Lepidosaurian diversity in the Mesozoic-Palaeogene: the potential roles of sampling biases and environmental drivers

Abstract

Lepidosauria is a speciose clade with a long evolutionary history, but there have been few attempts to explore its taxon richness through time. Here we estimate patterns of terrestrial lepidosaur genus diversity for the Triassic-Palaeogene (252-23 Ma), and compare observed and sampling-corrected richness curves generated using Shareholder Quorum Subsampling and classical rarefaction. Generalized least-squares regression (GLS) is used to investigate the relationships between richness, sampling and environmental proxies. We found low levels of richness from the Triassic until the Late Cretaceous (except in the Kimmeridgian-Tithonian of Europe). High richness is recovered for the Late Cretaceous of North America, which declined across the K-Pg boundary but remained relatively high throughout the Palaeogene. Richness decreased following the Eocene-Oligocene Grande Coupure in North America and Europe, but remained high in North America and very high in Europe compared to the Late Cretaceous; elsewhere data are lacking. GLS analyses indicate that sampling biases (particularly, the number of fossil collections per interval) are the best explanation for long-term face-value genus richness trends. The lepidosaur fossil record presents many problems when attempting to reconstruct past diversity, with geographical sampling biases being of particular concern, especially in the Southern Hemisphere.

Keywords: Grande Coupure; Lepidosauria; Mesozoic; Palaeogene; species-richness; subsampling.

Figure 1.

Terrestrial lepidosaur global generic diversity from Triassic–Palaeogene using (a) uncorrected (raw) diversity data and (b) subsampled (SQS) diversity data at quorum 0.4. Numbers on (b) indicate number of collections drawn for each time bin subsampled, as an indicator of underlying data quality. Time bin durations are explained in table 1.

Figure 2.

Subsampled terrestrial lepidosaur generic diversity from Triassic–Palaeogene at quorum 0.4, for individual continents: AS, Asia; EU, Europe; IM, Indo-Madagascar, NAm, North America; SA, South America. Also included is the combined Jehol Group. Numbers indicate number of collections drawn for each time bin subsampled, as an indicator of underlying data quality. Arrows indicate approximate timings of major finds in the lepidosaur record: 1, earliest squamate fossils; 2, earliest snake fossils; 3, earliest amphisbaenian fossils. Time bin durations are explained in table 1.

Figure 3.

Palaeolatitudinal distribution of terrestrial lepidosaur occurrences from Triassic–Palaeogene; light grey circles indicate non-lepidosaur tetrapod occurrences. ‘Lepido. indet’, lepidosaur taxa of basal or unknown placement.

Figure 4.

Distribution of North American lepidosaur localities in bins Pg2 and Pg3. Darker shades indicate layered localities.