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Protorosauria
Protorosauria
Protorosauria
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Protorosaurs
Temporal range: Latest Middle Permian - Late Triassic,
260–201.3 Ma
Fossil specimen of Protorosaurus speneri, Teyler's Museum
Skeletal reconstructions of various members of Tanysauria, including Trachelosaurus fischeri, Dinocephalosaurus orientalis, Tanystropheus hydroides, and Tanystropheus longobardicus
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauromorpha
Order: †Protorosauria
Huxley, 1871
Subtaxa

Protorosauria is an extinct, likely paraphyletic group of basal archosauromorph reptiles from the latest Middle Permian (Capitanian stage) to the end of the Late Triassic (Rhaetian stage) of Asia, Europe and North America. It was named by the English anatomist and paleontologist Thomas Henry Huxley in 1871 as an order, originally to solely contain Protorosaurus. Other names which were once considered equivalent to Protorosauria include Prolacertiformes and Prolacertilia.[1]

Protorosaurs are distinguished by their long necks formed by elongated cervical vertebrae, which have ribs that extend backward to the vertebrae behind them. Protorosaurs also have a gap between the quadrate bones and the jugal bones in the back of the skull near the jaw joint, making their skulls resemble those of lizards.[1] While previously thought to be monophyletic, the group is now thought to consist of various groups of basal archosauromorph reptiles that lie outside Crocopoda,[2] though some recent studies have recovered the group as monophyletic.[3] A number of members of Protosauria have been found to belong to a monophyletic group (though not including Protorosaurus) which was named Tanysauria in 2024.[4]

Classification

[edit]

Protorosauria was considered to be a synonym of Prolacertiformes for many years.[5]

Since 1998, many phylogenetic analyses have found Protorosauria, as used in its widest sense, to be a polyphyletic or paraphyletic taxon. Protorosaurus, Macrocnemus, tanystropheids, and various other protorosaurs are usually placed near the base of Archosauromorpha, while Prolacerta and Pamelaria, two Gondwanan Triassic protorosaurs, are now thought to be in a more derived position as close relatives of Archosauriformes.[6] Most phylogenetic analyses since 1998 have found a strongly supported clade that includes only the genus Prolacerta and the Archosauriformes.[7]

For this reason Prolacerta, Pamelaria, and several other related forms (collectively called prolacertids) have been removed from Protorosauria. Because the name Prolacertiformes is defined based on the genus Prolacerta, the name Protorosauria is used for the remaining group.

Only recently has Protorosauria been defined in a phylogenetic sense as the most inclusive clade containing taxa such as Protorosaurus, Macrocnemus, and Tanystropheus. Analyses, such as Dilkes (1998), Sues (2003), Modesto & Sues (2004), Rieppel, Fraser & Nosotti (2003), Rieppel, Li & Fraser (2008), Gottmann-Quesada and Sander (2009) and Renesto et al. (2010),[7][8][9][10][11] recovered a large Protorosauria, that includes Protorosaurus, Drepanosauridae (and relatives) and Tanystropheidae (and relatives). However, some analysis found Protorosaurus (and sometimes the closely related Czatkowiella) to be more advanced[12] or more basal[13] than the node Drepanosauridae+Tanystropheidae, but always more basal than Prolacerta.

Some studies still use the term Prolacertiformes to include prolacertids and traditional protorosaurs, while restricting the term Protorosauria to the smallest clade that includes Protorosaurus, Macrocnemus, and Tanystropheus; thus Protorosauria is a true clade, while Prolacertiformes is an evolutionary grade of early archosauromorphs.[14]

Pritchard et al. (2015),[15] Nesbitt et al. (2015),[16] Ezcurra (2016)[17] and Spiekman et al., 2021[2] found that even this definition of Protorosauria, like Prolacertiformes, was an unnatural group of various non-Crocopodan archosauromorphs. These studies found that tanystropheids were archosauromorphs more closely related to crocopods than to Protorosaurus. Nevertheless, Ezcurra noted that archosauromorph systematics required further study, and that phylogenetic support for Protorosauria being a natural group was only barely weaker than the support for the group being unnatural.

Included groups

[edit]

The Protorosauria includes the Permian genus Protorosaurus, closely related to Czatkowiella.[18] A wide variety of Permian and Triassic reptiles have been classified within Protorosauria, including the arboreal gliding reptile Sharovipteryx and the aquatic tanystropheids, which have extremely long necks.

Another enigmatic group of Triassic reptiles, the Drepanosauromorpha, have often been classified as belonging to the Protorosauria.[19]

Pterosaurs have also been proposed as protorosaurs or close relatives of them,[20] although they are now regarded as a more derived group of archosaurs.

While Senter (2004) reassigned the bizarre, arboreal drepanosaurids and Longisquama to a group of more primitive diapsids called Avicephala,[21] subsequent studies failed to find the same result, instead supporting the hypothesis that they were protorosaurs.

Cladogram

[edit]

The following cladogram shows the position of Protorosauria among the Sauria sensu Sean P. Modesto and Hans-Dieter Sues (2004).[7]

Most recent studies have recovered Protorosauria as a whole as a paraphyletic, cladogram after Spiekman et al. 2021[2]

Diapsida

Although Protorosauria as a whole is often found to be a paraphyletic, a large group of former "protorosaurs" (excluding Protorosaurus) is frequently found to be monophyletic. This clade was given the name "Tanysauria" by Spiekman et al. in 2024.[4]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Protorosauria

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Protorosauria is an extinct, polyphyletic group of basal archosauromorph reptiles that flourished from the latest Middle Permian (Capitanian stage) to the end of the ( stage), spanning approximately 265 to 201 million years ago, and is characterized by diverse morphologies including notably elongated necks in many taxa due to specialized . These reptiles, often lizard-like in build with slender limbs and skulls, occupied a range of ecological niches from terrestrial to aquatic habitats across , with fossils documented in , , , and (e.g., ). Although historically recognized as a monophyletic within —stem-group reptiles more closely related to archosaurs (such as crocodilians and birds) than to lepidosaurs ( and snakes)—recent phylogenetic analyses have demonstrated its , with constituent taxa scattering across the archosauromorph tree rather than forming a single lineage. Key defining features include elongated that span multiple vertebrae, a "U"-shaped frontal-parietal suture in the , and recurved marginal teeth adapted for grasping prey, though not all members exhibit extreme neck elongation. The group's diversity encompasses over a dozen genera, with early Permian forms like Protorosaurus speneri representing the most basal members—small, agile predators about 2 meters long from what is now —while Middle to Late Triassic taxa show greater specialization. Prominent later representatives include Macrocnemus species (e.g., M. fuyuanensis from and M. bassanii from Europe), terrestrial runners up to 1.2 meters in length with moderately long necks; Tanystropheus, renowned for its extreme neck comprising up to 13 elongated vertebrae that could reach three times the body length, likely functioning in ambush predation in coastal environments; Dinocephalosaurus orientalis, an aquatic form from with a serpentine neck and evidence of , suggesting adaptations for ; and Austronaga minuta (described in 2025), a small aquatic form from . Other notable genera are Prolacerta broomi from , a basal form closer to , and Langobardisaurus from , which may have exhibited bipedal capabilities. Phylogenetic studies using extensive morphological datasets (e.g., 307 characters across 42 operational taxonomic units) place protorosaurs as successive outgroups to more derived archosauromorphs like rhynchosaurs and allokotosaurs, highlighting their role in early experimentation with body plans that foreshadowed success. Despite their by the end of the , possibly linked to environmental upheavals preceding the end- mass , protorosaurs provide critical insights into the evolutionary radiation of reptiles during the era's dawn.

Definition and etymology

Etymology

The name Protorosauria derives from the genus Protorosaurus, combining the Greek words prōtos ("first" or "primitive") and sauros ("lizard" or "reptile"), reflecting its perception as an early representative of saurian reptiles. The term was coined by the English anatomist and paleontologist Thomas Henry Huxley in 1871 as an ordinal name within Sauropsida, initially encompassing only the genus Protorosaurus speneri from the Late Permian of Germany, which was viewed as a primitive form bridging earlier reptiles and more advanced saurians. Huxley's classification positioned Protorosauria alongside other orders like Sauropterygia and Placodontia under Sauropsida, emphasizing shared diapsid skull features and suggesting an ancestral role for Protorosaurus in the radiation of Mesozoic reptiles during the early 20th-century understanding of reptilian evolution. This reflected the limited fossil record at the time, with Protorosaurus—first described in 1832—serving as the type genus based on specimens from the Zechstein Formation. Over time, the term evolved from Huxley's monotypic order to a broader grouping in mid-20th-century classifications, incorporating diverse forms such as tanystropheids and prolacertids, often as a grade or paraphyletic assemblage of basal archosauromorphs rather than a strictly monophyletic . Modern phylogenetic analyses retain Protorosauria for convenience to denote non-archosaurian, non-lepidosauromorph archosauromorphs, though its exact boundaries remain debated due to ongoing revisions of early relationships.

Taxonomic definition

Protorosauria is formally defined as a stem-based within , consisting of all taxa more closely related to speneri than to Archosauria or Rhynchosauria. This definition anchors the group to its while excluding more crownward archosauromorph lineages, emphasizing Protorosauria's position as a basal assemblage of early reptiles. The was originally proposed by Huxley in 1871 and has been refined through phylogenetic analyses to capture its evolutionary boundaries relative to other major archosauromorph branches. Key synapomorphies diagnosing Protorosauria include elongated bearing backward-projecting that overlap onto subsequent vertebrae, facilitating an extended neck region, as well as the presence of antorbital and mandibular fenestrae that contribute to a lightweight skull structure. These features are evident in basal members like and support the clade's distinction from other archosauromorphs, where cervical elongation is less pronounced or lack such extensive posterior projection. Additional vertebral traits, such as parallelogram-shaped in cervico-dorsal regions and well-developed prezygodiapophyseal laminae, further characterize the group and underscore its adaptive morphology for terrestrial or semi-aquatic lifestyles. Recent phylogenetic studies have challenged the of Protorosauria, suggesting it may instead represent a paraphyletic grade of basal archosauromorphs rather than a cohesive . For instance, analyses from 2021 recover as a distinct monophyletic group outside a core Protorosauria, excluding these highly specialized forms with extreme cervical elongation and thereby rendering the traditional assemblage polyphyletic. A 2025 study on the skull of Protorosaurus speneri further supports its position as one of the earliest-diverging archosauromorphs, reinforcing the polyphyletic nature of the group. Such findings highlight ongoing debates in archosauromorph , where varying sampling and character sets influence whether Protorosauria is upheld as monophyletic or viewed as a sequential grade leading toward more derived archosauromorph diversity.

Description

Skull and dentition

Protorosaurs possess a skull configuration characterized by the presence of an and two e: a larger upper bordered by the postorbital, squamosal, parietal, and postfrontal, and a smaller lower formed by the postorbital, jugal, and squamosal. The generally features large temporal openings that accommodate robust jaw adductor muscles, contributing to a strong bite, while the orbits vary in size but are typically laterally directed for . In advanced forms, such as hydroides, the postorbitals are triradiate and fuse to form a complete posterior margin of the orbit, enhancing structural integrity. Specific cranial features include a variably elongated rostrum and fused postorbitals in some taxa, with the infratemporal bar often incomplete. For instance, speneri exhibits a robust with a short rostrum, an open lower temporal bar, and a large postfrontal contribution to the upper , reflecting its basal position within the group. In contrast, Macrocnemus bassanii displays a lighter build with a slender, anteroposteriorly elongated rostrum and large orbits bordered by the prefrontal, postfrontal, postorbital, and jugal, suggesting adaptations for agility and enhanced . hydroides further exemplifies elongation trends, with a dorsoventrally flattened measuring approximately 138 mm in length and dorsally positioned external nares that reduce hydrodynamic drag. Dentition in protorosaurs is typically acrodont or pleurodont, with teeth that are conical, homodont or , and often slightly recurved for grasping prey. In , the teeth are sharp, single-pointed, and fused to the margins of their sockets, indicating a pleurodont implantation suited to a generalist predatory lifestyle. Macrocnemus bassanii has homodont dentition with about 12–13 premaxillary teeth and up to 30 maxillary teeth, all slightly recurved and thecodont in shallow alveoli, providing a uniform piercing function. species show arrangements, featuring large, recurved fangs on the (up to six, with the anterior three largest) and smaller conical posterior teeth that form a "fish-trap" mechanism, with evidence of piscivory from associated scales in contents. Variations across Protorosauria reflect temporal evolution, with Permian taxa like displaying more robust jaws and shorter rostra adapted to terrestrial predation, while forms such as and Macrocnemus exhibit progressive rostral elongation and lighter cranial builds, correlating with aquatic or semi-aquatic habits. This trend underscores the group's diversification from generalized archosauromorphs to specialized long-necked forms.

Postcranial skeleton

The postcranial skeleton of protorosaurs is characterized by an elongated vertebral column, particularly in the cervical region, which varies in extent across taxa but consistently features hyperelongate and that project posteriorly to overlap multiple vertebrae, forming a stiffening mechanism along the 's ventral surface. In basal forms like speneri, the cervical series comprises seven vertebrae with moderate elongation, while more derived tanystropheids exhibit greater specialization: Macrocnemus aff. M. fuyuanensis has eight elongated cervicals (centra lengths ~20 mm), longobardicus possesses 13 hyperelongate cervicals (mid-cervical length-to-height ratio >15), and the aquatic Dinocephalosaurus orientalis reaches at least 32 short but numerous cervicals, contributing to a longer than the body and tail combined. These , often dichocephalic and spanning 2–3 intervertebral joints, provide structural rigidity without extensive fusion, as seen in Macrocnemus where the longest reach ~64 mm. The trunk region is moderately developed, with dorsal vertebrae typically numbering 17–19 and featuring amphicoelous centra, higher neural spines, and associated ribs that expand the thoracic cage to accommodate expanded lungs, supporting the respiratory demands of elongated necks. Sacral ribs are robust and sutured to the centra of the two sacral vertebrae, reinforcing the pelvic girdle attachment. Tails are often long and flexible, with ~46 caudals in Tanystropheus (including pleurodont processes on anterior vertebrae) and similarly extended in Protorosaurus, aiding balance but lacking the autotomy septa seen in some archosauromorphs. Appendicular elements are generally slender, reflecting terrestrial or semi-aquatic habits, with limb girdles and bones indicating a sprawling to semi-erect posture via broad scapulae (~29 mm in Macrocnemus), curved clavicles, and wing-like iliac blades. Forelimbs show humeri longer than radii (ratio ~21–22% in Macrocnemus), while hindlimbs have tibiae subequal to femora; in fully aquatic Dinocephalosaurus, limbs are pillar-like with reduced, solid elements and flipper-like autopodia featuring simple circular carpals and tarsals. Unlike some contemporaneous archosaurs, protorosaurs lack osteoderms, resulting in a lightly armored .

Evolutionary history

Origins

Protorosauria originated during the Middle Permian as part of the cryptic early diversification of archosauromorph reptiles in the northern regions of Pangea, which were positioned near the at the time. This emergence followed the Kungurian stage (approximately 283–272 Ma) and was influenced by the ecological opportunities arising after regional extinctions that affected earlier tetrapod faunas, allowing basal diapsids to occupy new terrestrial niches. The earliest records of the clade date to the late Middle Permian ( stage, approximately 265–259 Ma), though these are fragmentary, primarily from European localities such as those in , and their assignment to Protorosauria remains tentative. More substantial evidence appears in the Late Permian (Wuchiapingian stage, ~259 Ma), exemplified by Protorosaurus speneri from the Kupferschiefer Formation in , a well-preserved site yielding multiple articulated skeletons that highlight the group's initial radiation. No confirmed protorosaur fossils predate the Permian, underscoring a gap in the pre-Middle Permian record. As a derived lineage within basal , Protorosauria shared key ancestral traits with other early forms, such as the skull configuration with paired temporal fenestrae and preliminary adaptations in limb morphology for enhanced terrestrial mobility, including elongated humeri and robust tarsals. These features positioned protorosaurs as agile, lizard-like predators in floodplain and semi-arid environments. The sparse pre-Triassic fossil record, limited to a handful of Eurasian sites, suggests origins in equatorial Pangea, with potential undiscovered material in under-sampled continental interiors.

Temporal and geographic distribution

Protorosauria first appeared in the fossil record during the Middle Permian, specifically the Capitanian stage (approximately 265–259 million years ago), though definitive fossils are known from the subsequent Late Permian, with the earliest well-established representative being Protorosaurus speneri from the Kupferschiefer Formation in Germany (~257 Ma). The group persisted through the Early and Middle Triassic, achieving its peak diversity during the Anisian and Ladinian stages of the Middle Triassic, around 247 to 237 million years ago, when multiple genera such as Tanystropheus, Macrocnemus, and Dinocephalosaurus coexisted across various depositional environments. The temporal range extended into the Late Triassic, with the youngest records from the Norian stage, approximately 227 to 201 million years ago, after which the clade appears to have gone extinct by the Rhaetian. Fossils of Protorosauria are predominantly known from Laurasian landmasses, with the majority of well-preserved specimens originating from and . In , significant occurrences include Protorosaurus from the Late Permian Kupferschiefer Formation at localities near Kupferzell in southwestern Germany, and Tanystropheus longobardicus from the Middle Triassic Besano Formation at , spanning the Swiss-Italian border. In , notable finds come from southern , such as Dinocephalosaurus orientalis from the Middle Triassic Guanling Formation (Zhenzhuchong locality) in Province, highlighting the clade's presence in eastern Tethyan marine settings. Historical attributions of isolated remains to protorosaurs have been reported from in the Late Triassic of , but these are now considered to belong to other archosauromorph groups such as allokotosaurs. Many of these deposits, particularly in the , represent marginal marine or coastal environments, indicating that protorosaurs inhabited nearshore habitats throughout much of their evolutionary history. The decline of Protorosauria began in the , with diversity dropping sharply after the stage, and no unequivocal records surviving the end-Triassic around 201 million years ago. This extinction may have been influenced by increasing ecological competition from rising and lineages, which diversified rapidly in terrestrial and aerial niches during the Norian-Rhaetian, potentially outcompeting the more specialized protorosaurs.

Classification

History of classification

The genus was first described in 1710 by Christian Maximilian Spener based on a slab from the Permian Kupferschiefer of , initially interpreted as a preserved in stone. In the early , Hermann Friedrich von Meyer recognized the remains as belonging to an extinct , formally naming the species speneri in 1832 and providing a detailed in 1856. Thomas Henry Huxley established the taxon Protorosauria in 1871 as an order within , originally encompassing only and emphasizing its primitive features. Early 20th-century workers expanded the group; for instance, Osborn (1903) included Palaeohatteria and Kadaliosaurus, while Peyer (1931, 1937) added and Macrocnemus based on shared vertebral and limb morphologies. By the mid-20th century, Protorosauria was often placed within the informal assemblage "" or as prolacertiforms, with debates centering on affinities to saurians ( and relatives) versus rhynchocephalians; Camp (1945) notably included Prolacerta and favored Protorosauria over the broader "Eosuchia". Romer (1956) and others viewed it as a stem-group to archosaurs, while Kuhn (1961) formalized it as a suborder with squamate-like traits. Cladistic analyses in the marked a significant shift, nesting Protorosauria firmly within as a basal ; (1984, 1986) and Benton (1985) highlighted its position as sister to more crownward archosauromorphs based on and postcranial synapomorphies. Late 20th-century phylogenetic studies further refined this, with Benton and Allen (1997) and Jalil (1997) using character matrices to support , though Rieppel et al. (2003) began questioning it through combined datasets that suggested . Key debates included the inclusion of drepanosaurs, which some analyses grouped with protorosaurs due to elongated necks but others excluded based on differences in morphology and overall . In the 2010s and 2020s, accumulating evidence from expanded matrices confirmed the paraphyly of Protorosauria, with taxa like Prolacerta aligning closer to and tanystropheids forming a distinct non-protorosaurian ; Spiekman et al. (2021) explicitly advocated abandoning the name due to its polyphyletic nature in light of Tanystropheidae's revised phylogeny. This modern view reframes Protorosauria as a historical grade of early archosauromorphs rather than a natural group.

Included taxa

Protorosauria encompasses a diverse assemblage of early archosauromorph reptiles, primarily defined by shared derived traits such as elongated with low neural spines and thin , which distinguish them from unrelated long-necked forms like pterosaurs (e.g., azhdarchids). Approximately 10–15 valid genera are currently assigned to the group, though its remains debated, with some analyses suggesting or . Inclusion relies on these vertebral synapomorphies and skull features, excluding taxa like rhynchocephalians (e.g., Sapheosaurus) or allokotosaurs (e.g., Malerisaurus). Taxa like Prolacerta broomi are sometimes included in the grade but align more closely with in recent phylogenies.

Permian genera

The only well-established Permian protorosaur is Protorosaurus speneri, the type genus of Protorosauria, known from the late Permian (Wuchiapingian) of and . This slender, quadrupedal reptile reached lengths of about 2 meters and is inferred to have been a terrestrial based on its and limb proportions. Fragmentary remains from the same period in represent Aenigmastropheus parringtoni, a basal member with notochordal vertebrae and prezygodiapophyseal laminae, though its full morphology remains poorly known. Eorasaurus olsoni from the late –Wuchiapingian of is another early , characterized by parallelogram-shaped vertebral centra, but its assignment is tentative due to limited material.

Triassic genera

Triassic protorosaurs are more diverse and geographically widespread, spanning the Early to Late periods across , , , and . Tanystropheus, from the (–) of (, , ), includes species like T. longobardicus (up to 2 m, terrestrial with tricuspid teeth) and T. hydroides (over 3 m, long-necked with aquatic adaptations). Macrocnemus (e.g., M. bassanii from and M. fuyuanensis from ) was an agile, terrestrial form about 2 m long, with facultatively bipedal capabilities and plesiomorphic skull features. Dinocephalosaurus orientalis, from the of , measured around 5 m and exhibited extreme neck elongation suited to an aquatic lifestyle within the Dinocephalosauridae clade. Other notable genera include Prolacerta broomi (, and , ~1 m, terrestrial), Pectodens zhenyuensis (, , long-necked with potential gliding traits), Ozimek volans (, , , long-necked with potential gliding traits), Sclerostropheus fossai (, , based on ), and Jesairosaurus lehmani (, , basal with traits).

Dubious or excluded taxa

Several taxa have been historically assigned to Protorosauria but are now considered dubious or excluded. Otischalkia elderae from the of () was initially described as a rhynchosaur but is now regarded as a due to insufficient material for diagnosis. Trachelosaurus fischeri (Early–Middle Triassic, ) remains poorly understood due to fragmentary vertebrae, with uncertain affinities outside core protorosaurs. Exclusions like Malerisaurus reflect its placement among allokotosaurs rather than archosauromorphs sharing protorosaur synapomorphies. Pamelaria dolichotrachela (, ), originally considered a prolacertiform, is now classified as an allokotosaur (azendohsaurid).

Cladogram

The cladogram for Protorosauria reflects its status as a paraphyletic grade of basal archosauromorphs, based on a comprehensive phylogenetic analysis incorporating 47 taxa and 279 morphological characters. This topology positions Protorosauria as sequentially branching lineages basal to the clade comprising Rhynchosauria and Archosauria, with Protorosauridae (including ) as the most basal member, followed by tanystropheids such as Macrocnemus, , and the more derived Dinocephalosaurus. A simplified representation of the relevant portion of the is as follows:

[Archosauromorpha](/page/Archosauromorpha) ├── [Protorosauria](/page/Protorosauria) (paraphyletic grade) │ ├── [Protorosaurus](/page/Protorosaurus) (Protorosauridae) │ ├── Macrocnemus │ └── [Tanystropheidae](/page/Tanystropheidae) │ ├── [Tanystropheus](/page/Tanystropheus) │ └── Dinocephalosaurus └── Archosauria + Rhynchosauria (sister [clade](/page/Clade))

[Archosauromorpha](/page/Archosauromorpha) ├── [Protorosauria](/page/Protorosauria) (paraphyletic grade) │ ├── [Protorosaurus](/page/Protorosaurus) (Protorosauridae) │ ├── Macrocnemus │ └── [Tanystropheidae](/page/Tanystropheidae) │ ├── [Tanystropheus](/page/Tanystropheus) │ └── Dinocephalosaurus └── Archosauria + Rhynchosauria (sister [clade](/page/Clade))

Key nodes within this arrangement highlight the of through successive branching, where represents the basalmost and Dinocephalosaurus the most crownward within tanystropheids, immediately sister to stem Archosauria. Character support for these relationships includes moderate to strong backing at major nodes, such as the synapomorphic elongated and bifurcating that unite tanystropheids, with overall tree consistency index of 0.42 indicating robust resolution despite . Alternative topologies in earlier analyses have occasionally included drepanosaurs (e.g., Hypuronector) within as part of a monophyletic grouping with and tanystropheids, based on shared features like elongated necks, though more recent matrices exclude them as unrelated archosauromorphs.

Locomotion and habitat

Protorosaurians exhibited a range of locomotor adaptations inferred from postcranial skeletal features, with terrestrial forms generally employing a sprawling typical of early diapsids. In , the slender limb bones and robust proximal elements support quadrupedal with limbs held in a sprawling posture, facilitating movement through Permian woodlands near coastal margins. In contrast, more derived Triassic protorosaurians like displayed semi-aquatic adaptations, with limb proportions indicating limited swimming capabilities, likely punting along the substrate in shallow waters, though lacking adaptations for efficient open-water swimming. The elongated, buoyant neck of , composed of hyperelongate , likely aided in maintaining balance and reaching prey during predation in coastal environments. This neck stiffness was reinforced by extensive zygapophyseal articulations and overlapping , limiting lateral flexibility while permitting limited dorsoventral motion for or browsing. Habitat preferences varied temporally within Protorosauria, reflecting shifts from Permian terrestrial settings to marginal marine zones. Early forms such as inhabited forested lowlands adjacent to shallow seas in the Late Permian of , as evidenced by fossils from the Kupferschiefer Formation's coastal plain deposits. Later taxa, including , occupied coastal lagoons and nearshore environments, with abundant specimens from the bituminous shales of the Besano Formation indicating a brackish to marine setting at . Direct evidence of locomotion is limited, as trackways attributable to protorosaurians are rare, but limb proportions in genera like Macrocnemus—featuring gracile, elongate hindlimbs with a tibia longer than the femur—suggest cursorial capabilities suited to agile terrestrial traversal in Triassic woodlands or floodplains.

Diet and ecology

The diet of basal protorosaurs such as Protorosaurus speneri remains uncertain, with anatomical and taphonomic evidence suggesting a primarily carnivorous or insectivorous strategy despite the presence of plant material in some specimens. The sharp, recurved teeth with cutting edges in Protorosaurus are consistent with predation on small vertebrates or arthropods, aligning with the homodont dentition typical of early archosauromorph insectivores. However, gut contents from a well-preserved specimen from the Upper Permian Kupferschiefer of Germany include numerous ovules of the conifer Ullmannia frumentaria, indicating that plant material formed at least part of the diet, though the dentition shows no specialized adaptations for herbivory. This combination suggests possible omnivory in low-diversity Permian coastal faunas, where Protorosaurus may have occupied an apex or mid-level predatory role, exploiting limited prey resources including arthropods and occasional vegetation. In contrast, elongated-neck protorosaurs from the exhibit more specialized piscivorous diets, inferred from direct evidence of prey remains and cranial adaptations. For hydroides, stomach contents preserve ganoid scales and belemnoid hooklets, confirming a diet dominated by aquatic vertebrates and in Middle coastal habitats of the Tethys Sea. The "fish-trap" , featuring long curved fangs and reduced posterior teeth, further supports ambush predation on via ram-feeding in shallow marine environments. Similarly, Dinocephalosaurus orientalis from the Middle of contains well-preserved remains in the gastric region of specimens, indicating a piscivorous lifestyle facilitated by its flexible, serpentine neck for capturing prey in murky coastal waters. Recent findings also indicate in Dinocephalosaurus orientalis, with embryos preserved within specimens, supporting its adaptation to a fully marine lifestyle without needing to return to land for reproduction. No evidence of herbivory exists across protorosaur taxa, with tooth morphology and associated fossils consistently pointing to carnivory. Ecologically, protorosaurs filled mid-level predatory niches in Permian and coastal and marginal marine ecosystems, preying on , cephalopods, and smaller invertebrates while coexisting with emerging archosaurs. In the , protorosaurs such as partitioned resources through size differences among species or individuals and habitat preferences, with larger individuals targeting schooling and smaller ones exploiting crustaceans or softer prey to reduce . This specialization likely minimized overlap with terrestrial early archosaurs, such as rauisuchians, in nearshore settings, while Permian forms like dominated sparse faunas without direct competitors for or small prey. Niche separation from herbivorous rhynchosaurs, which occupied grazing roles in the same landscapes, further highlights protorosaurs' role as aquatic or semi-aquatic carnivores in diverse coastal communities.

References

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  20. https://anatomypubs.onlinelibrary.wiley.com/doi/10.1002/ar.25392
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  26. https://doi.org/10.1016/j.cub.2020.07.025
  27. https://riviste.unimi.it/index.php/RIPS/article/view/9541
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  29. https://www.sciencedirect.com/science/article/abs/pii/S0031018217301669
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  31. https://www.researchgate.net/publication/248153032_Morphological_evidence_for_bipedalism_in_the_Late_Triassic_prolacertiform_reptile_Langobardisaurus
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC2827974/
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  35. https://www.sci.news/paleontology/dinocephalosaurus-orientalis-12716.html
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