voluntary bites to restrained bites of Pygocentrus nattereriPanel of indentation tests for intact and removed Corydoras trilineatus scutesPanel of indentation tests for Corydoras trilineatus body region.

Synopsis

There is an evolutionary arms race between predators and prey. In aquatic environments, predatory fishes often use sharp teeth, powerful bites, and/or streamlined bodies to help capture their prey quickly and efficiently. Conversely, prey are often equipped with antipredator adaptations including: scaly armor, sharp spines, and/or toxic secretions. This study focused on the predator-prey interactions between the armored threestripe cory catfish (Corydoras trilineatus) and juvenile red-bellied piranha (Pygocentrus nattereri). Specifically, we investigated how resistant cory catfish armor is to a range of natural and theoretical piranha bite forces and how often this protection translated to survival from predator attacks by Corydoras. We measured the bite force and jaw functional morphology of P. nattereri, the puncture resistance of defensive scutes in C. trilineatus, and the in situ predatory interactions between the two. The adductor mandibulae muscle in juvenile P. nattereri is robust and delivers an average bite force of 1.03 N and maximum bite force of 9.71 N, yet its prey, C. trilineatus, survived 37% of confirmed bites without any damage. The C. trilineatus armor withstood an average of nine bites before puncture by P. nattereri. Predation was successful only when piranhas bit unarmored areas of the body, at the opercular opening and at the caudal peduncle. This study used an integrative approach to understand the outcomes of predator-prey interactions by evaluating the link between morphology and feeding behavior. We found that juvenile P. nattereri rarely used a maximal bite force and displayed a net predation success rate on par with other adult vertebrates. Conversely, C. trilineatus successfully avoided predation by orienting predator attacks toward their resilient, axial armor and behavioral strategies that reduced the predator's ability to bite in less armored regions of the body.

Biomechanical analyses of pterygotid sea scorpion chelicerae uncover predatory specialisation within eurypterids

Eurypterids (sea scorpions) are extinct aquatic chelicerates. Within this group, members of Pterygotidae represent some of the largest known marine arthropods. Representatives of this family all have hypertrophied, anteriorly-directed chelicerae and are commonly considered Silurian and Devonian apex predators. Despite a long history of research interest in these appendages, pterygotids have been subject to limited biomechanical investigation. Here, we present finite element analysis (FEA) models of four different pterygotid chelicerae-those of Acutiramus bohemicus, Erettopterus bilobus, Jaekelopterus rhenaniae, and Pterygotus anglicus-informed through muscle data and finite element models (FEMs) of chelae from 16 extant scorpion taxa. We find that Er. bilobus and Pt. anglicus have comparable stress patterns to modern scorpions, suggesting a generalised diet that probably included other eurypterids and, in the Devonian species, armoured fishes, as indicated by co-occurring fauna. Acutiramus bohemicus is markedly different, with the stress being concentrated in the proximal free ramus and the serrated denticles. This indicates a morphology better suited for targeting softer prey. Jaekelopterus rhenaniae exhibits much lower stress across the entire model. This, combined with an extremely large body size, suggests that the species likely fed on larger and harder prey, including heavily armoured fishes. The range of cheliceral morphologies and stress patterns within Pterygotidae demonstrate that members of this family had variable diets, with only the most derived species likely to feed on armoured prey, such as placoderms. Indeed, increased sizes of these forms throughout the mid-Palaeozoic may represent an 'arms race' between eurypterids and armoured fishes, with Devonian pterygotids adapting to the rapid diversification of placoderms.

Adaptive divergence of lateral plate ultrastructure in threespine stickleback

The lateral plates of threespine stickleback, Gasterosteus aculeatus Linnaeus, 1758, are well-studied for their adaptive morphological responses to predators, yet it is unknown whether habitat influences plate ultrastructure. We investigate using scanning electron microscopy the lateral plate ultrastructure (tubercles and ridges) of stickleback (<i>N</i> = 61 adult fish) from nine Haida Gwaii (coastal British Columbia) wild-type populations, two experimental transplants, and two lab-reared cohorts reared from source populations. Tubercle density, but not ridge density, differed significantly across habitat types and populations. Among wild-type fish, tubercle densities were greatest in dystrophic habitats containing predatory fish, and lowest in weakly dystrophic systems featuring bird–invertebrate predation and marine populations with diverse predatory fish. No differences in tubercle density were detected between source and transplant populations, despite major habitat shifts. Lab-reared fish exhibited significantly lower tubercle densities than their source populations (< one generation). Tubercle density differences across habitat types may reflect adaptation to divergent predation regimes, with tooth-bearing predators selecting for denser tubercles that disperse point forces. Conservation of ridge density across populations suggests an essential function in dispersing forces applied to dorsal spines during predator manipulation. Lateral plate ultrastructure in threespine stickleback thus results from both heritable effects and developmental plasticity.

Swimming and defence: competing needs across ontogeny in armoured fishes (Agonidae)

Biological armours are potent model systems for understanding the complex series of competing demands on protective exoskeletons; after all, armoured organisms are the product of millions of years of refined engineering under the harshest conditions. Fishes are no strangers to armour, with various types of armour plating common to the 400-500 Myr of evolution in both jawed and jawless fishes. Here, we focus on the poachers (Agonidae), a family of armoured fishes native to temperate waters of the Pacific rim. We examined armour morphology, body stiffness and swimming performance in the northern spearnose poacher (Agonopsis vulsa) over ontogeny. As juveniles, these fishes make frequent nocturnal forays into the water column in search of food, while heavily armoured adults are bound to the benthos. Most armour dimensions and density increase with body length, as does body stiffness. Juvenile poachers have enlarged spines on their armour whereas adults invest more mineral in armour plate bases. Adults are stiffer and accelerate faster than juveniles with an anguilliform swimming mode. Subadults more closely approximate adults more than smaller juveniles, with regards to both swimming and armour mechanics. Poacher armour serves multiple functions over ontogeny, from facilitating locomotion, slowing sinking and providing defence.

Bite marks and predation of fossil jawless fish during the rise of jawed vertebrates

Although modern vertebrate diversity is dominated by jawed vertebrates, early vertebrate assemblages were predominantly composed of jawless fishes. Hypotheses for this faunal shift and the Devonian decline of jawless vertebrates include predation and competitive replacement. The nature and prevalence of ecological interactions between jawed and jawless vertebrates are highly relevant to both hypotheses, but direct evidence is limited. Here, we use the occurrence and distribution of bite mark type traces in fossil jawless armoured heterostracans to infer predation interactions. A total of 41 predated specimens are recorded; their prevalence increases through time, reaching a maximum towards the end of the Devonian. The bite mark type traces significantly co-occur with jawed vertebrates, and their distribution through time is correlated with jawed vertebrate diversity patterns, particularly placoderms and sarcopterygians. Environmental and ecological turnover in the Devonian, especially relating to the nekton revolution, have been inferred as causes of the faunal shift from jawless to jawed vertebrates. Here, we provide direct evidence of escalating predation from jawed vertebrates as a potential contributing factor to the demise and extinction of ostracoderms.

Mediation of the Acute Stress Response by the Skeleton

We hypothesized that bone evolved, in part, to enhance the ability of bony vertebrates to escape danger in the wild. In support of this notion, we show here that a bone-derived signal is necessary to develop an acute stress response (ASR). Indeed, exposure to various types of stressors in mice, rats (rodents), and humans leads to a rapid and selective surge of circulating bioactive osteocalcin because stressors favor the uptake by osteoblasts of glutamate, which prevents inactivation of osteocalcin prior to its secretion. Osteocalcin permits manifestations of the ASR to unfold by signaling in post-synaptic parasympathetic neurons to inhibit their activity, thereby leaving the sympathetic tone unopposed. Like wild-type animals, adrenalectomized rodents and adrenal-insufficient patients can develop an ASR, and genetic studies suggest that this is due to their high circulating osteocalcin levels. We propose that osteocalcin defines a bony-vertebrate-specific endocrine mediation of the ASR.

All the better to see you with: eyes and claws reveal the evolution of divergent ecological roles in giant pterygotid eurypterids

Pterygotid eurypterids have traditionally been interpreted as active, high-level, visual predators; however, recent studies of the visual system and cheliceral morphology of the pterygotid Acutiramus contradict this interpretation. Here, we report similar analyses of the pterygotids Erettopterus, Jaekelopterus and Pterygotus, and the pterygotid sister taxon Slimonia. Representative species of all these genera have more acute vision than A. cummingsi. The visual systems of Jaekelopterus rhenaniae and Pterygotus anglicus are comparable to that of modern predatory arthropods. All species of Jaekelopterus and Pterygotus have robust crushing chelicerae, morphologically distinct from the weaker slicing chelicerae of Acutiramus. Vision in Erettopterus osiliensis and Slimonia acuminata is more acute than in Acutiramus cummingsi, but not to the same degree as in modern active predators, and the morphology of the chelicerae in these genera suggests a grasping function. The pterygotids evolved with a shift in ecology from generalized feeder to specialized predator. Pterygotid eurypterids share a characteristic morphology but, although some were top predators, their ecology differs radically between genera.

Discriminating signal from noise in the fossil record of early vertebrates reveals cryptic evolutionary history

The fossil record of early vertebrates has been influential in elucidating the evolutionary assembly of the gnathostome bodyplan. Understanding of the timing and tempo of vertebrate innovations remains, however, mired in a literal reading of the fossil record. Early jawless vertebrates (ostracoderms) exhibit restriction to shallow-water environments. The distribution of their stratigraphic occurrences therefore reflects not only flux in diversity, but also secular variation in facies representation of the rock record. Using stratigraphic, phylogenetic and palaeoenvironmental data, we assessed the veracity of the fossil records of the jawless relatives of jawed vertebrates (Osteostraci, Galeaspida, Thelodonti, Heterostraci). Non-random models of fossil recovery potential using Palaeozoic sea-level changes were used to calculate confidence intervals of clade origins. These intervals extend the timescale for possible origins into the Upper Ordovician; these estimates ameliorate the long ghost lineages inferred for Osteostraci, Galeaspida and Heterostraci, given their known stratigraphic occurrences and stem–gnathostome phylogeny. Diversity changes through the Silurian and Devonian were found to lie within the expected limits predicted from estimates of fossil record quality indicating that it is geological, rather than biological factors, that are responsible for shifts in diversity. Environmental restriction also appears to belie ostracoderm extinction and demise rather than competition with jawed vertebrates.

Evolutionary Physiology of Bone: Bone Metabolism in Changing Environments

Bone evolved to serve many mechanical and physiological functions. Osteocytes and bone remodeling first appeared in the dermal skeleton of fish, and subsequently adapted to various challenges in terrestrial animals occupying diverse environments. This review discusses the physiology of bone and its role in mechanical and calcium homeostases from an evolutionary perspective. We review how bone physiology responds to changing environments and the adaptations to unique and extreme physiological conditions.

IX.—The Postcranial Skeleton of Ensthenopteron foordi Whiteaves

Well preserved material of the crossopterygian fish Eusthenopteron enables fresh reconstructions and interpretations of its postcranial skeleton to be given. Comparisons throughout with other bony fishes show that it may be primitive in many features. Similarities with early amphibians such as the screw-shaped glenoid, the form of the humerus (on which an attempt to restore the pectoral musculature is based), the dorsal bicipital ribs and the possibility of a sacral attachment, throw much light on the origin of the tetrapod postcranial skeleton, particularly of the cheiropterygium. A functional analysis of the skeleton of Ensthenopteron is attempted, suggesting that it resembled the pike (Esox) in its mode of life and that it may have been capable of short journeys “walking” overland. The possible selective factors stimulating the evolution of such a fish, and further evolution to the tetrapod stage are discussed.

VI.—Features of Placoderm Diversification and the Evolution of the Arthrodire Feeding Mechanism

The initial adaptive radiation of the Placodermi took place rapidly following the development of the basic placoderm adaptive complex after the ancestral scale covering of the trunk fused into a rigid shield, and not long before the group appears in the fossil record in the Lower Devonian. The radiation was mainly concerned with different ways of living in the benthos of a variety of marine and fresh-water environments; a few nektonic species appear late in the history of the Arthrodira. The fossil record shows the evolution of the orders in their adaptive zones. The zones become increasingly distinct as the orders evolve and become more specific in their adaptations, and the arthrodire, antiarch and rhenanid zones segregate into successively occupied sub-zones. The evolution of the Placodermi has been previously described in terms of improvements in the locomotor mechanism by an analysis of changes in the trunk-armour and pectoral fins. A more detailed description can be given by considering the feeding mechanism as well; this is particularly true of the largest order, the Arthrodira. Study of the feeding mechanism involves the cervical joints as well as the jaws and gnathals. The cervical joints had the same functions in feeding as the anterior part of the vertebral column (“the neck”) in many higher fish. In arthrodires jaw action involved vertical movements of the mandibular lever; the upper jaw apparatus is comparable to the rigid palatoquadrate-maxillary complex of primitive bony fish. The mandible was transformed into a bent lever inBrachyosteusby the development of a small “coronoid” process, but the arthrodire jaw apparatus remained undeveloped in comparison with Actinopterygii and Elasmobranchii. Arthrodire jaw suspension was autostylic. Evidence from the Rhenanida and Ptyctodontida has been interpreted to suggest that this condition was secondary, and that primitive placoderms had an elasmobranch or holocephalan-like palatoquadrate with hyostylic suspension. This view is not entirely supported by the state of the palatoquadrate in primitive arthrodires, but there is no good evidence that placoderms had a complete, open spiracular gill-slit (the aphethoyoid condition). Arthrodire phylogeny cannot yet be described in vertical lines, but four successive levels of organization of increasing efficiency can be recognized; the actinolepid, phlyctaenaspid, coccosteomorph and pachyosteomorph levels. These levels can be defined by simple characters relating to broad adaptations in the locomotor and feeding mechanisms. Evolutionary trends in the Arthrodira include the enlargement of the scapulocoracoid and base of the pectoral fin and the reduction of the spinal plate and flank armour, as the fish gain better control in the water and more myomeres become available for use in swimming; and the enlargement of the nuchal gap and development of the cranio-thoracic joint as powerful muscles are developed to raise the head to give a wide gape, accompanied by the specialization of the gnathals for different modes of feeding. Some of these trends are reversed in compressed, nektonic species. The description of arthrodire phylogeny in terms of changes that can be understood from a functional point of view reveals interesting examples of mosaic and parallel evolution.Parabelosteusn.gen. is erected.

Cope's Rule and Romer's theory: patterns of diversity and gigantism in eurypterids and Palaeozoic vertebrates

Gigantism is widespread among Palaeozoic arthropods, yet causal mechanisms, particularly the role of (abiotic) environmental factors versus (biotic) competition, remain unknown. The eurypterids (Arthropoda: Chelicerata) include the largest arthropods; gigantic predatory pterygotids (Eurypterina) during the Siluro-Devonian and bizarre sweep-feeding hibbertopterids (Stylonurina) from the Carboniferous to end-Permian. Analysis of family-level originations and extinctions among eurypterids and Palaeozoic vertebrates show that the diversity of Eurypterina waned during the Devonian, while the Placodermi radiated, yet Stylonurina remained relatively unaffected; adopting a sweep-feeding strategy they maintained their large body size by avoiding competition, and persisted throughout the Late Palaeozoic while the predatory nektonic Eurypterina (including the giant pterygotids) declined during the Devonian, possibly out-competed by other predators including jawed vertebrates.

Materials design principles of ancient fish armour

Knowledge of the structure-property-function relationships of dermal scales of armoured fish could enable pathways to improved bioinspired human body armour, and may provide clues to the evolutionary origins of mineralized tissues. Here, we present a multiscale experimental and computational approach that reveals the materials design principles present within individual ganoid scales from the 'living fossil' Polypterus senegalus. This fish belongs to the ancient family Polypteridae, which first appeared 96 million years ago during the Cretaceous period and still retains many of their characteristics. The mechanistic origins of penetration resistance (approximating a biting attack) were investigated and found to include the juxtaposition of multiple distinct reinforcing composite layers that each undergo their own unique deformation mechanisms, a unique spatial functional form of mechanical properties with regions of differing levels of gradation within and between material layers, and layers with an undetectable gradation, load-dependent effective material properties, circumferential surface cracking, orthogonal microcracking in laminated sublayers and geometrically corrugated junctions between layers.

Giant claw reveals the largest ever arthropod

The fossil record has yielded various gigantic arthropods, in contrast to their diminutive proportions today. The recent discovery of a 46 cm long claw (chelicera) of the pterygotid eurypterid ('sea scorpion') Jaekelopterus rhenaniae, from the Early Devonian Willwerath Lagerstätte of Germany, reveals that this form attained a body length of approximately 2.5 m-almost half a metre longer than previous estimates of the group, and the largest arthropod ever to have evolved. Gigantism in Late Palaeozoic arthropods is generally attributed to elevated atmospheric oxygen levels, but while this may be applicable to Carboniferous terrestrial taxa, gigantism among aquatic taxa is much more widespread and may be attributed to other extrinsic factors, including environmental resources, predation and competition. A phylogenetic analysis of the pterygotid clade reveals that Jaekelopterus is sister-taxon to the genus Acutiramus, and is among the most derived members of the pterygotids, in contrast to earlier suggestions.

Origin and early evolution of vertebrate skeletonization

Data from living and extinct faunas of primitive vertebrates imply very different scenarios for the origin and evolution of the dermal and oral skeletal developmental system. A direct reading of the evolutionary relationships of living primitive vertebrates implies that the dermal scales, teeth, and jaws arose synchronously with a cohort of other characters that could be considered unique to jawed vertebrates: the dermoskeleton is primitively composed of numerous scales, each derived from an individual dental papilla; teeth are primitively patterned such that they are replaced in a classical conveyor-belt system. The paleontological record provides a unique but complementary perspective in that: 1) the organisms in which the skeletal system evolved are extinct and we have no recourse but to fossils if we aim to address this problem; 2) extinct organisms can be classified among, and in the same way as, living relatives; 3) a holistic approach to the incorporation of all data provides a more complete perspective on early vertebrate evolution. This combined approach is of no greater significance than in dealing with the origin of the skeleton and, combined with recent discoveries and new phylogenetic analyses, we have been able to test and reject existing hypotheses for the origin of the skeleton and erect a new model in their place.

Stages in the origin of vertebrates: analysis by means of scenarios

Vertebrates lack an epidermal nerve plexus. This feature is common to many invertebrates from which vertebrates differ by an extensive set of shared-derived characters (synapomorphies) derived from the neural crest and epidermal neurogenic placodes. Hence, the hypothesis that the developmental precursor of the epidermal nerve plexus may be homologous to the neural crest and epidermal neurogenic placodes. This account attempts to generate a nested set of scenarios for the prevertebrate-vertebrate transition, associating a presumed sequence of behavioural and environmental changes with the observed phenotypic ones. Toward this end, it integrates morphological, developmental, functional (physiological/behavioural) and some ecological data, as many phenotypic shifts apparently involved associated transitions in several aspects of the animals. The scenarios deal with the origin of embryonic and adult tissues and such major organs as the notochord, the CNS, grills and kidneys and propose a sequence of associated changes. Alternative scenarios are stated as the evidence often remains insufficient for decision. The analysis points to gaps in comprehension of the biology of the animals and therefore suggests further research.

Major steps in vertebrate evolution

We have come to the end of our story-a long one, covering some half a billion years, it appears. A modern man or other higher vertebrate has traveled far from the simply built insensate type of creature seen in his ultimate metazoan ancestor among the pterobranchs. The course of this evolutionary progression is far from direct and simple, as some might believe to be the case; it is a trail with many twists and turns. Nor is there the slightest reason to attempt a teleological interpretation; there is no trace of design and direction toward an obvious goal. Quite in contrast, it seems clear in many stages of the series that the changes which have taken place are immediately beneficial ones, strongly subject to selection. Obvious, too, is the fact that special environmental factors, biological and physical, have added unexpected quirks to the story. The development of a motile "tadpole" larva at an early chordate stage led to a sharp shift in an evolutionary sequence which otherwise might have simply ended in a sedate filtering form of tunicate type. The development of plant life on the continents opened up to motile chordates a new environment into which few invertebrates could enter and in which the chordates flourished to progress to the vertebrate level. The need for armor as defense against eurypterid enemies appears to have initiated the development of bony skeletal structures, without which the higher vertebrates could never have developed. The widespread late Paleozoic condition of seasonal drought favored progressive developments which, with the attainment of a reptilian stage, had the happy accidental result of the vertebrate conquest of the land, a conquest aided by the emergence of the insects as a basic food supply. The long period of dinosaur dominance seems to have been responsible for the sharpened wits which made the mammalian descendants of the therapsids competent for terrestrial dominance when the reign of the ruling reptiles ended. The arboreal life of primates was finally abandoned by man, but tree-dwelling had endowed his ancestors with advances in brain, eyes, and hands that were highly advantageous when this relatively feeble creature descended to the ground. It has been a long and tortuous journey; but every stage of it shows its effects in the structures and functions of such an end product as ourselves (Fig. 4).