Docodonta

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 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]

Docodont skulls are generally fairly low, and in general form are similar to other early mammliaforms such as morganucodonts. The snout is long and has several plesiomorphic traits: the paired 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 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.[5][8]

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.[2] 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.[9] Vertebrae at the base of the tail often have expanded 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 olecranon process of the ulna is flexed inwards.[10][3][9] 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.[10][9] The ankle is distinctive, with a downturned calcaneum and a stout 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]

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]

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]

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]

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 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]

• Superfamily †Docodontoidea
  • Family †Docodontidae (Marsh 1887) Simpson 1929[22]
    • †Agilodocodon scansorius Meng et al. 2015[4]
    • †Borealestes Waldman & Savage 1972
      • †B. cuillinensis Panciroli et al. 2021[7]
      • †B. serendipitus Waldman & Savage 1972[35]
    • †Castorocauda lutrasimilis Ji et al. 2006[2]
    • †Cyrtlatherium canei Freeman 1979 sensu Sigogneau-Russell 2001 (dubious) [Simpsonodon oxfordensis Kermack et al. 1987]
    • †Dobunnodon mussettae [Borealestes mussetti] Sigogneau-Russell 2003 sensu Panciroli et al. 2021[7]
    • †Docodon Marsh 1881 [Dicrocynodon Marsh in Osborn, 1888; Diplocynodon Marsh 1880 non Pomel 1847; Ennacodon Marsh 1890; Enneodon Marsh 1887 non Prangner 1845]
      • †D. apoxys Rougier et al. 2014
      • †D. victor (Marsh 1880) [Dicrocynodon victor (Marsh 1880); Diplocynodon victor Marsh 1880]
      • †D. striatus Marsh 1881 [disputed]
      • †D. affinis (Marsh 1887) [Enneodon affinis Marsh 1887] [disputed]
      • †D. crassus (Marsh 1887) [Enneodon crassus Marsh 1887; Ennacodon crassus (Marsh 1887)] [disputed]
      • †D. superus Simpson 1929 [disputed]
    • †Docofossor brachydactylus Luo et al. 2015[3]
    • †Dsungarodon zuoi Pfretzschner et al. 2005 [Acuodulodon Hu, Meng & Clark 2007; Acuodulodon sunae Hu, Meng & Clark 2007]
    • †Gondtherium dattai Prasad & Manhas 2007[29] [disputed]
    • †Haldanodon exspectatus Kühne & Krusat 1972 sensu Sigoneau-Russell 2003[16]
    • †Hutegotherium yaomingi Averianov et al. 2010[33]
    • †Itatodon tatarinovi Lopatin & Averianov 2005 [disputed, possibly a shuotheriid][14]
    • †Khorotherium yakutensis Averianov et al. 2018[27]
    • †Krusatodon kirtlingtonensis Sigogneau-Russell 2003
    • †Microdocodon gracilis Zhou et al. 2019[5]
    • †Paritatodon kermacki (Sigogneau-Russell, 1998) [disputed, possibly a shuotheriid][14]
    • †Peraiocynodon Simpson 1928
      • †P. inexpectatus Simpson 1928 [possible synonym of Docodon][36]
      • †P. major Sigogneau-Russell 2003 [disputed]
    • †Sibirotherium rossicus Maschenko, Lopatin & Voronkevich 2002
    • †Simpsonodon Kermack et al. 1987
      • †S. splendens (Kühne 1969)
      • †S. sibiricus Averianov et al. 2010
    • †Tashkumyrodon desideratus Martin & Averianov 2004
    • †Tegotherium gubini Tatarinov 1994