Dinosaurs and Other Reptiles from the Mesozoic of Mexico

8. Mexican Pterosaurs—Rare Jewels in the Fossil Record

Mexican Pterosaurs 127 the extremities, probably analogous to modern Dermaptera and flying squirrels. Such an ancestor of pterosaurs was suggested by Peters and Gutmann (1985) and Wellnhofer (1991). With the elongation of the lateral digits in manus and pes, the gliding membrane could be enlarged without hindering arboreal locomotion. With the evolution of a fibrous support of the wing membrane, the actinofibrils (Tischlinger and Frey, 2002; Frey et al., 2003b), the wingspan could be enlarged by further elongation of the lateral manual digit. This was the start of active flight that allowed an early pterosaur to leave its forest habitat for areas having better fossilization potential, such as lagoons, archipelagos, and, later, Cretaceous fluvial systems. Due to this fossilization bias against the earliest Pterosauria, the early evolution may remain cryptic. However, the postulated mechanical constraints of a proto­pterosaur may help identify possible relationships better than any character­based analysis of pterosaurs. The presence of three differ­ ent pterosaurian taxa at the very beginning of the pterosaur fossil record suggests that these animals must have evolved during the Early to early Middle Triassic, approximately coeval with the origin of the Dinosauria. Shortly after their first appearance, pterosaurs rapidly colonized the planet. The early taxa are characterized by a tail that is at least twice as long as the body and a short neck that at best reached the length of the skull (Wellnhofer, 1991). The wing membrane was controlled by a long bipartite fifth toe that extended across the width of the deep uropata­ gium between the hind legs. This group was commonly referred to as Rhamphorhynchoidea, a group that according to modern systematics is no longer valid because the Pterodactyloidea evolved from them. There­ fore, the “Rhamphorhynchoidea” and Pterodactyloidea do not share a common ancestor, rendering the “Rhamphorhynchoidea” polyphyletic (Unwin, 2003; Lü and Ji, 2006). The long­tailed pterosaurs died out during the Late Jurassic, whereas two other types of Pterosauria appeared, both characterized by tails that do not even reach half the length of the body: the so­called frogmouth pterosaurs or Anurognathidae, which formally belong to the long­tailed pterosaurs, and the short­tailed pterosaurs sensu stricto, which are the Pterodactyloidea (Wellnhofer, 1991; Unwin, 2006). The Anurognathidae differ from the Pterodactyloidea in having a short skull that is almost semicircular in dorsal view. With the so­called Rhamphorhynchoidea they have in common a pair of nasal apertures, which are separate from the antorbital fenestrae, a neck that is as long as the skull, and a long, bipartite fifth toe that spanned a deep uropatagium. The Anurognathidae appear during the Late Jurassic and become extinct during the Early Cretaceous. With a maximum wingspan of 0.4 m, they represent a group of small pterosaurs that most likely hunted insects on the wing and caught them with their wide, bristly “frogmouth” (Welln­ hofer, 1991; Unwin, 2006; Bennett, 2007). Pterosaur Systematics Frey and Stinnesbeck 128 Pterosaurs of the genus Darwinopterus from the Middle Jurassic Tiaojishan Formation in China are long­tailed and have a bipartite fifth toe that is longer than the other pedal digits. However, the skull is typical for a pterodactyloid short­tailed pterosaur in that the nasal aperture is con­ joined with the antorbital fenestra into a single nasoantorbital fenestra. For Darwinopterus and its allies plus the Pterodactyloidea sensu stricto, the name Monofenestrata was erected (Lü et al., 2010). The history of the Pterodactyloidea begins in the Middle Jurassic (Bathonian­Callovian) with Archaeositiodactylus (Lü and Fucha, 2011), a pterosaur of which the taxonomic position is doubtful (Martill, pers. comm.) and gnathosaurine pterodactyloids (Unwin, 1996; Buffetaut and Jeffrey, 2012). It ends in the latest Cretaceous with gigantic forms such as Hatzegopteryx tambaena (Buffetaut et al., 2002) or Arambourgiania philadelphiae (Frey and Martill, 1996), with wingspans of 10 m or more. Pterodactyloidea are characterized by a neck and skull that each exceeds the length of the trunk. The fifth toe is minute and may have lost its distal element (Wellnhofer, 1980,1991). As a consequence the uropatagium in pterodactyloid pterosaurs is narrow and deeply V­shaped. Pterodactyloi­ dea are furthermore characterized by enormous and sometimes bizarre cranial crests that may represent sexual dimorphism (Frey et al., 2003a, 2003b; Naish and Martill, 2003; Elgin et al., 2008), although this has not yet been positively proven. Crests likely also occurred in most long­tailed pterosaurs (e.g., Pterorhynchus, Austriadactylus, and Rhaeticodactylus; Czerkas and Ji, 2002; Dalla Vecchia et al...