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Postosuchus

Postosuchus is an extinct of large, carnivorous rauisuchid pseudosuchian that lived during the stage of the period, approximately 227 to 208 million years ago, in what is now the southwestern and . The includes two : the P. kirkpatricki, described from multiple specimens including skulls and postcrania from the Dockum Group in , and P. alisonae, known from a partial from the Deep River Basin in . These animals were apex predators, reaching lengths of 4 to 6 meters and estimated masses of 250 to 450 kilograms, with robust skulls featuring serrated, blade-like ziphodont teeth suited for tearing flesh. In Postosuchus kirkpatricki, the measures up to about 54 cm in length, with 13 maxillary and 4 premaxillary teeth, and features such as a prominent antorbital and a palpebral over the . The postcranial skeleton includes a long cervical series, a with three fused vertebrae, and disproportionately short forelimbs relative to the hindlimbs, indicating a semi-erect . Locomotor studies suggest Postosuchus was facultatively bipedal, capable of both quadrupedal walking and bipedal running or standing, adaptations that enhanced its predatory capabilities in and riverine environments. Phylogenetically, Postosuchus belongs to within , the crocodylian-line archosaurs, and is closely related to other large carnivores like Saurosuchus and Prestosuchus. As one of the largest non-dinosaurian predators of the , it coexisted with early dinosaurs, phytosaurs, and aetosaurs, occupying a key ecological role before the end-Triassic . Fossils of Postosuchus provide critical insights into the and of pseudosuchians, highlighting their dominance in terrestrial ecosystems prior to the rise of dinosaurs in the .

Taxonomy

Etymology and species

The genus name Postosuchus derives from "Post," referencing the town of Post, Texas, near the type locality of the holotype, combined with the Greek suffix -suchus, meaning "crocodile" and evoking the ancient Egyptian crocodile god Sobek, in allusion to its pseudosuchian affinities. The type and only originally described species is P. kirkpatricki, named in 1985 after quarry discoverer E. C. Kirkpatrick; its holotype (TTU-P 9000) comprises a nearly complete skull, assorted teeth, and partial postcranial skeleton of an adult individual from the Tecovas Formation of the Dockum Group in Texas. A second species, P. alisonae, was erected in 2008 based on holotype UNC 15575, a partial postcranial skeleton including most of the axial column and some limb elements from the Vinita Member of the Chatham Group in the Deep River Basin, North Carolina; it is named for fossil collector Alison Chambers and differs from P. kirkpatricki in several postcranial features. These two species represent the only valid taxa currently recognized within the genus, with no established synonyms; Angistorhinus, a contemporary but unrelated phytosaur genus, has occasionally been misassociated in non-technical contexts but is taxonomically distinct.

Classification and phylogeny

Postosuchus was originally classified within , a now-abandoned paraphyletic group encompassing various basal archosaurs, when first described in 1985 as a large carnivorous thecodontian from the of . The advent of cladistic methods in the late reclassified it within the monophyletic Archosauria, specifically the crocodylian-lineage clade , which includes all archosaurs more closely related to crocodilians than to birds. Within , Postosuchus belongs to the derived subclade , characterized by osteoderms and specialized ankle morphology, and is further nested in the family . The broader group , historically used for large predatory pseudosuchians like Postosuchus, is now recognized as a paraphyletic grade leading to more derived forms rather than a . In contrast, is monophyletic, comprising hypercarnivorous quadrupeds from the to , with Postosuchus as the primary North American representative alongside taxa like P. alisonae. A comprehensive phylogenetic analysis by Nesbitt (2011) recovered as the to , supported by four unequivocal synapomorphies: a rugose lateral ridge on the nasal, a subrectangular lateral in dorsal view, a quadrate with an expanded dorsomedial margin contacting the quadratojugal, and a prominent ridge on the dorsal surface of the surangular. Within Rauisuchidae, Postosuchus kirkpatricki forms a with Teratosaurus suevicus and Polonosuchus silesiacus, united by synapomorphies including a deep pit on the squamosal and a rugose on the skull roof; this subgroup is positioned basally relative to other rauisuchids like Rauisuchus tiradentes. Earlier pre-2000s classifications sometimes erroneously allied Postosuchus closely with crocodylomorphs or even dinosaurs due to incomplete material and outdated paraphyletic groupings like , but modern analyses have resolved these as stem pseudosuchians without direct crocodylomorph affinity beyond the shared loricatan ancestry. Recent studies, including those building on Nesbitt's dataset (e.g., Rezende et al., 2022), continue to affirm the of and Postosuchus's position as a derived member, emphasizing its role in North American faunas.

Description

Skull

The skull of Postosuchus is robust and deep, measuring approximately 54 cm in length in the of P. kirkpatricki (TTU-P 9000), with a narrower anterior region expanding posteriorly to accommodate powerful jaw musculature. It exhibits multiple that reduce weight while providing space for muscle attachments and sensory organs, including a large, wedge-shaped occupying much of the snout's lateral surface and a mandibular fenestra formed by the surangular and prearticular bones. A prominent runs along the parietals, serving as an attachment site for temporalis muscles to facilitate strong bite force. Dentition is heterodont and ziphodont, characterized by laterally compressed, serrated crowns adapted for slicing flesh. The upper bears 17 teeth total, with 4 in the (conical and procumbent, functioning as incisors) and 13 in the (including enlarged canines anteriorly and recurved, blade-like posterior teeth). The accommodates over 30 teeth, with the dentary alone supporting 15–16, exhibiting similar heterodonty and serrations for efficient prey dispatch. Sensory adaptations include large, keyhole-shaped orbits bordered by the postorbital and jugal bones, indicating enhanced for predation. Impressions of the olfactory bulbs on the frontal suggest a well-developed , potentially augmented by vomerine structures associated with the organ for chemical detection. The species P. alisonae ( UNC 15575) is known from fragmentary cranial elements, including portions of the nasal and other bones, which exhibit similarities to P. kirkpatricki in preserved features such as rugose ornamentation, with minor differences in tooth morphology, though full comparisons are limited by preservation.

Postcranial skeleton

Postosuchus kirkpatricki attained a total body length of up to 5–6 meters, with an estimated body mass of 250–300 based on skeletal proportions and comparisons to related archosaurs. This size underscores its role as a large predator, with the postcranial elements preserving a robust framework adapted for terrestrial locomotion and predation. The is characterized by an elongated neck comprising 9–10 , which are elongated and amphicoelous, facilitating flexibility in head movement. There are 15–16 vertebrae with low neural arches and strong ventral keels, contributing to a stiffened for support. The sacral region features ribs fused to the ilium, enhancing stability at the hip joint, while the consists of over 30 caudal vertebrae with haemal arches (chevrons) that allow for lateral flexibility and balance. Rectangular osteoderms, arranged in paravertebral rows along the and lateral surfaces, provide dermal armor; these thin, keeled plates overlap to form a protective shield over the presacral region without restricting movement. The pectoral girdle includes a robust scapula-coracoid complex, with the scapula exhibiting a broad process and the forming a tight suture, indicative of strong shoulder musculature for support. In the , the ilium is broad and elongate with a prominent supraacetabular confluent with its margin, providing extensive attachment sites for muscles and reflecting adaptations for . The pubis and are slender, contributing to an open that accommodates semi-erect .

Limbs and posture

Postosuchus possessed markedly unequal limb proportions, with elongated hindlimbs adapted for and comparatively reduced forelimbs. The measured approximately 40–50 cm in length in adult specimens, while combined forelimb elements ( plus /) were roughly 60% of hindlimb length, rendering the forelimbs slender and less robust. The pes was four-toed, featuring a symmetrical structure with digits II and III of subequal length and digit I notably reduced, consistent with a mesaxonic foot emphasizing the central for weight distribution. The posture of Postosuchus has been subject to debate, centered on its degree of bipedality. A 2013 of the postcranial by Weinbaum indicated potential for bipedality, inferred from the pronounced hindlimb-forelimb disparity and highly reduced manual digits, which limited forelimb utility for . In contrast, a 2022 study by Hartman et al. proposed a more versatile locomotor strategy, with Postosuchus capable of both bipedal and quadrupedal based on variation in femoral across specimens, though without evidence of an ontogenetic shift from quadrupedal juveniles to bipedal adults. Several skeletal adaptations underscored Postosuchus's locomotor efficiency as a predator. It maintained a stance, with limbs positioned directly beneath the body to facilitate an upright posture and efficient stride. The elevated calcaneal served as an analog to a strong mechanism, enhancing elastic energy storage for rapid acceleration, while forelimb reduction further emphasized hindlimb reliance, minimizing anterior during high-speed movement.

Discovery and naming

Initial discovery

The earliest fossils later attributed to Postosuchus were discovered in 1920 in Crosby County, western , within the Upper Triassic Dockum Group, part of the broader Texas Red Beds. These consisted of isolated elements, including a braincase (UMMP 7473) and fragments of pelvic bones (UMMP 7244), which paleontologist Ermine Cowles Case described in 1922 as potentially belonging to a new reptile of uncertain affinities, possibly a primitive crocodylomorph. The fragmentary nature of this material limited early interpretations, with Case noting resemblances to known archosaurs but unable to assign it to an established group. In 1943, Case reported additional isolated fragments from the Dockum Group, including a partial (UMMP 23127) featuring a footed pubis, which he interpreted as evidence of a novel parasuchid () species adapted for terrestrial locomotion. Like the 1922 specimens, these remains were too incomplete for definitive , leading to tentative with aquatic or semi-aquatic archosaurs common in the region, and they remained overlooked in broader studies for decades. A major advance occurred in the summer of 1980, when a field expedition uncovered a productive locality near in Garza County, , within the Cooper Canyon Formation of the Dockum Group. This site yielded the specimen (TTU-P 9000), a nearly complete articulated representing the first substantial Postosuchus material, alongside other remains that highlighted the site's taphonomic complexity. Initial analysis of the 1980 finds began in 1985 under Sankar , who recognized the new skeleton's diagnostic features and re-evaluated the earlier Case specimens, confirming their referral to the same taxon and challenging prior crocodylomorph or identifications due to the superior completeness of the . This breakthrough resolved longstanding uncertainties from the pre-1980 fragments, sparking renewed interest in rauisuchian diversity in North American ecosystems.

Named species

The genus Postosuchus comprises two valid species, both known from the of . The , P. kirkpatricki, was formally named and described by Sankar in 1985 based on material from the Dockum Group in . The specific epithet honors the Kirkpatrick Quarry, the type locality near the town of Post. The (TTU-P 9000) consists of a partial , including a well-preserved , partial axial column, and elements of the limb girdles and limbs, representing approximately 70% completeness of the . In 1995, Robert A. Long and Phillip A. Murry emended the hypodigm of P. kirkpatricki by excluding misidentified elements originally referred by , recognizing that the original assemblage included material from at least three distinct rauisuchian taxa, thereby refining the diagnosis to focus on the core and specimens. The second species, P. alisonae, was named in by Karen Peyer and colleagues based on a more complete specimen from the in . The specific epithet honors Alison L. Chambers for her contributions to outreach. The holotype (UNC 15575) includes a partial with cranial elements (such as nasals and frontals), vertebrae, , osteoderms, and portions of the pectoral and pelvic girdles along with fore- and hindlimb bones. Subsequent taxonomic revisions have addressed synonymy debates surrounding Postosuchus material. In 2006, Sterling J. Nesbitt and Mark A. Norell resolved uncertainties by synonymizing the genus Chatterjeea (previously considered part of the Postosuchus hypodigm) with Shuvosaurus, confirming that P. kirkpatricki represents a distinct rauisuchid without overlap from those taxa. More recent work in 2016 by Emily R. Lessner and colleagues described new rauisuchid material from the Dockum Group, including the genus Vivaron, which differs morphologically from Postosuchus and supports the validity of both P. kirkpatricki and P. alisonae by highlighting greater diversity among North American rauisuchids during the Norian stage.

Putative occurrences

Fossil material attributable to Postosuchus has been recovered primarily from deposits in the southwestern and , spanning the to stages (approximately 237–208.5 Ma). The , P. kirkpatricki, is known from multiple specimens in the Norian-age Cooper Canyon Formation of the Dockum Group, Garza and Crosby Counties, , including the and from the Post Quarry. Referred skeletal elements, including vertebrae, limb bones, and osteoderms, have been identified from the Norian in ( Quarry, Petrified Forest Member) and ( Whitaker Quarry, siltstone member). The second species, P. alisonae, is represented by a partial articulated (including vertebrae, ribs, manus, pes, and osteoderms) from the –early Pekin Formation (Deep River Basin, ) in , collected from a near Genlee. This specimen extends the geographic range of the eastward, though it exhibits subtle morphological differences from P. kirkpatricki, such as a unique on metacarpal II. Putative occurrences outside these confirmed sites remain debated. No unequivocal Postosuchus material has been documented from or other global localities, limiting its known distribution to . Outdated referrals, such as postcranial elements once included under the phytosaur Angistorhinus grandis from the Dockum Group, have been clarified as non-Postosuchus based on revised diagnoses and phylogenies. Post-2010 studies have not added new confirmed referrals, reinforcing the primary focus in the southwest with the single eastern outlier. Certain Triassic ichnofossils, including Chirotherium tracks from the and equivalent units, have been tentatively linked to Postosuchus-like rauisuchids due to pentadactyl manus impressions and quadrupedal gait patterns, though direct attribution remains provisional.

Paleoecology

Habitat and environment

Postosuchus inhabited the of , specifically during the stage (approximately 227–208 million years ago), within the of the (, ) and the equivalent Dockum Group of and eastern New Mexico, as well as the Deep River Basin of . These formations represent tropical biomes characterized by seasonal floodplains, meandering , lakes, and wetlands, as indicated by the interbedded fluvial sandstones, siltstones, and mudstones that preserved the fossils. The was warm and humid, influenced by a megamonsoon system that brought heavy seasonal rainfall, fostering lush riparian along systems while upland areas experienced periodic dryness. Sedimentological evidence from paleosols and channel deposits points to a dynamic fluvial with frequent flooding and , transitioning toward greater aridity in the later stage. Vegetation in these ecosystems was dominated by ferns (such as Clathropteris and Phlebopteris), horsetails (Neocalamites and Equisetites), and gymnosperms including conifers (Araucarioxylon), ginkgoales (Ginkgoites), and cycads/bennettitaleans (Nilssonia and Zamites). Petrified forests in Arizona's Petrified Forest National Park preserve extensive silicified conifer logs up to 40 meters long, highlighting the prevalence of tall gymnosperm woodlands in floodplain settings. The fauna formed a diverse, mixed archosaur-dominated assemblage, coexisting with aetosaurs like , therapsids such as the dicynodont , and early dinosaurs including , alongside phytosaurs and metoposaurs in semi-aquatic niches. Community structure reflected a of terrestrial to aquatic interactions across alluvial plains, with stable trophic networks despite faunal turnovers between early and late intervals.

Diet and interactions

Postosuchus was a hypercarnivorous , with a diet inferred to include large herbivores such as the Placerias, armored aetosaurs like , and smaller reptiles including early dinosaurs and cynodonts. Its featured ziphodont teeth—laterally compressed, serrated blades ideal for slashing flesh and inflicting deep wounds on prey—distinguishing it from blunt-toothed herbivores and omnivores in its . This feeding strategy aligns with its role in the food web, where it targeted abundant medium- to large-bodied vertebrates. Hunting behavior in Postosuchus likely involved ambush tactics or short pursuits, facilitated by its facultatively bipedal posture and powerful hindlimbs for rapid acceleration, though it may have also scavenged opportunistically on carcasses. These adaptations underscore its specialization as a terrestrial hunter in fluvial environments. As the dominant terrestrial predator, Postosuchus occupied the top , exerting predation pressure on diverse herbivores while minimizing overlap with smaller carnivores like Coelophysis, which may have evaded confrontation through pack hunting or niche partitioning toward smaller prey. It competed with other rauisuchians and phytosaurs for resources, as indicated by co-occurrence patterns in assemblages showing spatial separation along gradients. Predation traces, including shallow ziphodont bite marks on aetosaur osteoderms from the Chinle Formation, provide direct evidence of failed or successful attacks by large pseudosuchians like Postosuchus. Bonebeds, such as those from the Dockum Group, further reveal multi-individual accumulations potentially linked to predation events or territorial disputes among top carnivores.

References

  1. [1]
    Postosuchus, a new Thecodontian reptile from the Triassic of Texas ...
    Postosuchus kirkpatricki (ng, n.sp.) is a large, gracile, carnivorous thecodontian from the late Triassic Dockum Group of Texas.
  2. [2]
    [PDF] OSTEOLOGY AND RELATIONSHIPS OF Postosuchus kirkpatricki ...
    In 1985 Chatterjee described Postosuchus kirkpatricki from the remains of twelve individuals. Two large (4-6 meter) animals and ten smaller, animals (2-3 ...
  3. [3]
    [PDF] ARTICLE A NEW SUCHIAN ARCHOSAUR FROM THE UPPER ...
    Diagnostic features of this genus include an axis with two ventral keels; postaxial cervical centra with strongly developed single ventral keels that are.
  4. [4]
    [PDF] The skull of Postosuchus kirkpatricki (Archosauria - UC Berkeley
    Feb 18, 2011 · Chatterjee (1985) described Postosuchus kirkpatricki from the remains of twelve individuals: two large specimens (estimated length 4–6 meters) ...
  5. [5]
    Femoral specializations to locomotor habits in early archosauriforms
    Nov 28, 2021 · Postosuchus and Riojasuchus also were estimated as either bipedal or quadrupedal in our analysis depending on the specimen (Table 2).
  6. [6]
    The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida)
    Most studies recover Postosuchus from the Upper Triassic of Texas and/or other 'rauisuchians' as close relatives to crocodylomorphs (CitationGauthier 1986; ...
  7. [7]
  8. [8]
    The Early Evolution of Archosaurs: Relationships and the Origin of ...
    Aug 7, 2025 · This study presents a thorough phylogenetic analysis of 80 species-level taxa ranging from the latest Permian to the early part of the Jurassic using a dataset ...
  9. [9]
    [PDF] The taxonomy and anatomy of rauisuchian archosaurs from the Late ...
    Phylogenetic position of. Teratosaurus and Polonosuchus. The higher−level phylogeny of basal archosaurs has been a consistent subject of disagreement for ...
  10. [10]
    Postcranial anatomy of Prestosuchus chiniquensis (Archosauria ...
    Feb 1, 2024 · (1990). The phylogenetic analysis follows the dataset of Rezende et al. (2022), which was based on the matrix originally proposed by Nesbitt ( ...
  11. [11]
    [PDF] The skull anatomy and cranial endocast of the pseudosuchid ...
    Jan 24, 2019 · This paper provides the first detailed description of a complete, well-preserved skull and braincase of Prestosuchus chiniquensis, a ' ...
  12. [12]
  13. [13]
    (PDF) Postosuchus, a New Thecodontian Reptile from the Triassic of ...
    Aug 6, 2025 · Postosuchus kirkpatricki (n.g., n.sp.) is a large, gracile, carnivorous thecodontian from the late Triassic Dockum Group of Texas.Missing: etymology | Show results with:etymology
  14. [14]
    (PDF) The skull of Postsuchus kirkpatricki (Archosauria
    Aug 6, 2025 · (by monotypy). Holotype—TTUP 9000 Postosuchus kirkpatricki: almost. complete skull, assorted teeth, third ...
  15. [15]
    (PDF) Extreme convergence in the body plans of an early suchian ...
    Aug 6, 2025 · Here, we report a new Late Triassic suchian archosaur with unusual, highly specialized features that are convergent with ornithomimid dinosaurs.
  16. [16]
    A new rauisuchid (Archosauria, Pseudosuchia) from the Upper ...
    Sep 6, 2016 · Rauisuchids are large (2–6 m in length), carnivorous, and quadrupedal pseudosuchian archosaurs closely related to crocodylomorphs.
  17. [17]
    Conjuring Chirotherium | National Geographic
    Feb 22, 2013 · Rauisuchids - similar to the fearsome Postosuchus - are the probable Chirotherium trackmakers. Soergel illustrated a new, speculative ...
  18. [18]
    (PDF) Postcranial skeleton of Postosuchus kirkpatricki (Archosauria
    Aug 5, 2025 · Postosuchus kirkpatricki is a Late Triassic (Norian) 'rauisuchid' archosaur from North America. The initial description of the Postosuchus type material ...
  19. [19]
    [PDF] Assessing Ecological Relationships Among Late Triassic ...
    The Chinle Formation is an Upper Triassic (237 – 201.3 MYA) continental geologic formation, composed of interbedded sandstones, siltstones, and mudstones ( ...
  20. [20]
    Vertebrate paleoecology of the Chinle formation (Late Triassic) of ...
    The Late Triassic Chinle Formation is an extensive series of fluvial and lacustrine sediments exposed mostly in the Colorado Plateau region.
  21. [21]
    Collapse of the Late Triassic megamonsoon in western equatorial ...
    Aug 6, 2025 · The lower part of the Chinle Formation was deposited under humid climatic conditions with seasonality, which may have been caused by ...
  22. [22]
    Hematite reconstruction of Late Triassic hydroclimate over the ...
    Feb 9, 2021 · The Late Triassic Chinle Formation is largely a record of fluvial deposits accumulated within a shallow basin originating as a back-bulge of the ...Missing: Dockum | Show results with:Dockum
  23. [23]
    [PDF] Ferns From the Chinle Formation (Upper Triassic) in the Fort ...
    The Chinle Formation of Late Triassic age, which is widely exposed in parts of the Southwestern United. States, contains fossil leaves and fertile structures at.Missing: horsetails gymnosperms
  24. [24]
    [PDF] LATE TRIASSIC DINOSAURS FROM THE WESTERN UNITED ...
    All. Upper Triassic strata are assigned to the Chinle Group which yields four successive, dinosaur-bearing faunas. Otischalkian (early Tuvalian) dinosaur ...
  25. [25]
    BITE MARKS ON AN AETOSAUR (ARCHOSAURIA, SUCHIA ...
    Feb 1, 2021 · Trace fossils such as bite marks provide rare, direct evidence of animal behavior, including predator-prey interactions.
  26. [26]
  27. [27]
    Functional morphology of the Triassic apex predator Saurosuchus ...
    Aug 16, 2023 · Saurosuchus had a strong cranium but a surprisingly weak bite, possibly defleshing carcasses, and was a keystone species.
  28. [28]
    [PDF] Report - Ohio University
    Sep 22, 2016 · ... Postosuchus sp. cf. P. kirkpatricki (UMMP 7473), Desmatosuchus ... Paleontology and paleoecology of the Dockum Group, Upper Triassic,.
  29. [29]
    Osteohistological insight into the growth dynamics of early dinosaurs ...
    Apr 3, 2024 · We studied femoral bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation.