Mark A. Purnell

ORIENTATION AND ANATOMICAL NOTATION IN CONODONTS

Abstract All aspects of conodont paleontology rely on the identification and description of homologous anatomical units or elements. But the current schemes of anatomical notation and terms for orientation were formulated at a time when little was known of conodont anatomy or skeletal architecture, resulting in some confusion and difficulties in their application. With improving knowledge of conodonts, these problems are becoming increasingly acute. In an attempt to address current problems, we introduce new terms for orientation in conodonts and their elements, and a modified scheme of anatomical notation. The principal axes of the conodont body are identified as rostrocaudal, dorsoventral, and mediolateral, with opposite lateral sides designated dextral and sinistral. Anatomical notation is defined according to topological relationships between elements with reference to the principal axes of the body and takes the form of letters with numeric subscripts (e.g., P1, P2, S0-S4). The ozarkodinid apparatus serves as a standard, but the Pn-Sn scheme can be applied rigorously to all taxa that are known from natural assemblages or where an hypothesis of topological homology can be inferred from secondary morphological criteria.

Non-random decay of chordate characters causes bias in fossil interpretation.

Exceptional preservation of soft-bodied Cambrian chordates provides our only direct information on the origin of vertebrates. Fossil chordates from this interval offer crucial insights into how the distinctive body plan of vertebrates evolved, but reading this pre-biomineralization fossil record is fraught with difficulties, leading to controversial and contradictory interpretations. The cause of these difficulties is taphonomic: we lack data on when and how important characters change as they decompose, resulting in a lack of constraint on anatomical interpretation and a failure to distinguish phylogenetic absence of characters from loss through decay. Here we show, from experimental decay of amphioxus and ammocoetes, that loss of chordate characters during decay is non-random: the more phylogenetically informative are the most labile, whereas plesiomorphic characters are decay resistant. The taphonomic loss of synapomorphies and relatively higher preservation potential of chordate plesiomorphies will thus result in bias towards wrongly placing fossils on the chordate stem. Application of these data to Cathaymyrus (Cambrian period of China) and Metaspriggina (Cambrian period of Canada) highlights the difficulties: these fossils cannot be placed reliably in the chordate or vertebrate stem because they could represent the decayed remains of any non-biomineralized, total-group chordate. Preliminary data suggest that this decay filter also affects other groups of organisms and that ‘stem-ward slippage’ may be a widespread but currently unrecognized bias in our understanding of the early evolution of a number of phyla.

Distinguishing heat from light in debate over controversial fossils

Fossil organisms offer our only direct insight into how the distinctive body plans of extant organisms were assembled. However, realizing the potential evolutionary significance of fossils can be hampered by controversy over their interpretation. Here, as a guide to evaluating palaeontological debates, we outline the process and pitfalls of fossil interpretation. The physical remains of controversial fossils should be reconstructed before interpreting homologies, and choice of interpretative model should be explicit and justified. Extinct taxa lack characters diagnostic of extant clades because the characters had not yet evolved, because of secondary loss, or because they have rotted away. The latter, if not taken into account, will lead to the spurious assignment of fossils to basally branching clades. Conflicting interpretations of fossils can often be resolved by considering all the steps in the process of anatomical analysis and phylogenetic placement, although we must accept that some fossil organisms are simply too incompletely preserved for their evolutionary significance to be realized.

Dietary specializations and diversity in feeding ecology of the earliest stem mammals

The origin and radiation of mammals are key events in the history of life, with fossils placing the origin at 220 million years ago, in the Late Triassic period. The earliest mammals, representing the first 50 million years of their evolution and including the most basal taxa, are widely considered to be generalized insectivores. This implies that the first phase of the mammalian radiation—associated with the appearance in the fossil record of important innovations such as heterodont dentition, diphyodonty and the dentary–squamosal jaw joint—was decoupled from ecomorphological diversification. Finds of exceptionally complete specimens of later Mesozoic mammals have revealed greater ecomorphological diversity than previously suspected, including adaptations for swimming, burrowing, digging and even gliding, but such well-preserved fossils of earlier mammals do not exist, and robust analysis of their ecomorphological diversity has previously been lacking. Here we present the results of an integrated analysis, using synchrotron X-ray tomography and analyses of biomechanics, finite element models and tooth microwear textures. We find significant differences in function and dietary ecology between two of the earliest mammaliaform taxa, Morganucodon and Kuehneotherium—taxa that are central to the debate on mammalian evolution. Morganucodon possessed comparatively more forceful and robust jaws and consumed ‘harder’ prey, comparable to extant small-bodied mammals that eat considerable amounts of coleopterans. Kuehneotherium ingested a diet comparable to extant mixed feeders and specialists on ‘soft’ prey such as lepidopterans. Our results reveal previously hidden trophic specialization at the base of the mammalian radiation; hence even the earliest mammaliaforms were beginning to diversify—morphologically, functionally and ecologically. In contrast to the prevailing view, this pattern suggests that lineage splitting during the earliest stages of mammalian evolution was associated with ecomorphological specialization and niche partitioning.

Decay of vertebrate characters in hagfish and lamprey (Cyclostomata) and the implications for the vertebrate fossil record

The timing and sequence of events underlying the origin and early evolution of vertebrates remains poorly understood. The palaeontological evidence should shed light on these issues, but difficulties in interpretation of the non-biomineralized fossil record make this problematic. Here we present an experimental analysis of decay of vertebrate characters based on the extant jawless vertebrates (Lampetra and Myxine). This provides a framework for the interpretation of the anatomy of soft-bodied fossil vertebrates and putative cyclostomes, and a context for reading the fossil record of non-biomineralized vertebrate characters. Decay results in transformation and non-random loss of characters. In both lamprey and hagfish, different types of cartilage decay at different rates, resulting in taphonomic bias towards loss of ‘soft’ cartilages containing vertebrate-specific Col2α1 extracellular matrix proteins; phylogenetically informative soft-tissue characters decay before more plesiomorphic characters. As such, synapomorphic decay bias, previously recognized in early chordates, is more pervasive, and needs to be taken into account when interpreting the anatomy of any non-biomineralized fossil vertebrate, such as Haikouichthys, Mayomyzon and Hardistiella.

Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding.

Understanding the feeding mechanisms and diet of nonavian dinosaurs is fundamental to understanding the paleobiology of these taxa and their role in Mesozoic terrestrial ecosystems. Various methods, including biomechanical analysis and 3D computer modeling, have been used to generate detailed functional hypotheses, but in the absence of either direct observations of dinosaur feeding behavior, or close living functional analogues, testing these hypotheses is problematic. Microscopic scratches that form on teeth in vivo during feeding are known to record the relative motion of the tooth rows to each other during feeding and to capture evidence of tooth–food interactions. Analysis of this dental microwear provides a powerful tool for testing hypotheses of jaw mechanics, diet, and trophic niche; yet, quantitative analysis of microwear in dinosaurs has not been attempted. Here, we show that analysis of tooth microwear orientation provides direct evidence for the relative motions of jaws during feeding in hadrosaurid ornithopods, the dominant terrestrial herbivores of the Late Cretaceous. Statistical testing demonstrates that Edmontosaurus teeth preserve 4 distinct sets of scratches in different orientations. In terms of jaw mechanics, these data indicate an isognathic, near-vertical posterodorsal power stroke during feeding; near-vertical jaw opening; and propalinal movements in near anterior and near posterior directions. Our analysis supports the presence of a pleurokinetic hinge, and the straightness and parallelism of scratches indicate a tightly controlled occlusion. The dominance of scratched microwear fabrics suggests that Edmontosaurus was a grazer rather than a browser.

The interrelationships of ‘complex’ conodonts (Vertebrata)

Synopsis Little attention has been paid to the suprageneric classification for conodonts and existing schemes have been formulated without attention to homology, diagnosis and definition. We propose that cladistics providesan appropriate methodology to test existingschemes of classification and in which to explore the evolutionary relationships of conodonts. The development of a multielement taxonomy and a concept of homology based upon the position, not morphology, of elements within the apparatus provide the ideal foundation for the application of cladistics to conodonts. In an attempt to unravel the evolutionary relationships between ‘complex’ conodonts (prioniodontids and derivative lineages) we have compiled a data matrix based upon 95 characters and 61 representative taxa. The dataset was analysed using parsimony and the resulting hypotheses were assessed using a number of measures of support. These included bootstrap, Bremer Support and double‐decay; we also compared levels of homoplasy to those expected given the size of the dataset and to those expected in a random dataset. The dataset was analysed in three hierarchical tranches, representing three levels of certainty concerning multi‐element reconstructions and positional homologies. There is much agreement between the results derived from the three partitions, but some inconsistency, particularly in the precise composition of the three main evolutionary grades traditionally recognised (Prioniodontida, Prioniodinina, Ozarkodinina). This is considered to result from (a) the progressive inclusion of data that is increasingly uncertain and (b) the inclusion of increasingly distantly related taxa, introducing spurious hypotheses of homology. We tested for these by partitioning the dataset into the three main evolutionary grades and in each instance resolution was seen to increase substantially, especially among prioniodinins. Our concluding scheme of relationships is a tree derived from a compilation of the three component subtrees, which is directly compatible with the most‐parsimonious trees derived from the initial second tranche analysis with the exception of the position of Hibbardella. This is compared in detail to the main extant schemes of suprageneric classification. A formal scheme of suprageneric classification is presented and the distribution of characters with respect to component clades is considered as a basis for identifying diagnostic characters.

Feeding in extinct jawless heterostracan fishes and testing scenarios of early vertebrate evolution

How long–extinct jawless fishes fed is poorly understood, yet interpretations of feeding are an important component of many hypotheses concerning the origin and early evolution of vertebrates. Heterostracans were the most diverse clade of armoured jawless vertebrates (stem gnathostomes), and the structure of the mouth and its use in feeding are the subjects of long–standing and heated controversy. I present here evidence that heterostracan feeding structures exhibit recurrent patterns of in vivo wear, are covered internally by microscopic oral denticles, and that the mouth may have been less flexible than has been thought. These data, particularly the absence of wear at the tips of oral plates, and the evidence that the mouth was lined with delicate outwardly directed denticles, effectively falsify all but one hypothesis of feeding in heterostracans: heterostracans were microphagous suspension feeders. This has a direct bearing on hypotheses that address ecological aspects of early vertebrate diversity and evolution, contradicting the widespread view that the pattern of early vertebrate evolution reflects a long–term trend towards increasingly active and predatory habits.

The conodont controversies

The discovery of fossilized conodont soft tissues has led to suggestions that these enigmatic animals were among the earliest vertebrates and that they were macrophagous, using their oropharyngeal skeletal apparatus to capture and process prey. These conclusions have proved controversial. There is now a consensus that conodonts belong within the chordates, but their position within the clade is hotly debated. Resolution of these questions has major implications for our understanding of the origin of the vertebrates and the selective pressures that led to the development of the vertebrate skeleton.

Feeding mechanisms in conodonts and the function of the earliest vertebrate hard tissues

The outcome of the debate over conodont feeding mechanisms has important implications for hypotheses of early vertebrate relations and the evolution of vertebrate hard tissues. The lack of consensus regarding feeding and element function stems from the lack of thorough testing of functional hypotheses. The results of analysis of conodont skeletal ontogeny demonstrate that conodonts were not suspension feeders and support hypotheses of a toothlike function.