Ivan J. Sansom

HISTOLOGY OF THE GALEASPID DERMOSKELETON AND ENDOSKELETON, AND THE ORIGIN AND EARLY EVOLUTION OF THE VERTEBRATE CRANIAL ENDOSKELETON

Abstract The histological composition of the galeaspid cephalothoracic skeleton has been much debated: here we attempt to resolve this through the analysis of well-preserved remains of galeaspids from Yunnan Province, and Tarim Basin, Xinjiang Uygur Autonomous Region, China. Our results indicate that the galeaspid dermoskeleton is dominantly composed from an acellular laminar bone in which the mineral is organised into cylindrical crystal bundles that are arranged into three orthogonal sets with associated extrinsic fiber spaces, a unique histology for which the term galeaspedin is coined. This is permeated by a coarse vascular plexus that divides the dermoskeleton into upper and lower zones, and the upper zone into distinct tesserae which, like the bounding vascular network, are polygonal in outline. The outer surface of the dermoskeleton is ornamented by a series of tubercles centered on tesserae, the latter composed partly from galeaspedin, and partly from a capping layer of microspherulitic, acellular bone, similar to the limiting layer of bone of elasmoid scales. Neither dentine nor enameloid is present, nor do the tissue compositions or their arrangement indicate an odontogenic origin. The endoskeleton is composed of an outer zone of globular calcified cartilage in contact with the dermoskeleton through a poorly mineralized intermediate zone. The inner zone is finely laminated, resulting from progressive zones of calcification embracing the calcospherites in a direction away from the dermoskeleton. There is no persuasive histological evidence for the presence of appositional perichondral bone. As in osteostracans, the galeaspid endoskeleton is interpreted as an expanded neurocranium. However, the presence of a calcified cartilaginous neurocranium in galeaspids in the absence of a perichondral bone layer indicates that these two histogenic components have distinct evolutionary origins. The presence of perichondral bone is a synapomorphy of osteostracans and jawed vertebrates, while the presence of a mineralized neurocranium unites galeaspids to this clade (possibly also including pituriaspids).

Spines and tissues of ancient sharks

The ‘spine-brush complex’ of the extinct, mid-Palaeozoic primitive chondrichthyan shark Stethacanthus is one of the strangest vertebrate appendages known. Its structure has never been defined, but here we reveal that the ‘brush’ is actually an enlarged, specialized extension of the fin baseplate (basal cartilage). It consists of an unusual type of globular calcified cartilage, a tissue that is often associated with pre-jawed primitive vertebrates.

Trace Fossils and Paleoenvironments of a Late Silurian Marginal-Marine/Alluvial System: the Ringerike Group (Lower Old Red Sandstone), Oslo Region, Norway

Abstract The Late Silurian Ringerike Group of southern Norway is a lower Old Red Sandstone megasequence that marks the regressive culmination of Cambro–Silurian marine deposition in the Oslo Region. The basal Sundvollen Formation represents deposition in a number of sub-environments of a broad, muddy coastal-plain setting, and is succeeded by fluvial deposition of the Stubdal and Store Arøya Formations. The terminal formation of the Ringerike Group, the Holmestrand Formation, represents deposition in sub-environments of a sandy beach setting. Both the marginal-marine Sundvollen and Holmestrand formations contain a wide variety of trace fossils (Arenicolites, Cruziana, Didymaulichnus, Diplichnites gouldi, Diplocraterion, Gordia marina, Margaritichnus, “Merostomichnites,” Oniscoidichnus, Paleohelcura, Palmichnium stoermeri, ?Polarichnus garnierensis, Rusophycus, Siskemia bipediculus, Skolithos, Steinsfjordichnus brutoni, Taenidium) that are discussed and analyzed, comprising the first complete Late Silurian study from the Baltic area. The trace fossils, which occur in facies-controlled assemblages, are combined with sedimentologic evidence to perform a high-resolution paleoenvironmental analysis of the Ringerike Group. Multivariate cluster analysis of the bulk Ringerike ichnofauna with similar-aged ichnofaunas from other paleocontinents supports the hypothesis that localized environmental factors outweighed provincialism as the dominant control on the composition of arthropod-trackway-dominated trace-fossil assemblages during the Siluro–Devonian.

Histology and affinity of the earliest armoured vertebrate.

Arandaspids are the earliest skeletonizing vertebrates known from articulated remains. Despite a wealth of data, their affinity remains questionable because they exhibit a random mixture of primitive and derived characteristics. We constrain the affinity of arandaspids by providing the first detailed characterization of their dermoskeleton which is revealed to be three-layered, composed of a basal laminated, cancellous middle and tubercular superficial layers. All three layers are composed of acellular bone but the superficial layer also includes dentine and enameloid, comprising the tubercles. As such, the composition of the arandaspid dermoskeleton is common to heterostracans and astraspids, supporting existing hypotheses of early vertebrate phylogeny. This emphasizes the peculiarity of existing interpretations of aranadaspid anatomy and there is need for a complete reappraisal of the existing anatomical data.

Ordovician vertebrate habitats: A Gondwanan perspective

Abstract The Middle–Upper Ordovician represents a significant period in the early evolution of fishes. During this time, many of the major lineages, including jawless and putative jawed taxa, made their first appearance in the fossil record, marking a series of diversification events. As a number of studies have focused on the habitat of Laurentian fish during this interval, work has been undertaken at a number of known Gondwanan vertebrate localities in order to provide new perspectives on the ecological preferences of early fish from the Southern Hemisphere. Ichnological and sedimentological data collated from these localities enable reconstructions of the habitats of a number of Ordovician fish, most notably those of the arandaspid-bearing successions of the Anzaldo Formation of Bolivia, the Stairway Sandstone of central Australia, and the Amdeh Formation of Oman, from which articulated or macroscopic fragmentary fossil remains are recorded. These data indicate that the arandaspids were constrained to very shallow marine habitats and prone to seasonal influxes of freshwater and terrigenous sediment. It is proposed that this narrow paleoecological range may be used as a prospecting tool to search for other Ordovician vertebrate-bearing horizons.

Diversity of the dermal skeleton in Ordovician to Silurian vertebrate taxa from North America: Histology, skeletogenesis and relationships

Abstract All three Ordovician agnathans from Canon City, Colorado, USA, possess dentine and an enamel or enameloidcovering layer. Two have extensive bony trabeculae of aspidin below the tubercles, as part of the tesserae, whereas the other has cellular bone of attachment as the scale base. Each of the three has a different and distinctive dentine and tissue arrangement. Two have enameloid caps ( Astraspis and Vertebrate Indeterminate A), the former is above a microtubular dentine, the latter above a cellular mesodentine. In Eriptychius the dentine is a macrotubular form, none conforms to the type of orthodentine as found in later heterostracans.

The tail of the Ordovician fish Sacabambaspis

The tail of the earliest known articulated fully skeletonized vertebrate, the arandaspid Sacabambaspis from the Ordovician of Bolivia, is redescribed on the basis of further preparation of the only specimen in which it is most extensively preserved. The first, but soon discarded, reconstruction, which assumed the presence of a long horizontal notochordal lobe separating equal sized dorsal and ventral fin webs, appears to have considerable merit. Although the ventral web is significantly smaller than the dorsal one, the presence of a very long notochordal lobe bearing a small terminal web is confirmed. The discrepancy in the size of the ventral and dorsal webs rather suggests that the tail was hypocercal, a condition that would better accord with the caudal morphology of the living agnathans and the other jawless stem gnathostomes.

A new pteraspidomorph from the Nibil Formation (Katian, Late Ordovician) of the Canning Basin, Western Australia

ABSTRACT Based upon fragmentary remains of dermal armor, a new form of arandaspid fish, Ritchieichthys nibili, gen et. sp. nov., is described from subsurface core material from the Katian (Late Ordovician) Nibil Formation of the Canning Basin, Western Australia. Ritchieichthys nibili represents the first documented record of a fish from the Ordovician of the Canning Basin. Allied to the previous descriptions of arandaspsids from the Amadeus and Warburton basins of the Northern Territory and New South Wales, respectively, this record extends the paleogeographic range of arandaspids across the hypothetical Ordovician Larapintine Seaway and increases the stratigraphic range of the Order Arandaspidiformes into the Katian. The hard tissue histology of Ritchieichthys nibili confirms the presence of a cellular dentine forming the bulk of the dermal armor ornament in arandaspids, a tissue that had not been directly observed previously, and confirms the presence of largely unconnected osteocytes within the dermal bone that forms the majority of the armor.

Ichnofacies of the Stairway Sandstone fish-fossil beds (Middle Ordovician, Northern Territory, Australia)

The Stairway Sandstone is a 30–560 m thick succession of Middle Ordovician siliciclastic sedimentary rocks within the Amadeus Basin of central Australia, deposited in the epeiric Larapintine Sea of northern peri-Gondwana. The Stairway Sandstone is significant as one of only two known Gondwanan successions to yield articulated arandaspid (pteraspidomorph agnathan) fish. Herein we use the ichnology of the Stairway Sandstone to reveal insights into the shallow marine habitat of these early vertebrates, and compare it with that of other known pteraspidomorph-bearing localities from across Gondwana. The Stairway Sandstone contains a diverse Ordovician ichnofauna including 22 ichnotaxa of Arenicolites, Arthrophycus, Asterosoma, Cruziana, Didymaulichnus, Diplichnites, Diplocraterion, ?Gordia, Lockeia, Monocraterion, Monomorphichnus, Phycodes, Planolites, Rusophycus, Skolithos and Uchirites. These ichnofauna provide a well-preserved example of a typical Ordovician epeiric sea assemblage, recording the diverse ethologies of tracemakers in a very shallow marine environment of flashy sediment accumulation and regularly shifting sandy substrates. New conodont data refine the age of the Stairway Sandstone to the early Darriwilian, with ichnostratigraphic implications for the Cruziana rugosa group and Arthrophycus alleghaniensis.

Cutting the first ‘teeth’: a new approach to functional analysis of conodont elements

The morphological disparity of conodont elements rivals the dentition of all other vertebrates, yet relatively little is known about their functional diversity. Nevertheless, conodonts are an invaluable resource for testing the generality of functional principles derived from vertebrate teeth, and for exploring convergence in a range of food-processing structures. In a few derived conodont taxa, occlusal patterns have been used to derive functional models. However, conodont elements commonly and primitively exhibit comparatively simple coniform morphologies, functional analysis of which has not progressed much beyond speculation based on analogy. We have generated high-resolution tomographic data for each morphotype of the coniform conodont Panderodus acostatus. Using virtual cross sections, it has been possible to characterize changes in physical properties associated with individual element morphology. Subtle changes in cross-sectional profile have profound implications for the functional performance of individual elements and the apparatus as a whole. This study has implications beyond the ecology of a single conodont taxon. It provides a basis for reinterpreting coniform conodont taxonomy (which is based heavily on cross-sectional profiles), in terms of functional performance and ecology, shedding new light on the conodont fossil record. This technique can also be applied to more derived conodont morphologies, as well as analogous dentitions in other vertebrates and invertebrates.