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Protorosaurus

Protorosaurus is an extinct of basal archosauromorph known from the late Permian period (Wuchiapingian stage, approximately 259.9–255.7 Ma), primarily from the Kupferschiefer Formation in and the Marl Slate Formation in . This slender, quadrupedal reached lengths of up to 2 meters, featuring a long neck, elongated tail, and lizard-like build adapted for . Numerous well-preserved specimens, including complete skulls and articulated skeletons, with at least 28 known as of 2025, provide detailed insights into its , making it one of the best-known early saurians. First documented in 1710 and formally named Protorosaurus speneri in 1832 by Hermann von Meyer, the genus was extensively described in the based on multiple finds, with modern redescriptions, including a 2025 micro-CT study of the , enhancing understanding of its and evolutionary position. The exhibits features typical of early archosauromorphs, with a reconstructed form showing resemblances to later prolacertiforms, though these are considered convergent traits. Postcranially, it possessed a robust , parallelogram-shaped vertebral centra, and an ectaxonic pes (hindfoot) with the fourth digit longest and a phalangeal formula of 2-3-4-5-4, potentially linking it to trackways like Synaptichnium. Phylogenetically, Protorosaurus represents a basal member of , positioned as a sister taxon to more derived forms like Megalancosaurus in some analyses, and contributing to reconstructions of early archosauromorph diversification before the Permian-Triassic extinction. Its discovery highlights the radiation of reptiles in the late , bridging synapsids and later archosaurs in the saurian lineage.

Discovery

Initial Description

The first known specimen of Protorosaurus was discovered around near Kupfersuhl in , , and was initially interpreted as the bone of a or similar large . This specimen, consisting of a partial skeleton including vertebrae and limb elements, was described in Latin by the Prussian court physician Christian Maximilian Spener in a in Miscellanea Berolinensia ad incrementum scientiarum, marking one of the earliest scientific accounts of a . Spener's description emphasized its reptilian affinities but lacked a formal taxonomic placement, reflecting the limited paleontological framework of the early 18th century. Over a century later, in 1830, German paleontologist Hermann von Meyer formally established the genus Protorosaurus based on this and additional material from the Late Permian Kupferschiefer formation, recognizing it as a distinct extinct reptile rather than a modern animal. Von Meyer named the type species Protorosaurus speneri in 1832, honoring Spener's pioneering work, and published a detailed description in Palaeontographica, highlighting its lizard-like features such as elongated vertebrae and limbs. The name Protorosaurus, meaning "first lizard," underscored von Meyer's view of it as an ancestral form bridging earlier amphibians and more advanced reptiles. In the early 19th century, Protorosaurus was often interpreted as a transitional in the from amphibians to reptiles, with its mix of primitive and derived traits—such as a sprawling and elongated body—positioning it as a potential progenitor in the emerging understanding of phylogeny. These views were influenced by contemporaries like , who examined the fossils and reinforced their reptilian nature, though debates persisted on its exact affinities until later redescriptions. Subsequent discoveries in during the mid-19th century expanded the known distribution but built directly on these foundational interpretations.

Fossil Material

The fossil record of Protorosaurus speneri consists of approximately 40 known specimens, predominantly partial skeletons recovered from the Late Permian Kupferschiefer Formation in and the equivalent Marl Slate in . These include the lectotype (NHMW 1943 I 4), an almost complete postcranial known as the Swedenborg specimen from the Kupferschiefer Formation in , which lacks cranial elements; this specimen was designated lectotype in 2009 as no original was specified. The Spener 1710 specimen, a partial from Kupfersuhl in , is distinct from the lectotype and possibly housed in the Hunterian Museum in . Notable English specimens encompass a partial from collected around 1870 and a more complete individual from Quarrington Quarry in 1993, the latter preserving fragments of the skull for the first time in British material. Preservation of these fossils is generally poor due to within fine-grained sedimentary layers, resulting in incomplete and distorted skeletons with missing extremities and axial elements. Many specimens exhibit surface damage from early mechanical preparation techniques, though recent applications of dilute acetic acid have successfully removed adhering matrix to expose finer details without excessive . Advancements in have significantly enhanced understanding of the material; a comprehensive redescription in 2009 by Gottmann-Quesada and Sander incorporated newly prepared specimens from the Kupferschiefer, revealing previously obscured postcranial features through combined mechanical and chemical methods. In 2025, micro-computed tomography (µCT) scanning of the near-complete specimen NMK S 180 from provided unprecedented views of internal cranial structures, including the braincase, which had been inaccessible via traditional preparation. Despite these efforts, no complete skeletons of Protorosaurus have been documented, with all known individuals represented by disarticulated or partially articulated remains. Cranial material remains scarce, limited to fragmentary mandibles and isolated elements prior to the 2025 µCT analysis, underscoring significant gaps in the record for holistic reconstructions.

Description

Overall Morphology

Protorosaurus speneri was a slender, quadrupedal archosauromorph characterized by a lizard-like , with a total length estimated at 1.5 to 2 meters. Its build was gracile overall, adapted for terrestrial locomotion in a Permian environment, featuring a proportionally long neck, elongated trunk, and extended tail that accounted for more than half of the body length. The postcranial skeleton exhibited primitive archosauromorph features, though detailed cranial traits are distinct from postcranial elements. The included seven forming a notably long neck, with low neural spines and elongated centra that contributed to the animal's overall streamlined appearance. The trunk comprised 18 or 19 dorsal vertebrae, providing an extended thoracic region typical of early archosauromorphs, while the caudal series was extensive, emphasizing the tail's role in balance and . were slender and associated with these vertebrae, supporting a flexible yet robust without evidence of extensive armor or osteoderms. The featured gracile limbs, with the forelimbs shorter and more robust relative to the hindlimbs, the latter being elongated to facilitate habits. Pectoral and pelvic girdles were lightly built, consistent with a terrestrial , and the overall proportions evoked comparisons to modern monitor lizards (Varanus spp.), though Protorosaurus retained more primitive traits such as less specialized digit configurations. impressions, where preserved, suggest a covering of small, non-overlapping scales, aligning with basal .

Skull and Dentition

The skull of Protorosaurus speneri exhibits an elongated , contributing to its overall lizard-like cranial profile. This structure includes a large and the characteristic temporal fenestrae of reptiles, with a robust rostrum, high zygoma, wide , narrow interorbital region, large , and elongated temporal region. Recent X-ray micro-computed tomography (µCT) analyses conducted in 2025 on referred specimens have revealed previously obscured details of the internal cranial anatomy, including a robust braincase and the presence of a sclerotic ring supporting the eye structure. These findings enhance understanding of the basicranial and palatal regions, confirming the absence of a secondary palate—a feature distinguishing Protorosaurus from more derived archosaurs. The comprises conical teeth that are slightly recurved, with subthecodont implantation, numbering numerous marginal teeth per jaw. Posterior teeth exhibit a triangular profile in side view at mid-crown height, while wear patterns on the teeth show evidence of . Sensory adaptations are evident in the large orbits, which indicate enhanced visual capabilities, and the possible presence of a pineal , potentially linked to . Prior to 2025, interpretations of cranial relied heavily on incomplete mandibles and surface preparations, limiting insights into internal features; the new µCT data provide a more complete picture of its primitive archosauromorph traits.

Classification

Historical Classifications

Protorosaurus speneri was first formally described and classified in the by Hermann von Meyer, who named the genus in 1830 and viewed it as a primitive -like reptile, the "first ," based on s from the Late Permian Kupferschiefer of . Contemporaries, including and Harry Govier Seeley, reinforced this interpretation, with Seeley in 1888 describing additional material and placing it within a group of early -like forms, though some early accounts suggested affinities to amphibian-reptile intermediates due to its slender build and presumed aquatic habits. This initial classification as a protorosaurid reflected the limited record and pre-Darwinian understanding of reptilian . In the early , Protorosaurus was incorporated into broader taxonomic schemes emphasizing its lizard affinities, often placed within Prolacertiformes, a group of small, agile diapsids from the Permian and . By the , Robert L. Carroll explicitly classified it as a basal squamate or lepidosauromorph in works such as his 1975 study on Permo- 'lizards' from the Basin, highlighting shared features like elongated limbs and a lightweight skull that suggested close ties to modern . Mid-20th-century debates saw varied placements, with some authors considering Protorosaurus a proganosaur—a hypothetical stem-group to more advanced reptiles—or a relative of varanid . Alfred S. Romer, in his 1956 monograph Osteology of the Reptiles, tentatively linked it to early within Protorosauria, proposing it as a side branch near the base of the line based on skeletal comparisons, though without the benefit of cladistic methods this remained speculative. Key shifts occurred in the 1980s and 1990s, when reexaminations repositioned Protorosaurus as a primitive outside crown-group reptiles, prefiguring modern views of its archosauromorph status. For instance, Michael J. Benton's 1985 phylogenetic analysis of included it in Prolacertiformes as an early archosauromorph, while Reinhold Wild's 1980 redescription emphasized its basal position among lacking rigorous phylogenetic support at the time. These classifications marked a transition from lepidosaur-centric views to recognizing its distinct evolutionary role.

Modern Phylogeny

In modern phylogenetic analyses, Protorosaurus speneri is consistently recovered as the earliest-diverging archosauromorph, positioned as the sister taxon to all remaining archosauromorphs and outside the clade Protorosauria. This placement stems from a comprehensive cladistic study by (2016), which utilized a emphasizing basal archosauromorph relationships and incorporated morphological characters from , axial, and appendicular elements to resolve early divergences within the group. The analysis highlights Protorosaurus as a stem-archosauromorph, distinct from more derived lineages such as those leading to archosaurs. Recent cladistic work has reinforced this basal position through expanded datasets and new anatomical data. Spiekman et al. (2025) integrated over 50 taxa in their phylogenetic analysis, confirming Protorosaurus as one of the earliest well-represented archosauromorphs from the late Permian, with a topology that excludes it from Protorosauria and emphasizes its role as an outgroup to subsequent archosauromorph radiations. A concurrent redescription by the same team (Schoch and Spiekman, 2025) provides updated cranial details, identifying autapomorphies such as a uniquely structured quadrate with a pronounced posterior and specialized basicranial features that support its non-protorosaurid status and basal placement. These updates address limitations in pre-2024 cladograms by incorporating high-resolution data, shifting emphasis from earlier tentative inclusions within Protorosauria to a more robust stem-archosauromorph interpretation. Regarding interrelationships, Protorosaurus shows a close affinity to the Polish taxon Czatkowiella harae, based on shared long-necked morphology and basal archosauromorph synapomorphies in the and skull, though Czatkowiella remains fragmentary and potentially chimeric due to its basis in isolated elements. In contrast, it is phylogenetically distant from more derived archosauromorph subgroups, such as tanystropheids (nested within Tanysauria) and rhynchosaurs (positioned crownward toward archosauromorphs with herbivorous adaptations), as evidenced by the lack of shared derived traits like elongated or specialized in these analyses.

Paleobiology

Diet and Feeding

The diet of Protorosaurus is inferred to have been omnivorous, based on direct evidence from preserved gut contents in a single specimen from the Upper Permian Kupferschiefer of Hessen, . This specimen, housed in the Senckenberg Museum, contains numerous ovules of the voltzialean Ullmannia frumentaria, a locally abundant in the Late Permian ecosystems of , indicating consumption of reproductive structures from . Alongside the plant material, several rounded pebbles were preserved within the , interpreted as gastroliths likely used to grind tough vegetative matter in the digestive tract. The dentition of Protorosaurus, characterized by conical, unserrated, peg-like teeth, initially suggested a primarily carnivorous or insectivorous lifestyle suited for piercing and grasping small prey. However, the presence of substantial plant remains in the gut contents contradicts this assumption, pointing instead to an opportunistic omnivorous diet that incorporated both vegetal and possibly animal matter, despite the lack of specialized herbivorous adaptations in the teeth. No definitive insect remains have been identified in the preserved gut material, though the tooth morphology remains compatible with occasional insectivory. Feeding mechanics likely involved the elongate and slender of Protorosaurus to probe into for ovules or to capture small, agile prey such as among foliage. The scarcity of gut content preservation—limited to this one specimen—precludes broad generalizations about the ' diet, and further analyses, such as on additional fossils, are needed to confirm the extent of processing capabilities.

Locomotion and Ecology

Protorosaurus speneri was a quadrupedal archosauromorph adapted for , with a slender build and limb proportions indicative of an agile, lifestyle similar to that of modern monitor lizards. The elongated hindlimbs and overall postcranial morphology supported efficient movement across terrains, lacking adaptations for or arboreal habits. No trackways attributable to Protorosaurus have been identified, limiting direct evidence of its or speed. Ecologically, Protorosaurus occupied a terrestrial niche in Permian wetland and floodplain environments, as evidenced by gut contents containing terrestrial plant remains and quartz pebbles interpreted as gastroliths. This suggests foraging behavior focused on ground-level vegetation or associated prey in vegetated lowlands, consistent with its basal archosauromorph position and the absence of specialized aquatic features. Its relative rarity in the fossil record of the Kupferschiefer formation points to a non-dominant role, likely as a mid-sized generalist filling gaps between smaller insectivores and larger herbivores in the ecosystem. Recent phylogenetic analyses reinforce its early-diverging status among archosauromorphs, implying moderate activity levels suited to opportunistic terrestrial exploitation rather than high-energy pursuits.

Paleoenvironment

Geological Setting

Protorosaurus fossils are primarily known from the Kupferschiefer Formation in and the equivalent Marl Slate in , both basal units of the Zechstein Group deposited during the late Permian ( epoch). The Kupferschiefer consists of finely laminated black shales, bituminous marls, and mudstones, typically less than 1 meter thick, formed in a vast epicontinental sea that extended from across to . This represented a transition from restricted marine to marginal marine and terrestrial settings, with the basin bordered by Variscan mountains and influenced by periodic marine incursions into shallower, lagoonal areas. The temporal range of these deposits spans approximately 260 to 254 million years ago, corresponding to the Wuchiapingian stage of the , predating the end-Permian mass extinction around 252 Ma. Recent stratigraphic correlations using biostratigraphy (e.g., Merrillina divergens and Mesogondolella britannica) and strontium isotope chemostratigraphy have refined this age estimate, confirming the Kupferschiefer's position within the upper Permian without significant revisions in the past decade. The paleoclimate was characterized by semi-arid conditions at mid-latitudes (10–30°N), with seasonal monsoons driving episodic humidity amid overall aridity, as evidenced by widespread deposits in overlying Zechstein cycles indicating hypersaline marine waters. However, Protorosaurus-bearing horizons occur at basin margins influenced by freshwater inputs, suggesting localized less saline conditions. Taphonomic preservation of Protorosaurus specimens is exceptional due to deposition in anoxic, sulfidic bottom waters of the bituminous shales, resulting from episodic anoxic events that minimized bioturbation and scavenging. Rapid burial in fine-grained, organic-rich sediments (with up to 10%) protected articulated skeletons from decay, often preserving them as part-and-counterpart slabs with minimal distortion, though some show or crushing from later . This mode of preservation biases the fossil record toward individuals from low-energy aquatic margins rather than fully terrestrial interiors.

Associated Biota

The associated with Protorosaurus habitats in the Late Permian Kupferschiefer consisted primarily of such as Walchia and seed ferns belonging to the order Peltaspermales, which formed forests adjacent to marginal environments. Gut contents preserved in Protorosaurus specimens include ovules and fragments from voltzialean like Ullmannia, confirming dietary incorporation of this dominant vegetation and supporting an omnivorous habit. The contemporaneous fauna featured a mix of synapsids and reptiles in low-diversity terrestrial assemblages, with abundant aquatic components in marginal settings. Synapsids included early therapsids such as cynodonts (Procynosuchus). Other reptiles encompassed gliding diapsids like Weigeltisaurus, pareiasaurs (Parasaurus), and early archosauromorphs, while fish (e.g., palaeonisciforms) and amphibians (e.g., dissorophids) dominated near-shore and lagoonal deposits. Protorosaurus, as a mid-sized reaching up to 2 meters in length, occupied a niche among larger herbivores like pareiasaurs and smaller carnivorous synapsids and reptiles, within an ecosystem of overall low tetrapod diversity preceding the end-Permian mass extinction. Recent analyses of Kupferschiefer material have identified additional protorosaurian specimens, increasing recognized diversity among early archosauromorphs in this biota. Recent discoveries include a new basal archosauromorph from late Permian deposits in , further highlighting archosauromorph diversity in this ecosystem.