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Docodonta

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Docodonta is an order of extinct Mesozoic mammaliaforms (advanced cynodonts closely related to true crown-group mammals). They were among the most common mammaliaforms of their time, persisting from the Middle Jurassic to the Early Cretaceous across the continent of Laurasia (modern-day North America, Europe, and Asia). They are distinguished from other early mammaliaforms by their relatively complex molar teeth. Docodont teeth have been described as "pseudotribosphenic": a cusp on the inner half of the upper molar grinds into a basin on the front half of the lower molar, like a mortar-and-pestle. This is a case of convergent evolution with the tribosphenic teeth of therian mammals. There is much uncertainty for how docodont teeth developed from their simpler ancestors. Their closest relatives may have been certain Triassic "symmetrodonts", namely Woutersia, and Delsatia.[1] The shuotheriids, another group of Jurassic mammaliaforms, also shared some dental characteristics with docodonts. One study has suggested that shuotheriids are closely related to docodonts,[2] though others consider shuotheriids to be true mammals, perhaps related to monotremes.[3]

Docodonts
Temporal range: Middle Jurassic-Early Cretaceous Bathonian–Aptian
Skeletal diagrams of Borealestes serendipitus (green) and B. cuillinensis (blue) Scale bars = 10 mm
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Clade: Mammaliaformes
Order: †Docodonta
Kretzoi, 1946
Genera

See text.

For much of their history of study, docodont fossils were represented by isolated teeth and jaws. The first docodont known from decent remains was Haldanodon, from the Late Jurassic Guimarota site of Portugal. Recently, exceptionally preserved skeletons have been discovered in the Jurassic Tiaojishan Formation of China. Chinese docodonts include otter-like,[4] mole-like,[5] and squirrel-like species,[6][7] hinting at impressive ecological diversity within the group. Many docodonts have muscular limbs and broad tail vertebrae, adaptations for burrowing or swimming. Like true mammals, docodonts have hair,[4] a saddle-shaped hyoid apparatus,[7] and reduced postdentary jaw bones which are beginning to develop into middle ear ossicles. On the other hand, the postdentary bones are still attached to the jaw and skull, the nares (bony nostril rims) have yet to fuse, and in most species the spine's thoracic-lumbar transition is rather subdued.[5][6]

Description

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Skeletal traits

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Jaw and ear

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Docodonts have a long and low mandible (lower jaw), formed primarily by the tooth-bearing dentary bone. The dentary connects to the cranium via a joint with the squamosal, a connection which is strengthened relative to earlier mammaliaforms. The other bones in the jaw, known as postdentary elements, are still connected to the dentary and lie within a groove (the postdentary trough) in the rear part of the dentary's inner edge. Nevertheless, they are very slender, hosting hooked prongs which start to converge towards an oval-shaped area immediately behind the dentary. The ectotympanic bone, also known as the angular, fits into a deep slot on the dentary which opens backwards, a characteristic unique to docodonts. The malleus (also known as the articular) sends down a particularly well-developed prong known as the manubrium, which is sensitive to vibrations. The incus (also known as the quadrate) is still relatively large and rests against the petrosal bone of the braincase, a remnant of a pre-mammalian style jaw joint. In true mammals, the postdentary elements detach fully and shrink further, becoming the ossicles of the middle ear and embracing a circular eardrum.[8][4][6][7]

Cranium and throat

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Docodont skulls are generally fairly low, and in general form are similar to other early mammaliforms such as morganucodonts. The snout is long and has several plesiomorphic traits: the nares (bony nostril holes) are small and paired, rather than fused into a single opening, and the rear edge of each naris is formed by a large septomaxilla, a bone which is no longer present in mammals. The nasal bones expand at the back and overlook thick lacrimals. The frontal and parietal bones of the skull roof are flat and broad, and there is no postorbital process forming the rear rim of the orbit (eye socket).[8][5][9]

Docodonts also see the first occurrence of a mammalian-style saddle-shaped complex of hyoids (throat bones). Microdocodon has a straight, sideways-oriented basihyal which connects to two pairs of bony structures: the anterior hyoid cornu (a jointed series of rods which snake up to the braincase), and the posterior thyrohyals (which link to the thyroid cartilage). This hyoid system affords greater strength and flexibility than the simple, U-shaped hyoids of earlier cynodonts. It allows for a narrower and more muscular throat and tongue, which are correlated with uniquely mammalian behaviors such as suckling.[7][10]

Postcranial skeleton

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The oldest unambiguous fossil evidence of hair is found in a well-preserved specimen of the docodont Castorocauda, though hair likely evolved much earlier in synapsids.[4] The structure of the vertebral column is variable between docodonts, as with many other mammaliaforms. The components of the atlas are unfused, attaching to the large and porous occipital condyles of the braincase.[11] Vertebrae at the base of the tail often have expanded transverse processes (rib pedestals), supporting powerful tail musculature.[4][6][11] Most docodonts have gradually shrinking ribs, forming a subdued transition between the thoracic and lumbar regions of the spine. However, this developmental trait is not universal. For example, Agilodocodon lacks lumbar ribs, so it has an abrupt transition from the thoracic to lumbar vertebrae like many modern mammals.[5][6]

The forelimbs and hindlimbs generally have strong muscle attachments, and the olecranon process of the ulna is flexed inwards.[12][5][11] All limb bones except the tibia lack epiphyses, plate-like ossified cartilage caps which terminate bone growth in adulthood. This suggests that docodont bones continued growing throughout their lifetime, like some other mammaliaforms and early mammals.[12][11] The ankle is distinctive, with a downturned calcaneum and a stout astragalus which connects to the tibia via a trochlea (pulley-like joint).[5][6][7][11] The only known specimen of Castorocauda has a pointed spur on its ankle, similar to defensive structures observed in male monotremes and several other early-branching mammals.[4][13]

Teeth

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Toothed jaw fragments and molar teeth of Khorotherium yakutensis, from the Early Cretaceous of Russia. Right maxilla (upper two rows) and left dentary (lower four rows)

Like other mammaliaforms, docodont teeth include peg-like incisors, fang-like canines, and numerous interlocking premolars and molars. Most mammaliaforms have fairly simple molars primarily suited for shearing and slicing food. Docodonts, on the other hand, have developed specialized molars with crushing surfaces. The shape of each molar is defined by a characteristic pattern of conical cusps, with sharp, concave crests connecting the center of each cusp to adjacent cusps.[1]

Upper molars

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Molar teeth of Haldanodon expectatus, from the Late Jurassic of Portugal. Upper (maxilla) molars are pink and lower (dentary) molars are blue. Anterior is to the right.
* Left side: right maxilla molar and left dentary molar in occlusal view (looking onto the teeth). Cusp nomenclature is labelled.
* Right side: left maxilla and dentary molars in lingual view (from the perspective of the tongue, right).

When seen from below, the upper molars have an overall subtriangular or figure-eight shape, wider (from side to side) than they are long (from front to back). The bulk of the tooth makes up four major cusps: cusps A, C, X, and Y. This overall structure is similar to the tribosphenic teeth found in true therian mammals, like modern marsupials and placentals. However, there is little consensus for homologizing docodont cusps with those of modern mammals.[1]

Cusps A and C lie in a row along the labial edge of the tooth (i.e., on the outer side, facing the cheek). Cusp A is located in front of cusp C and is typically the largest cusp in the upper molars. Cusp X lies lingual to cusp A (i.e., positioned inwards, towards the midline of the skull). A distinct wear facet is found on the labial edge of cusp X, extending along the crest leading to cusp A. Cusp Y, a unique feature of docodonts, is positioned directly behind cusp X. Many docodonts have one or two additional cusps (cusps B and E) in front of cusp A. Cusp B is almost always present and is usually shifted slightly labial relative to cusp A. Cusp E, which may be absent in later docodonts, is positioned lingual to cusp B.[1]

Lower molars

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The lower molars are longer than wide. On average, they have seven cusps arranged in two rows. The labial/outer row has the largest cusp, cusp a, which lies between two more cusps. The other major labial cusps are cusp b (a slightly smaller cusp in front of cusp a) and cusp d (a much smaller cusp behind cusp a). The lingual/inner row is shifted backwards (relative to the labial row) and has two large cusps: cusp g (at the front) and cusp c (at the back).[1]

Two additional lingual cusps may be present: cusp e and cusp df. Cusp e lies in front of cusp g and is roughly lingual to cusp b. Cusp df (“docodont cuspule f”) lies behind cusp c and is lingual to cusp d. There is some variation in the relative sizes, position, or even presence of some of these cusps, though docodonts in general have a fairly consistent cusp pattern.[1]

Tooth occlusion

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A distinct concavity or basin is apparent in the front half of each lower molar, between cusps a, g, and b. This basin has been named the pseudotalonid. When the upper and lower teeth occlude (fit together), the pseudotalonid acts as a receptacle for cusp Y of the upper molar. Cusp Y is often termed the "pseudoprotocone" in this relationship. At the same time, cusp b of the lower molar shears into an area labial to cusp Y. Occlusion is completed when the rest of the upper molar slides between adjacent lower molar teeth, letting the rear edge of the preceding lower molar scrape against cusp X. This shearing-and-grinding process is more specialized than in any other early mammaliaform.[1]

"Pseudotalonid" and "pseudoprotocone" are names which reference the talonid-and-protocone crushing complex which characterize tribosphenic teeth. Tribosphenic teeth show up in the oldest fossils of therians, the mammalian subgroup containing marsupials and placentals. This is a case of convergent evolution, as therian talonids lie at the back of the lower molar rather than the front. The opposite is true for docodont teeth, which have been described as "pseudotribosphenic".[1]

Pseudotribosphenic teeth are also found in shuotheriids, an unusual collection of Jurassic mammals with tall pointed cusps. Relative to docodonts, shuotheriids have pseudotalonids which are positioned further forwards in their lower molars. This is potentially another case of convergent evolution, as shuotheriid are often considered true mammals related to modern monotremes.[3] Docodont and shuotheriid teeth are so similar that some genera, namely Itatodon and Paritatodon, have been considered members of either group.[14][15] A 2024 study, describing the new shuotheriid Feredocodon, even proposed that shuotheriids and docodonts were most closely related to each other among mammaliaforms. The study named a new clade, Docodontiformes, to encompass the two groups.[2]

Paleoecology and paleobiology

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Docodont ecological variation

Docodonts and other Mesozoic mammals were traditionally thought to have been primarily ground dwelling and insectivorous, but recent more complete fossils from China have shown this is not the case.[16] Castorocauda[4] from the Middle Jurassic of China, and possibly Haldanodon[17][18] from the Upper Jurassic of Portugal, were specialised for a semi-aquatic lifestyle. Castorocauda had a flattened tail, similar to that of a beaver, and recurved molar cusps, which suggests a possible diet of fish or aquatic invertebrates.[4] It was thought possible that docodonts had tendencies towards semi-aquatic habits, given their presence in wetland environments,[19] although this could also be explained by the ease with which these environments preserve fossils compared with more terrestrial ones. Recent discoveries of other complete docodontans such as the specialised digging species Docofossor,[5] and specialised tree-dweller Agilodocodon[6] suggest Docodonta were more ecologically diverse than previously suspected. Docofossor shows many of the same physical traits as the modern day golden mole, such as wide, shortened digits in the hands for digging.[5]

Metabolism and lifespan

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A 2024 study on adult and juvenile Krusatodon specimens found that docodonts had a slower metabolism and lower growth rates relative to modern mammals of the same size. The juvenile, which was 7 to 24 months old at the time of its death, was only 49% through the process of replacing its deciduous teeth with permanent (adult) teeth. Based on jaw length, the juvenile was 51-59% the weight of the 7-year-old adult. The closest comparisons among modern mammals were monotremes and hyraxes, though Krusatodon was much smaller than either, at fewer than 156 g in adult body mass. The authors propose that the standard condition in modern small mammals (very high metabolism, rapid growth, and short lifespan) would not be adopted until true mammals in the Jurassic mammals. In addition, docodonts contradict earlier assumptions that high metabolism evolved in sync with ecological diversity, since their diversity far outpaces their metabolism.[20]

Classification

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The lineage of Docodonta evolved prior to the origin of living mammals: monotremes, marsupials, and placentals. In other words, docodonts are outside of the mammalian crown group, which only includes animals descended from the last common ancestor of living mammals. Previously, docodonts were sometimes regarded as belonging to Mammalia, owing to the complexity of their molars and the fact that they possess a dentary-squamosal jaw joint. However, modern authors usually limit the term "Mammalia" to the crown group, excluding earlier mammaliaforms like the docodonts. Nevertheless, docodonts are still closely related to crown-Mammalia, to a greater extent than many other early mammaliaform groups such as Morganucodonta and Sinoconodon. Some authors also consider docodonts to lie crownward of the order Haramiyida,[5] though most others consider haramiyidans to be closer to mammals than docodonts are.[4][21][7] Docodonts may lie crownward of haramiyidans in phylogenetic analyses based on maximum parsimony, but shift stemward relative to haramiyidans when the same data is put through a Bayesian analysis.[22]

Cladogram based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs:[7]

Mammaliaformes

Docodont fossils have been recognized since the 1880s, but their relationships and diversity have only recently been well-established. Monographs by George Gaylord Simpson in the 1920s argued that they were specialized "pantotheres", part of a broad group ancestral to true therian mammals according to their complex molars.[23][24] A 1956 paper by Bryan Patterson instead argued that docodont teeth were impossible to homologize with modern mammals. He drew comparisons to the teeth of Morganucodon and other "triconodont" mammaliaforms, which had fairly simple lower molars with a straight row of large cusps.[25] However, re-evaluations of mammaliaform tooth homology in the late 1990s established that docodonts were not closely related to either morganucodonts or therians.[26][27] Instead, they were found to be similar to certain early "symmetrodonts", a broad and polyphyletic grouping of mammaliaforms with triangular upper molars.[27] In particular, the closest relatives of Docodonta have been identified as certain Late Triassic "symmetrodonts", such as Delsatia and Woutersia (from the Norian-Rhaetian of France).[1][28] These "symmetrodonts" have three major cusps (c, a, and b) set in a triangular arrangement on their lower molars. These cusps would be homologous to cusps c, a, and g in docodonts, which have a similar size and position. The lingual cusp (cusp X) is prominent in Woutersia.[1] Another proposed docodont relative, Tikitherium from India, was originally considered to have been a very early mammaliaform which lived during the Carnian stage of the Triassic. Later investigation found that Tikitherium was likely a misidentification of Neogene shrew teeth, completely unrelated to docodonts or any Mesozoic mammaliaforms.[29]

Unambiguous docodonts are restricted to the Northern Hemisphere, abruptly appearing in the fossil record in the Middle Jurassic. Very few docodonts survived into the Cretaceous Period; the youngest known members of the group are Sibirotherium and Khorotherium, from the Early Cretaceous of Siberia.[30][31] One disputed docodont, Gondtherium, has been described from India, which was previously part of the Southern Hemisphere continent of Gondwana.[32][1] However, this identification is not certain, and in recent analyses, Gondtherium falls outside the docodont family tree, albeit as a close relative to the group.[6][7] Reigitherium, from the Late Cretaceous of Argentina, has previously been described as a docodont,[33] though it is now considered a meridiolestidan mammal.[34] Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae),[35][14][36] but the monophyly of these groups (in their widest form) are not found in any other analyses, and therefore not accepted by all mammal palaeontologists.[37]

Cladograms based on phylogenetic analyses focusing on docodont relationships:

Species

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See also

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References

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