Loricata
Loricata is a clade of pseudosuchian archosaurs defined as all taxa more closely related to Crocodylus niloticus than to phytosaurs or aetosaurs.[1] It includes the only surviving pseudosuchians, the crocodylians (crocodiles, alligators, caimans, gharials, and their extinct relatives), as well as several extinct lineages of large, hypercarnivorous Triassic predators such as prestosuchids (e.g., Prestosuchus), rauisuchids (e.g., Postosuchus, Saurosuchus), and parapredosaurs.[2] The name Loricata, derived from the Latin lorica meaning "armor" or "corselet," refers to the characteristic dorsal armor of overlapping osteoderms present in most members.[3] Originally used in the 19th century for crocodilians and related reptiles, the clade was formally defined in modern cladistics by Sterling J. Nesbitt in 2011 as part of a comprehensive revision of archosaur phylogeny.[1] Loricatans originated in the Early Triassic following the Permian-Triassic extinction, diversified into apex predators during the Middle and Late Triassic, and dominated terrestrial ecosystems until the end-Triassic extinction, after which only the crocodylomorph lineage survived and persisted to the present.[4]Taxonomy
History of classification
The name Loricata was proposed in 1817 by Christian Friedrich Schumacher as a class within Mollusca, referring to the armored shell structure of chitons resembling a lorica (Latin for "armor" or "coat of mail").[5] This designation emphasized the distinctive eight-plated dorsal shell and was part of early 19th-century efforts to classify marine invertebrates based on shell morphology. In 1821, George Robert Gray established the class Polyplacophora, which quickly became the preferred name, rendering Loricata a synonym.[6] Early classifications grouped chitons with other mollusks like aplacophorans under Amphineura, but by the late 19th century, Polyplacophora was recognized as a distinct class due to unique features such as the serial gills and radula structure.[7] Fossil discoveries from the Late Cambrian onward confirmed their ancient origins, influencing revisions that separated extinct forms into Palaeoloricata. Modern taxonomy, informed by molecular phylogenetics, places Polyplacophora as a basal aculiferan clade sister to other mollusks.[7]Definition and diagnosis
Loricata, synonymous with Polyplacophora, is defined as the class of mollusks characterized by a dorsal shell composed of eight transverse, overlapping calcareous plates (valves) articulated by a girdle of muscular and calcareous tissue.[6] This class encompasses approximately 1,000 extant species and over 430 fossil species, primarily marine but with rare freshwater records.[7] Diagnostic features include a dorsoventrally flattened body with a broad ventral foot for adhesion and locomotion; a chitinous radula for grazing; and gills arranged in series along the mantle cavity for respiration.[7] The shell plates bear sensory aesthetes for light and chemical detection, and the girdle often features spines or scales for protection. These traits distinguish Polyplacophora from other mollusks, such as gastropods (which have a single coiled shell) or bivalves (with two valves). Early fossils like those from the Late Cambrian exhibit these features, confirming the clade's antiquity.[6]Included groups
Polyplacophora is divided into two main subclasses: the extant Neoloricata, which includes all living species (~940 as of 2023), and the extinct Palaeoloricata, known only from fossils dating from the Late Cambrian to the Late Cretaceous.[6] Neoloricata further comprises three orders: Lepidopleurida (deep-sea forms with smooth valves), Chitonida (shallow-water species with diverse ornamentation), and Callochitonida (intermediate forms).[8] Palaeoloricata lacks sutural laminae on the valves and represents primitive chitons, with genera like Matthevia from the Cambrian illustrating early diversification. The total known diversity includes about 20 families, predominantly in Neoloricata, reflecting adaptation to rocky substrates across global marine environments.[7]Description
Cranial features
The skulls of non-crocodylomorph loricatans exhibit an elongate rostrum adapted for a predatory lifestyle, featuring ziphodont teeth that are labiolingually compressed, recurved, and finely serrated along their mesial and distal carinae. In Postosuchus kirkpatricki, the premaxilla is subrectangular and slightly longer than deep, housing four such teeth, while the maxilla bears 13 alveoli with similar dentition, including approximately three serrations per millimeter on the posterior teeth, facilitating slashing and tearing of prey.[9] Prestosuchus chiniquensis displays comparable morphology, with four premaxillary teeth serrated at four denticles per millimeter and 13 maxillary teeth at three to four denticles per millimeter, emphasizing the hypercarnivorous adaptations shared across the group.[10] Cranial fenestration in loricatans supports robust jaw mechanics, with the antorbital fenestra typically wedge-shaped and present but variably proportioned relative to skull length, bordered by the maxilla, nasal, and lacrimal. The supratemporal fenestra is notably large and ovate, bordered by the parietal, postorbital, and squamosal, providing expansive attachment areas for the jaw adductor muscles such as the m. adductor mandibulae externus.[9][10] The quadrate bone is oriented anterodorsally to posteroventrally, with distinct medial and lateral condyles and a quadrate foramen, enabling a wide gape essential for ambushing large prey; palatal teeth on the pterygoids, when present, further aid in grasping by forming a secondary tooth row along the palate.[10][11] Variations in cranial morphology reflect evolutionary progression within Loricata. Basal forms like Decuriasuchus quartacolonia retain a more primitive, shorter rostrum with a subtriangular antorbital fenestra and 17 maxillary teeth, indicating less specialized elongation compared to derived taxa.[12] Advanced rauisuchids, such as Saurosuchus galilei, feature deeper skulls with rectangular lateral profiles and convex ventral margins, enhancing structural support despite relatively modest bite forces estimated at 1015–1885 N, suited for defleshing rather than bone-crushing.[13] Sensory adaptations include large, keyhole-shaped orbits in Postosuchus, bordered by the prefrontal, postfrontal, postorbital, and jugal, suggesting enhanced visual acuity; the forward-facing orientation of these orbits, combined with the infratemporal fenestra, likely permitted some degree of binocular vision for prey localization.[9]Postcranial skeleton
The postcranial skeleton of Loricata exhibits a robust axial column adapted for supporting large body sizes and facilitating movement. The vertebral column typically includes 7–9 cervical vertebrae in basal forms like Decuriasuchus and Prestosuchus, though some rauisuchids such as Postosuchus possess up to 10–12 elongated cervicals that enhance neck flexibility for prey manipulation.[14] Dorsal vertebrae number around 13–15, with pronounced ventral keels on cervicals and amphicoelous centra; presacral counts reach 25 in Prestosuchus.[15] Gastralia form a series of overlapping rods providing abdominal support and rigidity, as seen in Postosuchus specimens.[16] Limb morphology in Loricata varies but emphasizes powerful hindlimbs for propulsion. In large rauisuchids like Postosuchus, hindlimbs are pillar-like with straight femora and robust tibiae, supporting body weights up to several tons and enabling rapid terrestrial locomotion; forelimbs are notably reduced, with humeri shorter than femora, indicating facultative bipedality in some taxa.[17] Basal loricatans such as Decuriasuchus retain more equal fore- and hindlimb proportions, suggesting quadrupedality.[18] The pelvic girdle in basal Loricata features a crocodile-like, imperforate acetabulum formed by the ilium, ischium, and pubis, providing a stable articulation for the femur.[19] The ilium bears a prominent supra-acetabular crest for muscle attachment, while the pubis and ischium often show slight distal expansions; sacral vertebrae incorporate 2–3 elements, varying across the clade, to bolster pelvic stability.[14] In crocodylomorphs, the acetabulum evolves toward a more open configuration, enhancing hip mobility.[20] The tail in Loricata is long and muscular, comprising numerous caudals that aid in balance and propulsion. Caudal vertebrae elongate distally, with haemal arches (chevrons) beginning from the third vertebra; these Y-shaped chevrons, as in Prestosuchus, feature separated proximal facets and enhance tail flexibility while supporting lateral undulation.[14][16] Body sizes in Loricata span a wide range, from small basal taxa around 1–2 meters in length, such as certain Middle Triassic forms, to giants exceeding 7 meters like Fasolasuchus, reflecting diverse ecological roles within Pseudosuchia.[21][22]Phylogeny
Position within Mollusca
Polyplacophora, synonymous with Loricata, is a class of mollusks positioned within the phylum Mollusca as part of the Aculifera clade, which also includes the aplacophoran classes Solenogastres and Caudofoveata.[23] Aculifera forms one of the two major lineages of Mollusca, sister to the Conchifera (which encompasses gastropods, bivalves, cephalopods, and others), a topology supported by genomic, mitogenomic, and morphological data.[23][7] This placement highlights Polyplacophora's basal position among mollusks, with fossil evidence indicating an origin in the Late Cambrian, around 500 million years ago, predating the diversification of Conchifera.[23] The monophyly of Polyplacophora is well-established, characterized by synapomorphies such as the eight-plated dorsal shell (valves) and a creeping foot with a girdle.[24] Within Aculifera, Polyplacophora diverged early, with molecular clock estimates suggesting a split from other aculiferans around 440 million years ago during the Silurian.[23] Recent phylogenomic analyses, incorporating whole-genome data from species like Acanthochitona discrepans and Boreochiton ruber, confirm this deep divergence and reveal high rates of chromosomal rearrangements despite conserved morphology.[23] Polyplacophora's ancient lineage underscores its role as a key group for understanding early molluscan evolution, bridging fossil records with modern biodiversity of approximately 1,000 extant species.[24]Interrelationships
Internally, Polyplacophora is divided into two subclasses: the extinct Palaeoloricata, known from Cambrian to Jurassic fossils with more primitive shell structures, and the Neoloricata, which includes all extant species and features more derived valve articulation.[6] Within Neoloricata, phylogenomic and mitogenomic studies resolve three main orders: Lepidopleurida (basal, including families like Leptochitonidae), Callochitonida, and Chitonida.[24][7] Chitonida, the most diverse order, further splits into suborders Chitonina (encompassing Chitonoidea and Schizochitonoidea) and Acanthochitonina (including Mopalioidea and Cryptoplacoidea).[24] Callochitonidae is positioned as sister to the rest of Chitonida, while Lepidopleurida forms the basal grade.[7] These relationships are corroborated by analyses of 13 mitochondrial genomes and transcriptomic data, identifying gene rearrangements like tRNA inversions as synapomorphies for certain clades.[7] A simplified cladogram of Polyplacophora, based on recent phylogenomic datasets, illustrates these relationships as follows:This topology reflects the monophyletic nature of major chiton lineages and their evolutionary diversification from basal to more specialized forms.[23][24]Polyplacophora ├── Palaeoloricata (extinct) └── Neoloricata ├── Lepidopleurida ├── Callochitonida └── Chitonida ├── Chitonina │ ├── Chitonoidea │ └── Schizochitonoidea └── Acanthochitonina ├── Mopalioidea └── CryptoplacoideaPolyplacophora ├── Palaeoloricata (extinct) └── Neoloricata ├── Lepidopleurida ├── Callochitonida └── Chitonida ├── Chitonina │ ├── Chitonoidea │ └── Schizochitonoidea └── Acanthochitonina ├── Mopalioidea └── Cryptoplacoidea