A Morrison stem gekkotan reveals gecko evolution and Jurassic biogeography

Abstract

Geckos are a speciose and globally distributed clade of Squamata (lizards, including snakes and amphisbaenians) that are characterized by a host of modifications for nocturnal, scansorial and insectivorous ecologies. They are among the oldest divergences in the lizard crown, so understanding the origin of geckoes (Gekkota) is essential to understanding the origin of Squamata, the most species-rich extant tetrapod clade. However, the poor fossil record of gekkotans has obscured the sequence and timing of the assembly of their distinctive morphology. Here, we describe the first North American stem gekkotan based on a three-dimensionally preserved skull from the Morrison Formation of western North America. Despite its Late Jurassic age, the new species already possesses several key characteristics of the gekkotan skull along with retained ancestral features. We show that this new stem gekkotan, and several previously named species of uncertain phylogenetic relationships, comprise a widespread clade of early crown lizards, substantiating faunal homogeneity in Laurasia during the Late Jurassic that extended across disparate ecological, body-size and physiological classes.

Keywords: Gekkota; Squamata; biogeography; macroevolution; phylogenetics.

Figure 1.

CT Imagery of the skull of Helioscopos dickersonae (DINO 15914). (a,b) In situ skull in dorsal (a) and ventral (b) views. (c–f) Reconstructed skull in left lateral (c), right lateral (d), dorsal (e) and ventral (f) views. All scale bars 5 mm. bc, braincase; ‘cb’, fused postdentary elements (compound bone); d, dentary; f, frontal; j, jugal; m, maxilla; p, parietal; pa, palatine; pf, prefrontal; pof, postorbitofrontal; pt, pterygoid; sq, squamosal.

Figure 2.

Close-up photographs of Limnoscansor digitatellus (CM 4026). (a) Full body of specimen (dorsal view). (b) Close up of skull (dorsal view).

Figure 3.

Phylogenetic position of Helioscopos dickersonae under different analytical regimes. (a) Results of unconstrained implied weights parsimony, weighting strength K = 12. (b) Results of implied weights parsimony, weighting strength K = 12, with the topology of the modern taxa constrained to the molecular tree of [2]. Numbers on nodes are bootstrap values (1000 pseudoreplicates). (b) Results of tip-dated Bayesian analysis. Numbers on nodes are posterior probabilities, while the bars represent the 95% confidence intervals of the inferred age of the node. (d) Reduced evolutionary tree of Pan-Gekkota showing several key anatomical transitions in the skull roof and face.

Figure 4.

Evolution of pan-gekkotan anatomy and biogeography. Tree of Pan-Gekkota (from our weighted analysis K = 12 restricted to a molecular topology). Branch squares represent inferred continents of origin, with terminal branches based on recorded observations and internal branches being the results of our parsimony ancestral states reconstruction. Red, Europe; blue, North America; orange, Asia; green, Africa; purple, South America; yellow, Australian plate. New taxa in bold. Circles above branches represent key synapomorphies of Gekkota and Pan-Gekkota identified by [3]. Open circles represent the absence of features, closed circles represent the presence of features, and partially filled circles represent partially acquired features. 1, fusion of the frontals; 2, fusion of the frontal subolfactory processes into a tubular structure; 3, absence of a postorbital; 4, paired parietals; 5, posterior midline projection of the parietal; 6, absence of a pineal foramen; 7, prefrontal medial margin extending beneath frontal subolfactory process; 8, absence of a jugal postorbital bar; 9, edentulous palate (as gauged by the condition of the pterygoid); 10, dentary fused over Meckel's canal; 11, 31 or more maxillary and dentary teeth.

Figure 5.

Historical biogeography of Pan-Gekkota. (a) The distribution of sister taxa found at European and western North American sites. The red dot indicates the hypothesized European origin of Pan-Gekkota and the blue dot represents the previously unknown North American radiation. (b) The phylogeographic hypothesis for pygopodoids generated in this study. The green dot represents the estimated Asian origin of Gekkota and the yellow dot represents the estimated Australian origin of Pygopodoidea. White lines represent the two possible scenarios of pan-pygopodoid dispersal out of Asia supported by this study. Paleogeographic reconstructions constructed with GPlates.