Eichstaettisaurus Schroederi 76545

''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 geckoes. 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

The phylogenetic history of ''Eichstaettisaurus'' has been tumultuous like that of ''Ardeosaurus'', which has been a consequence of both incomplete data and its primitive form. von Meyer did not provide a placement for ''Ardeosaurus'' when he named the genus; Richard Lydekker and Karl Alfred von Zittel placed it and ''Homoeosaurus'' in the Rhynchocephalia, Baron Franz Nopcsa placed ''Ardeosaurus'' as a close relative of the skinks, and Charles Camp placed it close to the Gekkonidae. Based on the well-developed rims of the eye sockets and supratemporal fenestrae on the skull in the type specimen of ''E. schroederi'', Broili concurred with Nopcsa's conclusion in 1938. Within the Scincomorpha, he noted that the amphicoelous vertebrae of ''Ardeosaurus'' (including ''E. schroederi'') best resembled that of the Xantusiidae (night lizards), as most other skinks have procoelous (convex on the rear surface) vertebrae.

Upon separating ''schroederi'' into the genus ''Broilisaurus'', Hoffstetter retained it within the Xantusiidae in 1953, but he noticed that its skull curiously resembled those of geckoes. Since xantusiids and geckoes were then placed on the two major recognized branches of lizards, Autarchoglossa and Ascalabota, Hoffstetter speculated that it was close to the ancestral stock of lizards. Upon renaming it to ''Eichstaettisaurus'', Kuhn established the family Eichstaettisauridae, still in the Xantusiidae. In 1961 and then 1963, Cocude-Michel concurred with Camp that Ardeosauridae and Eichstaettisauridae were actually geckoes and not xantusiids, based on major differences in the postcranial skeleton, and placed both in the Gekkonomorpha. Hoffstetter accepted this identification in 1966, as did Richard Estes in 1983.

However, phylogenetic analyses conducted by Victor Reynoso in 1996 (not published), Evans and Luis Javier Barbadillo in 1998 and 1999, and Evans and Daniel Chure in 1998 suggested that both ''Eichstaettisaurus'' and ''Ardeosaurus'' were much more basal (early-diverging) squamates. Along with ''Bavarisaurus

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