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

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.

Structure, not Bias

“…Peter Wagner and Jonathan Marcot (2013) showed how, with a relatively simple segregation of their data in time bins on different continents, they could allow preservation probability to vary through time and space, producing superior estimates of divergence times. Others have had similar success (Sansom et al 2014, Silvestro et al 2014, and macrostratigraphy (Peters 2006) has great promise for allowing these kinds of approaches to be done more widely. All of these hinge on understanding and embracing the structure of the fossil record and the sedimentary record in which it is found.…”

The non-uniformity of fossil preservation

“…Estimates of outcrop area in general will incorporate many more environments than such near-shore ones, thereby overestimating the opportunities for fossil recovery and artificially shortening confidence intervals on the origin of clades. Indeed, when Sansom et al [103] accounted for habitat, the length of their confidence intervals increased, better reflecting the probability of recovery of these jawless vertebrates. If indirect proxies such as outcrop area, number of stratigraphic units and gap-bound packages were tailored to particular habitats, biomes and provinces, they could produce improved estimates of fossil recovery.…”

“…Indirect proxies should be used with caution as they can give a misleading picture of potential fossil recovery where not all strata are 'sampling opportunities' [5], that is, collections from which a clade of interest could have been sampled had they been present. For example, Early Palaeozoic jawless vertebrates have a strong affinity for near-shore terrigenous sandy substrates [103,[105][106][107]. Estimates of outcrop area in general will incorporate many more environments than such near-shore ones, thereby overestimating the opportunities for fossil recovery and artificially shortening confidence intervals on the origin of clades.…”

“…One possibility is rock-outcrop area (the area over which rocks of a given age are exposed at the Earth's surface), which correlates with fossil occurrences and diversity [98 -103]. For example, Sansom et al [103] used rock outcrop area and sea level to constrain the divergence times of jawless vertebrates in the lower Palaeozoic. They showed that ghost clades are explained substantially better by using confidence intervals on the oldest members of a clade that incorporate non-constant preservation.…”

Exploring phylogenetic relationships of Pteraspidiformes heterostracans (stem-gnathostomes) using continuous and discrete characters

“…A solid phylogenetic framework for the Heterostraci would allow identification of pleisiomorphic states and characters, which could in turn elucidate ancestral conditions on the gnathostome stem. Furthermore, a phylogeny would enable the incorporation of ghost ranges to studies of diversity through time (Sansom et al 2015) and a test for scenarios of palaeobiogeography (Sansom 2009;Blieck 2011;Zigait_ e & Blieck 2013). Historically, there are five major clades within the Heterostraci; Cyathaspididae, Amphiaspididae, Traquairaspididae, Psammosteidae and Pteraspidiformes, along with many incertae sedis genera of uncertain affinity (Halstead 1973;Janvier 1996).…”