Sequences, stratigraphy and scenarios: what can we say about the fossil record of the earliest tetrapods?

Abstract

Past research on the emergence of digit-bearing tetrapods has led to the widely accepted premise that this important evolutionary event occurred during the Late Devonian. The discovery of convincing digit-bearing tetrapod trackways of early Middle Devonian age in Poland has upset this orthodoxy, indicating that current scenarios which link the timing of the origin of digited tetrapods to specific events in Earth history are likely to be in error. Inspired by this find, we examine the fossil record of early digitbearing tetrapods and their closest fish-like relatives from a statistical standpoint. We find that the Polish trackways force a substantial reconsideration of the nature of the early tetrapod record when only body fossils are considered. However, the effect is less drastic (and often not statistically significant) when other reliably dated trackways that were previously considered anachronistic are taken into account. Using two approaches, we find that 95 per cent credible and confidence intervals for the origin of digitbearing tetrapods extend into the Early Devonian and beyond, spanning late Emsian to mid Ludlow. For biologically realistic diversity models, estimated genus-level preservation rates for Devonian digited tetrapods and their relatives range from 0.025 to 0.073 per lineage-million years, an order of magnitude lower than species-level rates for groups typically considered to have dense records. Available fossils of early digited tetrapods and their immediate relatives are adequate for documenting large-scale patterns of character acquisition associated with the origin of terrestriality, but low preservation rates coupled with clear geographical and stratigraphic sampling biases caution against building scenarios for the origin of digits and terrestrialization tied to the provenance of particular specimens or faunas.

First Articulated Phyllolepid Placoderm from North America, with Comments on Phyllolepid Systematics

“…A new chronology for tetrapod origins has been suggested based on the recent discoveries of putative tetrapod tracks and trackways in the basal Middle Devonian (Eifelian) of Poland (Niedźwiedzki et al, 2010). This chronology is further supported by recent analyses using molecular data from 13 proteins sequenced from 17 species to calculate divergence times (George and Blieck, 2011), and from statistical methods looking at predicted nodes for tetrapod divergence (Friedman and Brazeau, 2011). Both studies suggest an estimated Early Devonian time of origin for tetrapods.…”

Bias in phylogenetic measurements of extinction and a case study of end‐Permian tetrapods

“…For the majority of Palaeozoic and Mesozoic terrestrial vertebrate clades, particularly those which include many point occurrences, the sampling rate is likely to be on the order of 0.01 lmy −1 (Foote et al . ; Friedman & Brazeau ). Thus the simulation results at 0.01 ltu −1 are the most representative of our tetrapod dataset, and are therefore the best indicator of the bias that can be expected in our empirical results.…”

A Simple Method for Estimating Informative Node Age Priors for the Fossil Calibration of Molecular Divergence Time Analyses

“…A second approach would be to apply Alroy's [42], [50] two-timer rates method, which provides estimates of the preservation rate based on the ratio of different fossil range classes. There are many approaches for estimating preservation rates available (see for example [51]), and ultimately the users of this method will need to decide which approach is most appropriate for their taxonomic group of interest.…”