Abstract
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.
Highlights
The diversity and disparity of living vertebrates are dominated by the gnathostomes, with jawless vertebrates comprising just two conservative lineages, the hagfishes and lampreys
Examples include the hypotheses that the vertebrate skeleton evolved in response to predation by eurypterids [5], and that ostracoderms were competitively displaced by jawed vertebrates [6,7], both of which are based on coincident patterns of raw diversity
The reconstructed diversity curves for ostracoderms follow a similar pattern in each group; there was an early peak in diversity in the Silurian followed by a drop in the Upper Silurian and larger maximum peak in at the Early Devonian (Lochkovian) followed by gradual decline towards the end of the Devonian
Summary
The diversity and disparity of living vertebrates are dominated by the gnathostomes ( jawed vertebrates), with jawless vertebrates comprising just two conservative lineages, the hagfishes and lampreys. Jawed vertebrates only assumed dominance in the Devonian (419–359 Ma); for the preceding 100 million years (Myr), vertebrate communities were dominated by the ‘ostracoderms’, an entirely extinct grade of jawless fishes characterized by an extensive armour-like dermal skeleton. The evolutionary dynamics of events surrounding the emergence of jawed vertebrates have been read literally from the stratigraphic record [3,4]. Examples include the hypotheses that the vertebrate skeleton evolved in response to predation by eurypterids [5], and that ostracoderms were competitively displaced by jawed vertebrates [6,7], both of which are based on coincident patterns of raw diversity. It is widely acknowledged that the fossil record is biased by secular variation in the rock record, such that it has become appropriate to assume a null model that stratigraphic variation in rock
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