Dimorphodon macronyx
Age
Jurassic（Hettangian）
201.4-199.5 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Pterosauria
Family: Dimorphodontidae
Genus: Dimorphodon
Species: Dimorphodon macronyx
Morphological description
Mature individuals of Dimorphodon macronyx had a wingspan of approximately 1.5 m. The skull was elongate and deep, measuring about 20–22 cm in adult specimens and approaching one-quarter of the total body length, including the tail. The snout was blunt and the skull roof relatively flat, with a large external naris. The antorbital fenestra occupied the central region of the skull, while the orbit was shaped like an inverted teardrop and extended through nearly the entire vertical depth of the skull. Diagnostic features include an extremely narrow lacrimal with a bifurcated dorsal end, an elongate supraorbital process extending nearly to the middle of the orbit, and a nearly vertical, triangular quadrate with a large foramen near its base. Repreparation of NHMUK 41212 in 2001 also revealed previously obscured elements, including the prefrontal, palatine, ectopterygoid, and pterygoid. The pterygoid bears distinct grooves and a possible foramen.
The premaxilla bears four large, elongate, sharply pointed, slightly recurved, spear-like teeth. The maxillary teeth are conspicuously shorter and predominantly peg-like. The anterior end of the lower jaw likewise bears large spear-like teeth, followed abruptly by numerous minute conical teeth only about 1 mm high. When the jaws were closed, the large upper and lower teeth interlocked. The anterior end of the dentary is slightly downturned and bears a ventral rostral keel and a thin, posteriorly extending flange. An opening in the lower jaw was once interpreted as an external mandibular fenestra, but subsequent studies regarded it as the result of breakage. The combination of large spear-like anterior teeth, shorter maxillary teeth, and minute posterior dentary teeth is among the most distinctive features of the skull.
The vertebral column is estimated to have included at least about seven cervical vertebrae, 12 dorsal vertebrae, one lumbar vertebra, four sacrals, and approximately 30 caudal vertebrae. The cervical vertebrae are unusually robust relative to the dorsals and would have helped support the disproportionately large head. Pneumatic foramina are present in some cervical and dorsal vertebrae. The tail was extremely long: the tail preserved in NHMUK 41346 measures approximately 52 cm and contains 30 caudal vertebrae. The centra gradually increase in length through the middle of the tail before becoming shorter toward the tip, while the posterior caudals bear elongate, filamentous extensions of the zygapophyses and chevrons.
The pectoral girdle has a distinctly bird-like configuration. The scapula is long and slender, and in mature individuals the scapula and coracoid are fused. The humerus is short and robust, with a well-developed deltopectoral crest whose distal end curves dorsally, whereas the radius and ulna are considerably longer. The main wing membrane was supported by the greatly elongated fourth digit. Its four wing phalanges gradually taper and curve slightly anteriorly, giving the entire wing finger a gentle arc. Movement at the joints between the wing phalanges was limited, so the fourth digit functioned largely as a continuous supporting unit. The first three digits were much shorter but bore powerful claws, with the claw of the third digit being the longest. Sesamoid bones are also preserved near several of the manual unguals.
The pelvis is relatively small. The preacetabular and postacetabular processes of the ilium are similar in length, while the pubis and ischium form a broad plate. The acetabulum is shallow and imperforate. The femoral head is strongly directed medially, and the femur is slightly curved dorsally. The tibia is considerably longer than the femur, while the fibula is greatly reduced and partially fused with the tibia. Metatarsals I–IV form a compact but unfused unit. Metatarsal V is particularly short and stout and articulates with two extremely elongate phalanges that lack an ungual. The pedal claws are approximately half the size of the manual claws. The structure of the metatarsals and pedal digits has been interpreted as more consistent with a plantigrade stance and does not support habitual digitigrady.
Etymology
The genus name Dimorphodon is derived from di-, meaning "two," morph, meaning "form" or "shape," and odon, meaning "tooth," referring to the markedly different forms of teeth in the jaws.
The specific name macronyx means "large claw" or "long claw."
Biological description
Dimorphodon macronyx lived during the Hettangian Stage of the Early Jurassic. It is one of the earliest known Jurassic pterosaurs and was the first pterosaur from Britain to be formally described. The holotype, NHMUK R.1034, was discovered in 1828 by the renowned fossil collector Mary Anning near Lyme Regis in Dorset, England, in the alternating shale and limestone beds commonly known as the Blue Lias. William Buckland subsequently acquired the specimen and described it in 1829 as Pterodactylus macronyx. In 1858, Richard Owen obtained another specimen, NHMUK R.1035, which included a skull, and about a decade later acquired the nearly complete adult specimen NHMUK 41212. The skulls differed substantially from those of Pterodactylus then known from the Solnhofen deposits of Germany, leading Owen to establish the genus Dimorphodon in 1859.
During the later nineteenth century, additional material entered several museum collections. Between 1873 and 1881, Othniel Charles Marsh purchased a series of fossils from the London fossil dealer Bryce M. Wright. These specimens are now housed in the Peabody Museum of Natural History at Yale University and include humeri, ulnae, tibiae, fibulae, femora, wing phalanges, and skull fragments from juvenile individuals. The Yale specimens YPM 9182 and YPM 350 are substantially smaller than the large London specimens and have been interpreted as juveniles.
In terms of flight, Dimorphodon macronyx possessed the fully developed flight apparatus characteristic of pterosaurs. Functional analyses of its well-preserved humerus and pectoral girdle indicate that the forelimb was well suited to active flapping flight. The shoulder joint allowed the humerus to be retracted alongside the body when the wing was folded, to protract by approximately 90°, and to move through an additional arc of roughly 90° in elevation and depression. The elbow functioned largely as a hinge joint and showed clear functional similarities to the avian elbow. A highly mobile hinge was also present between metacarpal IV and the first wing phalanx.
How Dimorphodon macronyx moved on the ground has long been debated. Richard Owen originally reconstructed the animal as a quadruped with a semi-sprawling posture. A 1983 study reached a very different conclusion based on the articulations of the femur, tibia, ankle, and metatarsus. It proposed that the hindlimbs were fully erect and oriented in a parasagittal plane, and that the animal moved rapidly on the ground as a digitigrade biped. Under this interpretation, the hindlimbs were capable of supporting the body independently, whereas the restricted orientation of the humerus and hinge-like elbow made the forelimbs poorly suited to the sprawling terrestrial locomotion typical of many reptiles. The tibia being longer than the femur, together with the elongate metatarsus, was also interpreted as evidence of agility, rapid locomotion, and possibly some degree of cursorial ability.
A reassessment published in 2021 interpreted the same general problem differently. The medially directed femoral head, elongate tibia, reduced fibula, and mesotarsal ankle still indicate a relatively erect hindlimb configuration. The preserved relationship between the femoral head and acetabulum in NHMUK 41212 suggests that the legs may have been held close to the parasagittal plane during terrestrial locomotion, although with a slight lateral splay. However, metatarsals I–IV neither interlock nor fuse, and the form of their distal articulations restricts strong dorsiflexion of the toes. Habitual digitigrady is therefore considered unlikely, and the foot is more consistent with a plantigrade stance. This reconstruction consequently combines relatively upright hindlimbs with plantigrade feet.
The unusually large cervical vertebrae would have helped support the disproportionately massive head. The arrangement of the cervical zygapophyses differs from that of birds and may indicate a more restricted range of neck movement. The elongate supraorbital process projecting into the orbit may have helped protect the anterodorsal part of the eye and stabilise its position, while the sclerotic ring may have helped resist intraocular pressure and maintain the shape of the eyeball. Pneumatic foramina in the cervical and dorsal vertebrae demonstrate postcranial skeletal pneumaticity in Dimorphodon macronyx and support the view that skeletal pneumaticity may already have been present in the common ancestor of most pterosaurs.
Its respiratory mechanics have also been discussed. On the basis of the morphology of the prepubis, a muscle comparable to the crocodilian musculus diaphragmaticus may have been present, potentially producing anteroposterior displacement of the abdominal viscera during breathing. Expansion and compression of the thoracic region associated with the flight stroke may also have contributed to ventilation.
In traditional classifications, Dimorphodon macronyx was placed within Rhamphorhynchoidea because it possesses a long tail, relatively short cervical vertebrae, a short fourth metacarpal, two phalanges in the fifth pedal digit, and separate external nares and antorbital fenestrae. Later phylogenetic studies, however, demonstrated that the traditional Rhamphorhynchoidea does not form a monophyletic group and instead represents a series of early pterosaur lineages along the stem leading toward pterodactyloids.
Different phylogenetic analyses have recovered somewhat different positions for Dimorphodon macronyx. In the phylogenetic framework discussed in the 2021 study, D. macronyx and the Late Triassic Caelestiventus hanseni form a sister-group relationship. Together they constitute a monophyletic Dimorphodontidae positioned near the base of Macronychoptera.
（Author: Shui-Ye You）
References
Padian K. (1983). Osteology and functional morphology of Dimorphodon macronyx (Buckland) (Pterosauria: Rhamphorhynchoidea) based on new material in the Yale Peabody Museum. Postilla.
Sangster S. (2021). The osteology of dimorphodon macronyx, a non-pterodactyloid pterosaur from the lower jurassic of dorset, england. Monographs of the Palaeontographical Society.
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