A global overview of pterosaur ichnology: tracksite distribution in space and time

Abstract

At least 50 pterosaur track sites have been reported from Late Jurassic through Late Cretaceous localities in North America, Europe, East Asia and South America, plus one possible site from north Africa. Tracks from these sites have been assigned to 11 ichnospecies in four ichnogenera. Of these, Pteraichnus is by far the most prevalent, well-preserved, and represented by multiple (presently eight) ichnospecies. The majority of Pteraichnus tracksites are Late Jurassic or earliest Cretaceous (Berriasian) in age. In contrast, the other three ichnogenera – Purbeckopus, Haenamichnus and possibly Agadirichnus – are all represented by single ichnospecies from single localities and are based on relatively poorly-preserved, earliest through latest Cretaceous material. At least 16 Late Jurassic, well-preserved Pteraichnus samples from marginal marine deposits in western North America are dominated by small tracks (pes length ~2–10 cm), often in trackways. The two ichnospecies from this region (P. saltwashensis and P. stokesi) are both based on trackway segments and differ in style of preservation. Other Late Jurassic trackways from marginal marine deposits in France and Poland are also small (pes length typically ~3–5 cm); much larger tracks (pes length 18 cm) are known from a single locality in Asturias, Spain. It is unclear whether most Pteraichnus represent pterodactyloid or “rhamphorhynchoid” pterosaurs because the diagnostic impressions of pedal digit V are rarely clearly and unambiguously impressed. Six basal Cretaceous (Berriasian) Pteraichnus ichnospecies from a least a dozen sites in Soria, Spain are also mostly small (footprint length ~1–5 cm) and based on as yet insuffi ciently described, isolated footprints, not trackways. As a result, several of these ichnospecies are probably nomina dubia. The contemporaneous Purbeckopus from England is much larger (foot length ~19–22 cm). Although small, Pteraichnus-like tracks have been reported sporadically from the post-Berriasian Cretaceous, most are much larger (foot length 10–20 cm) and, in the case of Haenamichnus from Korea, reach 30–33 cm. It is unclear whether Agadirichnus from the Late Cretaceous of Morocco (foot length 10–12 cm) is pterosaurian. Most Cretaceous sites represent lacustrine, not marginal marine, habitats. Both Jurassic and Cretaceous assemblages often contain very high track densities and a range of track sizes associated with invertebrate traces. This suggests that diverse pterosaurian fl ocks may have congregated in large numbers to feed. Some assemblages reveal swim tracks that suggest pterosaurs fl oated in shallow water, touching the submerged substrate with only their hind feet. These swim track assemblages also contain possible beak traces that may indicate feeding.