title: Latitudinal diversity gradients in Mesozoic non-marine turtles.
abstract: The latitudinal biodiversity gradient (LBG)-the pattern of increasing taxonomic richness with decreasing latitude-is prevalent in the structure of the modern biota. However, some freshwater taxa show peak richness at mid-latitudes; for example, extant Testudines (turtles, terrapins and tortoises) exhibit their greatest diversity at 25° N, a pattern sometimes attributed to recent bursts of climatically mediated species diversification. Here, we test whether this pattern also characterizes the Mesozoic distribution of turtles, to determine whether it was established during either their initial diversification or as a more modern phenomenon. Using global occurrence data for non-marine testudinate genera, we find that subsampled richness peaks at palaeolatitudes of 15-30° N in the Jurassic, 30-45° N through the Cretaceous to the Campanian, and from 30° to 60° N in the Maastrichtian. The absence of a significant diversity peak in southern latitudes is consistent with results from climatic models and turtle niche modelling that demonstrate a dearth of suitable turtle habitat in Gondwana during the Jurassic and Late Cretaceous. Our analyses confirm that the modern testudinate LBG has a deep-time origin and further demonstrate that LBGs are not always expressed as a smooth, equator-to-pole distribution.
authors.affiliation: Department of Earth Sciences , The Natural History Museum , Cromwell Road, London SW7 5BD , UK.
authors.forename: David B
authors.identifier: 0000-0002-1296-5673
authors.initials: DB
authors.lastname: Nicholson
authors.affiliation: Museum of Paleontology , University of California , 1101 Valley Life Sciences Building, Berkeley, CA 94720 , USA.
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Holroyd
authors.affiliation: School of Geographical Sciences , University of Bristol , University Road, Bristol BS8 1SS , UK.
authors.forename: Paul
authors.initials: P
authors.lastname: Valdes
authors.affiliation: Department of Earth Sciences , The Natural History Museum , Cromwell Road, London SW7 5BD , UK.
authors.forename: Paul M
authors.identifier: 0000-0003-0412-3000
authors.initials: PM
authors.lastname: Barrett
keywords: Testudinata; genus richness; geographical range; latitudinal biodiversity gradient; subsampling