Docodonta

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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.

Docodonta is an

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, Delsatia, and Tikitherium.[1]

For much of their history of study, docodont fossils were represented by isolated teeth and jaws. The first docodont known from decent remains was

middle ear ossicles. On the other hand, the postdentary bones are still attached to the jaw and skull, the nostrils have yet to fuse, and in most species the spine's thoracic-lumbar transition is rather subdued.[3][4]

Description

Skeletal traits

Jaw and ear

Docodonts have a long and low

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 ecotympanic 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.[6][2][4][5]

Cranium and throat

Docodont skulls are generally fairly low, and in general form are similar to other early mammliaforms such as

nares (bony nostril holes) are small and separate, and their rear edge 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).[6][3][7]

Docodonts also see the first occurrence of a mammalian-style saddle-shaped complex of

suckling.[5][8]

Postcranial skeleton

The oldest unambiguous fossil evidence of

transverse processes (rib pedestals), supporting powerful tail musculature.[2][4][9] 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.[3][4]

The forelimbs and hindlimbs generally have strong muscle attachments, and the

astragalus which connects to the tibia via a trochlea (pulley-like joint).[3][4][5][9] 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.[2][11]

Teeth

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

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

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

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

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 another case of convergent evolution, as shuotheriid are true mammals related to modern monotremes.[12] Docodont and shuotheriid teeth are so similar that some genera, namely Itatodon and Paritatodon, have been considered members of either group.[13][14]

Paleoecology

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.[15] Castorocauda[2] from the Middle Jurassic of China, and possibly Haldanodon[16][17] from the Upper Jurassic of Portugal, were specialised for a semi-aquatic lifestyle. Castorocauda had a flattened tail and recurved molars, which suggests possible fish or aquatic invertebrate diet.[2] It was thought possible that docodonts had tendencies towards semi-aquatic habits, given their presence in wetland environments,[18] 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,[3] and specialised tree-dweller Agilodocodon[4] 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.[3]

Classification

The lineage of Docodonta evolved prior to the origin of living mammals:

phylogenetic analyses based on maximum parsimony, but shift stemward relative to haramiyidans when the same data is put through a Bayesian analysis.[20]

Cladogram based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs:[5]

Mammaliaformes

Docodont fossils have been recognized since the 1880s, but their relationships and diversity have only recently been well-established.

triconodont" mammaliaforms, which had fairly simple lower molars with a straight row of large cusps.[23] However, re-evaluations of mammaliaform tooth homology in the late 1990s established that docodonts were not closely related to either morganucodonts or therians.[24][25] Instead, they were found to be similar to certain early "symmetrodonts", a broad and polyphyletic grouping of mammaliaforms with triangular upper molars.[25] 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) and Tikitherium (from the Carnian of India).[1][26] 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. Tikitherium in particular is very similar to docodonts, as its wide upper molars have an apparent lingual cusp (cusp X) with a labial wear facet, though its cusp Y is comparatively underdeveloped. Cusp X is even more prominent in Woutersia, though it lacks a wear facet in that genus.[1]

Unambiguous docodonts are restricted to the

Cretaceous Period; the youngest known members of the group are Sibirotherium and Khorotherium, from the Early Cretaceous of Siberia.[27][28] One disputed docodont, Gondtherium, has been described from India, which was previously part of the Southern Hemisphere continent of Gondwana.[29][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.[4][5] Reigitherium, from the Late Cretaceous of Argentina, has previously been described as a docodont,[30] though it is now considered a meridiolestidan mammal.[31] Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae),[32][13][33] 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.[34]

Cladograms based on phylogenetic analyses focusing on docodont relationships:

Species

See also

References

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External links

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