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Cladotheria
Cladotheria
Cladotheria
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Cladotheres
Temporal range: Middle Jurassic–Recent
Fossil of the basal cladotherian Henkelotherium guimarotae
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Trechnotheria
Clade: Cladotheria
McKenna, 1975
Subgroups

Cladotheria is a clade (sometimes ranked as a legion)[2] of mammals. It contains modern therian mammals (marsupials and placental mammals) and several extinct groups, such as the "dryolestoids", amphitheriids and peramurids. The clade was named in 1975 by Malcolm McKenna. In 2002, it was defined as a node-based taxon containing "the common ancestor of dryolestids and living therians, plus all its descendants".[1] A different, stem-based definition was given in 2013, in which Cladotheria contains all taxa that are closer to Mus musculus (the house mouse) than to the "symmetrodont" Spalacotherium tricuspidens.[3]

Cladotheria incorporates a set of nested mammal clades culminating in Tribosphenida (also known as Boreosphenida), mammals with fully tribosphenic teeth such as therians and a few of their closest relatives. The clade Prototribosphenida includes "the common ancestor of Vincelestes and living therians, plus all of its descendants". Apart from tribosphenids, Prototribosphenida also includes amphitheriids and peramurids, as well as a few isolated genera such as Vincelestes. It excludes the various basal cladotherian groups which have been combined under the label "dryolestoids".[1] The clade Zatheria is even more exclusive, restricted to solely peramurids and tribosphenids. Zatheria is defined as "the common ancestor of Peramus and living marsupials and placental mammals, plus all of its descendants".[1]

Description

[edit]
Comparison of the jaw and tooth morphology of cladotherians and other mammals

Early cladotherians can be distinguished from other mammals by a number of derived traits (apomorphies). Their teeth differed from those of the "symmetrodonts" by the evolution of a talonid shelf (hypoflexid) on the lower molars, which occluded with the paracone of the corresponding upper molars. A true talonid basin, allowing for the crushing and grinding of food, was however absent in early-diverging groups like the dryolestoids, amphitheriids and peramurids. Cladotherians are also distinguished by a backwards-pointing angular process at the rear end of the dentary bone, below the jaw joint. The shape of this process indicates that early cladotherians had a more transverse (side-to-side) chewing motion than more basal mammal groups. The connection of the middle ear bones to the dentary through an ossified Meckel's cartilage appears to have been lost in cladotherians, but a cartilaginous connection may have been retained in early-diverging groups.[4]

Phylogeny

[edit]

Cladogram after Panciroli et al. 2018:[5]

Cladotheria

The cladogram after Lasseron et al 2022:[6]

Cladotheria

Cladogram after Magallanes et al, 2024:[7]

Cladotheria

References

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Cladotheria

View on Grokipedia
from Grokipedia
Cladotheria is a of mammals defined as the of dryolestids and living therians (the crown clade comprising marsupials and placentals) and all descendants of that ancestor. This group is characterized by key synapomorphies including the loss of tooth replacement in the postcanine dentition and advanced molar features such as a triangulated upper molar pattern with a functional lingual cusp (protocone precursor) and a multicusped lower talonid enabling precise occlusion between upper and lower molars. Originating in the around 160 million years ago, Cladotheria represents a pivotal lineage in mammalian , bridging early mammaliaforms to the dominant therian mammals of today. Phylogenetically, Cladotheria forms a major subgroup within , positioned as the sister taxon to groups like spalacotheriids in some analyses, and is supported by cladistic studies incorporating dental, cranial, and postcranial characters. Its principal subgroups include , encompassing families such as Dryolestidae (e.g., Dryolestes), Paurodontidae, and the South American endemic (e.g., , Coloniatherium cilinskii), and Zatheria, which includes stem therians like Peramus and Amphitherium leading directly to crown . Fossils of cladotherians are primarily known from Laurasian deposits in the and , but Gondwanan records, particularly from , reveal high diversity and endemic radiations during the , with some dryolestoids like Peligrotherium tropicalis persisting into the early . The evolutionary significance of Cladotheria lies in its role as the stem group for , showcasing innovations that facilitated the radiation of mammals before the dominance of therians in the . Dental advancements, such as the development of a functional talonid basin with three cusps (entoconid, hypoconid, hypoconulid) and dorsomedial jaw movements for efficient mastication, prefigured the tribosphenic molars of modern mammals. Additionally, early cladotherians like Henkelotherium guimarotae from the exhibited advanced structures, including a coiled spanning at least 270° and bony laminae supporting the , indicating the ancient origins of high-frequency hearing capabilities that enhanced sensory adaptations in nocturnal or crevice-dwelling lifestyles. Despite the end-Cretaceous mass extinction decimating many lineages, Cladotheria's legacy endures through the thriving therian mammals, which account for over 99% of extant mammalian species.

Taxonomy

Definition and Diagnosis

Cladotheria is a of mammals defined as the node-based comprising the most recent common ancestor of Dryolestoidea (including ) and , and all of its descendants. This definition encompasses a diverse array of and mammals, including extinct dryolestoids and the crown group (marsupials and placentals), positioning Cladotheria as a key intermediate in therian evolution. The clade was originally established by McKenna in 1975 as part of a broader phylogenetic framework for mammals. The name Cladotheria derives from the Greek words klados (branch) and therion (beast), coined by McKenna in 1975 to highlight its role as a branching lineage within the therian radiation. Key synapomorphies diagnosing Cladotheria include the reduction and medial inflection of the angular process on the dentary bone, the loss of postdentary elements (such as the articular, prearticular, and surangular) in adults, and the emergence of the tribosphenic molar pattern characterized by a well-developed protocone on upper molars that occludes with an expanded talonid basin on lower molars. These features reflect adaptations for enhanced mastication, including transverse jaw movements and grinding capabilities, distinguishing cladotherians from earlier mammaliamorphs. Formal diagnoses of Cladotheria in modern cladistics draw from comprehensive phylogenetic analyses, such as those in McKenna and Bell (1997), which integrate morphological data from cranial, dental, and postcranial elements to support the clade's monophyly. This framework was refined by Kielan-Jaworowska et al. (2004), who emphasized additional diagnostic traits like the posterior positioning of the angular process relative to the condyle and elevated lower molar talonids, based on extensive reviews of Mesozoic fossil material. These studies underscore Cladotheria's position as a pivotal group in mammalian diversification, bridging symmetrodontan-grade forms and advanced therians.

Classification History

In the early , diverse and mammals exhibiting generalized tribosphenic-like dentition were grouped under the informal category Pantotheria by in his systematic review of mammals. This classification lumped together forms now assigned to Cladotheria with other non-therian groups, reflecting limited evidence and a focus on morphological similarities rather than phylogenetic relationships. Mid-century refinements emerged with Malcolm C. McKenna's 1975 proposal of Cladotheria as a monophyletic comprising therians and their immediate relatives, explicitly excluding multituberculates due to differences in jaw articulation and mechanics, such as the transverse jaw movement enabling tribosphenic occlusion. McKenna's framework emphasized cladistic principles, positioning Cladotheria as a key branch within based on shared apomorphies in the dentary-squamosal joint. From the late 20th to early 21st century, phylogenetic analyses incorporating cladistic and molecular data further refined Cladotheria's boundaries, consistently including dryolestoids as basal members while excluding eutriconodonts as more primitive non-therians. For instance, Wible et al.'s 2007 broad-scale analysis of Cretaceous mammals supported dryolestoids within Cladotheria through shared petrosal and dental traits, reinforcing its position as the stem group to crown Theria. Key debates in the 1990s and 2000s centered on the inclusion of certain "symmetrodonts," particularly following discoveries like the Late Jurassic Fruitafossor windscheffeli, which exhibited ambiguous dental and postcranial features suggesting either symmetrodont affinities within Cladotheria or exclusion as a basal theriimorph. Initial descriptions placed Fruitafossor near symmetrodonts based on obtuse-angled molars, but subsequent analyses debated its cladotherian status due to primitive jugal and limb traits, highlighting ongoing uncertainties in Mesozoic mammal interrelationships.

Anatomy

Dental Features

Cladotheria is distinguished by advanced dental occlusion precursor to tribospheny, featuring upper molars with a lingual cusp (stylocone in basal forms) that shears against the multicusped talonid of the opposing lower molars, facilitating transverse grinding and diagonal shearing for efficient mastication. This precise occlusal mechanism contrasts with the simpler shearing in non-cladotherian mammals, allowing for more versatile processing of and matter. The functional integration of these structures, evidenced by facets on cusps, optimizes biomechanical efficiency during jaw movements. The evolutionary progression of cladotherian dentition traces from primitive pseudotribosphenic conditions in basal forms, such as dryolestoids, to the fully realized tribospheny in therians. In early cladotherians like those in Dryolestidae, upper molars exhibit a prominent stylocone—a lingual projection—lacking a true protocone, which shears against a reduced talonid basin on lower molars, providing initial enhancements in occlusion but limited grinding capacity. This progressed in therians through the expansion of the talonid into a broader basin and the addition of a hypocone on upper molars, enabling advanced of tougher foods. Variations in reflect ecological adaptations within Cladotheria, with dryolestids often displaying sectorial premolars specialized for slicing flesh, complemented by triangular molars for basic grinding. In contrast, therians evolved broader, bunodont molars with well-developed hypocones for pulverizing . counts vary, but adults typically retain 3–4 premolars and 3 molars, a reduction from the higher postcanine counts (e.g., 5 premolars and 6-7 molars) in primitive taxa like Amphitherium. The stylocone in basal forms and hypocone in derived therians play critical roles in , as biomechanical studies reveal through analysis of occlusal wear and cusp shear paths, underscoring adaptations for diverse diets.

Cranial and Skeletal Traits

Cladotherian mammals exhibit several derived cranial features that distinguish them from more basal mammaliaforms, including an enlarged braincase. Braincase volume shows a notable increase across the , from approximately 0.3 cm³ in Jurassic mammaliaforms like to around 2.11 cm³ in cladotherians such as Vincelestes neuquenianus, reflecting adaptations for improved sensory processing and neural complexity. This enlargement is linked to expanded telencephalic counts, estimated at up to 27.95 million in Vincelestes, which likely facilitated better integration of auditory and olfactory inputs. A key cranial innovation in Cladotheria is the reduction and detachment of reptilian jaw elements, with the articular bone evolving into the of the ossicle chain, freeing the dentary for primary function. This transition, evident in forms like Dryolestes leiriensis, involves the petrosal bone incorporating neural and vascular features ancestral to therians, including a coiled cochlear canal (approximately 270°) for enhanced hearing acuity. The inner ear structures, such as the primary bony lamina supporting the basilar membrane, represent the groundplan for modern therian audition, enabling resolution of higher sound frequencies. Jaw mechanics in Cladotheria are characterized by the dentary-squamosal articulation, which replaces the ancestral quadrate-articular joint and permits greater mobility, including increased yaw rotation during mastication. The angular process of the dentary is prominent and posteriorly oriented, providing enhanced mechanical advantage for jaw adduction via the masseter and pterygoid muscles, with yaw moment arms approximately 50% longer than in eutriconodontans. The coronoid process, serving as the attachment site for the temporalis muscle, maintains a consistent elevation (21.5–25.5% of jaw length) across early cladotherians, supporting efficient transverse and power strokes in feeding. In some dryolestoid cladotherians, Meckel's groove is reduced or absent, indicating a cartilaginous rather than ossified connection between the jaw and middle ear elements. Postcranial skeletal traits in Cladotheria reflect adaptations for diverse terrestrial lifestyles, with the pelvis featuring an epipubis (or epipubic bone) in many forms, homologous to that in marsupials and aiding in abdominal support during locomotion. In the Jurassic cladotherian Henkelotherium guimarotae, the epipubis is elongate and strap-like, articulating with the pubis and contributing to a lightweight pelvic girdle where the ilia form about 60% of its length. Limb proportions indicate an upright posture, with slender long bones; for example, in Henkelotherium, the femur measures 15.8 mm and exceeds the tibia (13.8 mm), while elongated phalanges (pedal phalangeal index of 153%) suggest agility in arboreal or scansorial habits. Early cladotherian forms generally display terrestrial adaptations, though some exhibit limb ratios compatible with semi-aquatic transitions in related lineages.

Evolution

Origins and Fossil Record

The origins of Cladotheria trace back to the , with the earliest definitive records dating to the stage around 167 million years ago from sites in , such as Guelb el Ahmar, where stem cladotherians including amphitheriids and cf. dryolestids have been found. These initial fossils, primarily isolated teeth and jaw fragments, were recovered from the in the , where genera such as Tinodon and Priacodon represent some of the most primitive known members of the , exhibiting early dental specializations indicative of cladotherian affinity. Subsequent discoveries have expanded the known stratigraphic distribution of Cladotheria into the , highlighting key fossil sites across . The Purbeck Group in has produced a diverse assemblage of (Berriasian) cladotherians, including symmetrodonts and early dryolestoids preserved in lagoonal deposits. In , the Guimarota mine within the Alcobaça Formation has yielded well-preserved specimens like Henkelotherium guimarotae, offering detailed postcranial evidence of locomotor adaptations in basal dryolestoids. Further east, the in northeastern documents (Aptian-Barremian) expansions, with taxa such as Zhangheotherium quinquecuspedens showcasing advanced symmetrodont features in a richly fossiliferous lacustrine environment. Transitional forms play a crucial role in understanding cladotherian emergence, with genera like Amphilestes from the Purbeck Group and various symmetrodonts (e.g., Gobiotheriodon) exhibiting primitive molar patterns that bridge earlier mammaliaforms and more derived cladotherians. These forms highlight potential basal positions within the , though their exact affinities remain debated based on dental morphology. Regarding extinction patterns, Cladotheria as a whole persisted through the Cretaceous-Paleogene (K-Pg) boundary approximately 66 million years ago, with therian lineages (including eutherians and metatherians) forming the primary survivors and undergoing post-extinction radiations in the . In contrast, non-therian branches like dryolestoids underwent a marked decline, with their diversity peaking in the before dwindling; the latest records, such as fragmentary remains from localities, date to the Eocene around 50 million years ago.

Major Diversifications

The major diversifications of Cladotheria occurred primarily during the and periods, marked by adaptive radiations among stem groups like dryolestoids and the emergence of crown therians. Dryolestoids, as pre-tribosphenic stem cladotherians, underwent a significant radiation from the onward, expanding ecologically across and with increasing dietary variety. Initially dominated by insectivory or insect-based omnivory in early forms, later dryolestoids shifted toward more specialized omnivory and carnivory, as evidenced by variations in molar morphology and jaw mechanics that supported broader prey capture. This diversification paralleled the early evolution of boreosphenidans (tribosphenic therians), which appeared by the and rapidly diversified in the , incorporating metatherians and eutherians with enhanced tribosphenic dentition for improved mastication of diverse foods. Boreosphenidan fossils from units like the Trinity Group indicate this phase involved stem therians adapting to varied insectivorous niches, setting the stage for further therian expansion. The Cretaceous-Paleogene (K-Pg) boundary represented a pivotal transition, with therians experiencing an explosive post-extinction radiation as they colonized niches vacated by non-avian dinosaurs and other vertebrates. Placentals, in particular, underwent rapid morphological evolution in the early , emphasizing increases in body size and diversification into herbivorous and carnivorous forms to exploit these opportunities. Meanwhile, marsupials dispersed southward into Gondwanan landmasses during the , achieving greater representation in and eventually by the Eocene through vicariance and limited overwater crossings via . This migration contributed to metatherian persistence in southern hemispheres, where they filled generalist roles amid placental dominance in the north. Adaptive peaks within Cladotheria highlighted specialized locomotor and dietary strategies, such as the adaptations in eutherians like Zalambdalestes, which featured elongated limbs and robust hindlegs suited for hopping and evasion in open terrains. Ecologically, cladotherians began as small-bodied generalists, with most forms estimated at 10–500 g and diets inferred from dental microwear to include , soft , and occasional vertebrates. By the , therian lineages expanded to larger sizes (up to several kilograms in early herbivores), reflecting broader ecological roles in recovering post-K-Pg ecosystems, though quantitative microwear analyses confirm persistent insectivory alongside emerging omnivory.

Phylogeny

Position Within Mammals

Cladotheria occupies a pivotal position within as the to , which encompasses multituberculates and related extinct lineages, together forming the while excluding eutriconodonts. This placement reflects shared derived features such as advanced dental occlusion and mandibular adaptations that distinguish from more basal mammaliforms. Within the broader mammalian tree, Cladotheria represents a major lineage leading toward crown-group mammals, with its supported by phylogenetic analyses incorporating osteological and dental characters across taxa. Key outgroups to Cladotheria include Morganucodontidae and , which exhibit more primitive traits and are positioned as pre-cladotherian stem groups within . Morganucodontids, such as , retain ancestral features like a postdentary trough and limited cochlear coiling, while docodonts like Haldanodon show intermediate advancements but lack the full suite of cladotherian specializations. Cladotheria itself is characterized by transformative modifications, including the detachment of postdentary elements to form the mammalian ossicles ( and ), marking a critical evolutionary shift toward the auditory configuration seen in crown mammals. In broader context, crown-group Mammalia, specifically (encompassing and ), is nested deeply within Cladotheria as a derived , with numerous stem cladotherians such as dryolestids bridging the gap to modern therians. The clade spans from the , with early records like amphitheriids from dated to approximately 167–163 million years ago, through the diversification of its subgroups, to the present day via surviving therians. Molecular phylogenetic studies corroborate this fossil-based positioning, aligning genomic data with paleontological evidence to estimate the divergence of cladotherian lineages from other early mammaliforms around 160 million years ago. For instance, analyses incorporating relaxed molecular clocks confirm the origin of therians within Cladotheria, reconciling molecular divergence times with key fossils like .

Internal Relationships

Cladotheria is characterized by a basal position for , which includes Dryolestidae and a paraphyletic assemblage of basal forms formerly classified as Paurodontidae, with nested within Dryolestoidea as a derived subgroup. These dryolestoids represent successive outgroups to more crownward therians, exhibiting primitive dental and cranial features that bridge earlier mammaliaforms and advanced tribosphenic mammals. More derived cladotherians fall within Zatheria, which encompasses stem therians leading to (a broader clade defined by the tribosphenic molar pattern). Boreosphenida, the northern lineage within Zatheria, leads to , the crown clade comprising (marsupials and their stem relatives) and (placentals); (including monotremes and southern hemisphere forms like Teinolophos) represents a separate Gondwanan clade with convergent tribosphenic , often resolved as sister to Boreosphenida outside Cladotheria. Stem therians, such as members of , occupy positions basal to the metatherian-eutherian divergence, often resolved as sister to Marsupialiformes within , highlighting a radiation of proto-marsupial lineages in . Phylogenetic consensus for these relationships derives from parsimony-based analyses using extensive morphological datasets, including 137 characters across 50 taxa, which yield strict consensus trees supporting 's basal placement and Zatheria's derivation toward . Bayesian approaches corroborate this topology, with dryolestoids consistently emerging as stepwise outgroups to , bolstered by bootstrap values above 70% at key nodes. Recent updates incorporating new South American fossils, such as Orretherium, reinforce Meridiolestida's inclusion within . Recent 2024 studies on therians from further highlight cladotherian diversification in , refining estimates of therian origins. Several nodes remain unresolved, particularly the exact placement of paurodontids, which appear paraphyletic as basal dryolestidans without clear synapomorphies uniting them. Similarly, "symmetrodonts" form a potentially paraphyletic grade of basal cladotherians, with groups like Zhangheotheriidae occupying ambiguous positions near the Dryolestida-Zatheria split due to conflicting dental homoplasies in parsimony trees. These uncertainties underscore the need for integrated molecular and additional fossil data to refine early cladotherian branching.

References

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