Eichstaettisaurus (meaning "Eichstätt lizard") is a genus of lizards from the Late Jurassic and Early Cretaceous of Germany, Spain, and Italy. With a flattened head, forward-oriented and partially symmetrical feet, and tall claws, Eichstaettisaurus bore many adaptations to a climbing lifestyle approaching those of geckos. The type species, E. schroederi, is among the oldest and most complete members of the Squamata, being known by one specimen originating from the Tithonian-aged Solnhofen Limestone of Germany. A second species, E. gouldi, was described from another skeleton found in the Matese Mountains of Italy. Despite being very similar to E. schroederi, it lived much later, during the Albian stage. Fossils of both species show exceptional preservation due to deposition in low-oxygen marine environments.
Initially named as a species of the genus Ardeosaurus by Ferdinand Broili in 1938, E. schroederi has had a convoluted taxonomic history. It was initially recognized as a relative of night lizards until research in the 1960s identified it and Ardeosaurus as geckoes. However, phylogenetic analyses conducted in the 1990s suggested that it was closer to the ancestor of all squamates. Various conflicting positions were found until the advent of analyses incorporating more species and better data, which resolved E. schroederi as a close relative of geckoes in the Gekkonomorpha. However, the position of Ardeosaurus in relation to E. schroederi remains uncertain: one species, A. digitatellus, has been recovered as its close relative in some analyses, but the other species A. brevipes has been found to be more distantly related.
Discovery and naming E. schroederi
In 1938, Ferdinand Broili described an exquisitely preserved specimen of lizard, preserved top-side-up, from Jurassic-aged rock deposits in the municipality of Wintershof, Eichstätt, Germany. The specimen came into the possession of the Bavarian State Collection of Paleontology and Historical Geology (BSPG), where it was prepared by L. Haimerl. It consists of a nearly-complete skeleton, albeit with a crushed skull, a tail missing the tail after the fifth vertebra (probably the result of amputation and replacement by cartilage), and several vertebrae preserved only as impressions (but their presence could still be verified by the associated ribs). A counterslab to the specimen also exists. Today, the rock deposits are known to belong to the Altmühltal Formation, and the specimen and its counterslab are stored under the accession numbers BSPG 1937 I 1a and 1b.
Although he was unable to examine it first-hand, Broili noted from Christian Erich Hermann von Meyer's description of the type specimen of Ardeosaurus brevipes that it bore similarities to this specimen, but also that they had important differences in snout and frontal bone shape, as well as vertebral counts. However, he hesitated to create a new genus due to the preservational state of the skull. Thus, he assigned it to the genus Ardeosaurus with reservations as a new species, A.? schröderi, named after his colleague J. Schröder. (The current specific name is schroederi: by Article 32.5.2.1 of the International Code of Zoological Nomenclature, umlauts in German names published before 1985 must be deleted and be replaced with an "e" inserted as the next letter.)
Robert Hoffstetter formally recognized the distinction between A.? schroederi and A. brevipes in 1953, and established a new generic name, Broilisaurus. However, Oskar Kuhn had already named a stegocephalian amphibian with the same name in 1938; thus, in 1958, he established another generic name for the species, Eichstättisaurus. (Again, due to the ICZN, the current generic name is Eichstaettisaurus.) In 1963, Marguerite Cocude-Michel opined that E. schroederi really represented the same species as Homoeosaurus digitatellus, which had previously been named as a non-type species of Homoeosaurus in 1914 by N.M. Grier. Thus, she created the new combination Eichstaettisaurus digitatellus.
Contrary to Cocude-Michel, Hoffstetter identified digitatellus as a member of Ardeosaurus separate from Eichstaettisaurus in 1964. Even though Cocude-Michel was aware of Hoffstetter's paper, in 1965 she reiterated her argument and made no attempt to address Hoffstetter. Hoffstetter responded in 1966, calling Cocude-Michel's synonymy "unbearable". He noted that it would entail the revision of the diagnostic characteristics of Eichstatettisaurus to a point where it would be practically indistinguishable from Ardeosaurus, which would contradict her family-level separation of the two. Subsequent literature has retained Eichstaettisaurus schroederi and Ardeosaurus digitatellus as separate, although they received little attention until Tiago Simões and colleagues published a redescription of both in 2017.
E. gouldi
For many years, Eichstaettisaurus was known from the single specimen of E. schroederi until Cretaceous-aged remains were found elsewhere in Europe. In 2000, Susan Evans and colleagues described a small lizard specimen in the Berriasian–Valanginian aged locality of Montsec in Catalonia, Spain. Despite its poor preservation, the specimen was clearly distinct from the more common lizard in the locality, Meyasaurus; Evans and colleagues suggested that it held affinities to Eichstaettisaurus.
In 2004, Evans and colleagues reported even younger remains of Eichstaettisaurus, which originated from the Albian-aged Pietraroja Plattenkalk in the locality of Pietraroia, which is located in the Matese Mountains of southern Italy. A specimen catalogued as MPN 19457 formed the type specimen of a new species, E. gouldi, which they named after the late biologist Stephen Jay Gould; it is a skeleton preserved bottom-side-up, which includes the skull and parts of the vertebral column and limbs. A second specimen consists of a disarticulated skull, which is preserved as part of the gut contents of an indeterminate rhynchocephalian catalogued as MPN A01/82. Both specimens are stored in the Museum of Palaeontology at the University of Naples (MPN).
Description Eichstaettisaurus was a relatively small animal. E. schroederi had a snout-vent length (measured from the tip of the snout to the opening of the cloaca) of , while E. gouldi had a snout-vent length of .
Skull
Eichstaettisaurus can be identified by flattened skulls with short, blunt, and rounded snouts. E. schroederi had large nostrils. The tooth-bearing bones of the snout, the premaxilla and the maxilla, were likely connected by a band of soft tissue in E. schroederi. The premaxilla had 6 or 7 teeth in E. gouldi, but the number in E. schroederi is unknown; the maxilla had approximately 22 teeth in E. schroederi, and at least 30 in E. gouldi. The teeth of both species were small, narrow, conical, and single-cusped; they had a pleurodont implantation, meaning that they were fused to the inner side of the jawbone. Like Ardeosaurus, both species lacked the lacrimal bone, usually a part of the eye socket. Also in the eye socket, the postfrontal and postorbital bones were separate unlike Ardeosaurus, and the rearward projection of the latter bones were relatively wide. In E. schroederi, the contact between these bones was further forward than most lizards. The squamosal bones were slender and projected downwards to meet the quadrate bones, as in most lizards; unlike the Iguania and Teiidae, however, they lacked upward projections. Living geckoes lack the postorbital and squamosal bones, and they also have relatively shorter jugal bones than Eichstaettisaurus. On the palate of E. schroederi, the ectopterygoid bones overlapped the pterygoid bones at their rear.
Unlike A. brevipes, E. schroederi lacked bone ornamentation and osteoderms on the skull roof. In both species, the frontal bones were fused and narrower between the eye sockets than they were at the front. The downward projections on the frontals known as subolfactory processes were well-developed in both species, but met at the midline only in E. schroederi. The suture between the frontal and parietal bones was convex in E. schroederi and concave in E. gouldi, unlike Ardeosaurus where it was straight. E. gouldi and most other lizards have fused parietal bones, but they were paired and unfused in E. schroederi. The parietals of both species lacked rearward projections and nuchal fossae. In E. schroederi, the outer edges of the parietals curved inwards, and the rearward projections known as the supratemporal processes were short, widely separated, and bore depressions. Also in E. schroederi, a pair of crests were present on the supraoccipital bone of the braincase, which were likely imprinted by the semicircular canals due to the skull's reduced ossification.
Vertebrae and ribs Compared to most iguanian, gekkotan, and scincomorphan lizards, which generally have 24–26 vertebrae in front of the sacrum (or hip), E. schroederi had 31, including 7 neck vertebrae and approximately 24 back vertebrae. E. gouldi had 6–7 neck vertebrae, and an unknown number of back vertebrae. In E. gouldi, the second (axis) and third neck vertebrae were connected to the next by elements known as intercentra. In E. schroederi, most of the neck vertebrae lacked cervical ribs, save for the last two. The upward-projecting neural spines on the back vertebrae were small, and the last few back vertebrae lacked ribs in the vein of lumbar vertebrae. Zygosphene-zygantrum articulations are present on the back vertebrae of both species. The ribs were single-headed and had circular articulating surfaces. In E. gouldi, the centra (main bodies) of the tail vertebrae were broad, cylindrical, amphicoelous (concave on both ends), and bore strongly developed transverse processes.
Limbs and limb girdles
Eichstaettisaurus had a well-developed acromion process, or the attachment for the clavicle, on the scapula (shoulder blade). The clavicles of E. schroederi did not curve forward, unlike most members of the Autarchoglossa, and also lacked rearward projections. As in Ardeosaurus, both species of Eichstaettisaurus had relatively stout forelimbs. In E. schroederi, the olecranon process of the ulna was well-developed. In E. gouldi, the carpal bones were likely not fully ossified. The five-digited hand of E. schroederi bore 2, 3, 4, 5, and 3 phalanges (finger bones) on each digit; the second-to-last phalanges from the tips were relatively long. E. gouldi appears to have had 4 phalanges on the fourth digit, and its claws were short, deep, and sharp, with well-developed flexor tubercles.
In the pelvis of E. schroederi, the expansion of the pubes into the fused pubic apron was relatively narrow; the pubes also lacked the forward projections known as the pubic tubercles. The ischia were enlarged and contacted each other extensively, and the rear expansion of the blade of the ilium was long. Both Eichstaettisaurus species had stout hindlimbs similar in length to the forelimbs. The femur had a straight shaft, unlike most other lizards where it is sigmoidal. In the foot, the first metatarsal bone was slightly shorter than the second to fourth metatarsal bones. Unusually, the fifth metatarsal bone was much shorter and hooked. In E. schroederi, the foot was five-digited like the hands, with 2, 3, 4, 5, and 4 phalanges on each digit, and the second-last phalanges and claws were similar. E. schroederi shared two features of the foot claws with the living Tokay gecko: the claws had expanded bottom ends; and the articulations between phalanges were simple concave-convex joints, instead of bicondylar (i.e. based on two rounded projections on the ends of the phalanges), unlike most other lizards.
Classification