Abstract
The skull of the Cretaceous pterosaur Istiodactylus latidens, a historically important species best known for its broad muzzle of interlocking, lancet-shaped teeth, is almost completely known from the broken remains of several individuals, but the length of its jaws remains elusive. Estimates of I. latidens jaw length have been exclusively based on the incomplete skull of NHMUK R3877 and, perhaps erroneously, reconstructed by assuming continuation of its broken skull pieces as preserved in situ. Here, an overlooked jaw fragment of NHMUK R3877 is redescribed and used to revise the skull reconstruction of I. latidens. The new reconstruction suggests a much shorter skull than previously supposed, along with a relatively tall orbital region and proportionally slender maxilla, a feature documented in the early 20th century but ignored by all skull reconstructions of this species. These features indicate that the skull of I. latidens is particularly distinctive amongst istiodactylids and suggests greater disparity between I. latidens and I. sinensis than previously appreciated. A cladistic analysis of istiodactylid pterosaurs incorporating new predicted I. latidens skull metrics suggests Istiodactylidae is constrained to five species (Liaoxipterus brachyognathus, Lonchengpterus zhoai, Nurhachius ignaciobritoi, Istiodactylus latidens and Istiodactylus sinensis) defined by their distinctive dentition, but excludes the putative istiodactylids Haopterus gracilis and Hongshanopterus lacustris. Istiodactylus latidens, I. sinensis and Li. brachyognathus form an unresolved clade of derived istiodactylids, and the similarity of comparable remains of I. sinensis and Li. brachyognathus suggest further work into their taxonomy and classification is required. The new skull model of I. latidens agrees with the scavenging habits proposed for these pterosaurs, with much of their cranial anatomy converging on that of habitually scavenging birds.
Highlights
The istiodactylid pterosaurs, a group of ornithocheiroid pterodactyloids best known for their unusual ‘cookie cutter’ dentition and broad, rounded snouts, were represented for over a century by a single species, Ornithodesmus ( = Istiodactylus) latidens Seeley, 1901 [2] from Lower Cretaceous Wealden deposits of Southern Britain [3]. The history of this species is typically convoluted for a pterosaur found in British soil during the late 1800s: the holotype (Natural History Museum, London specimen NHMUK R176) was once placed in a genus known to represent a small theropod dinosaur, Ornithodesmus [4]; its initial naming is problematic and risked classification as a nomen nudum, and some mystery surrounds when certain I. latidens specimens came to light, what they were, and who saw them [3]
The cranial morphology once only known from I. latidens is shared across a distinct pterosaur group, the Istiodactylidae, which may include up to seven taxa from Barremian – Aptian deposits of Europe and Asia: I. latidens; a second Istiodactylus species, I. sinensis Andres and Ji, 2006 [5]; Haopterus gracilis Wang and Lu, 2001 [6]; Liaoxipterus brachyognathus
Because many limb bones of I. latidens are missing parts of their diaphysis [15] and some doubt exists over the methods Hooley used to reconstruct the lengths of missing bones in NHMUK R3877, many characters using limb bone metrics were not scored for I. latidens despite their use in previous systematic analyses of this species [5]
Summary
There is, a third piece of the NHMUK R3877 skull and mandible, representing short lengths of the right maxilla and mandibular ramus (Figure 2). The separation of the ‘maxillonasal bar’ from these jaw fragments, presumably postdating Hooley’s illustration, may indicate that additional preparation of the specimen has taken place since Hooley’s work (one referee noted its absence from Arthaber’s I. latidens skull 1919 reconstruction [Figure 1B], suggesting it may have been removed shortly after the publication of Hooley’s monograph) and, possibly, permit modern workers to associate the skull components in a fashion denied to Hooley.
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