| Docodontans | |
|---|---|
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| Skeletal diagrams ofBorealestes serendipitus (green) andB. cuillinensis (blue) Scale bars = 10 mm | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Cynodontia |
| Clade: | Mammaliaformes |
| Order: | †Docodonta Kretzoi, 1946 |
| Genera | |
See text. | |
Docodonta is anorder of extinctMesozoicmammaliaforms (advancedcynodonts closely related to truecrown-groupmammals). They were among the most common mammaliaforms of their time, persisting from theMiddle Jurassic to theEarly Cretaceous across the continent ofLaurasia (modern-day North America, Europe, and Asia). They are distinguished from other early mammaliaforms by their relatively complexmolar teeth. Docodontan 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 amortar-and-pestle. This is a case ofconvergent evolution with thetribosphenic teeth of therian mammals. There is much uncertainty for how docodontan teeth developed from their simpler ancestors. Their closest relatives may have been certain Triassic "symmetrodonts", namelyWoutersia, andDelsatia.[1] Theshuotheriids, another group of Jurassic mammaliaforms, also shared some dental characteristics with docodontans. One study has suggested that shuotheriids are closely related to docodontans,[2] though others consider shuotheriids to be true mammals, perhaps related tomonotremes.[3]
For much of their history of study, docodontan fossils were represented by isolated teeth and jaws. The first docodontan known from decent remains wasHaldanodon, from theLate JurassicGuimarota site ofPortugal. Recently, exceptionally preserved skeletons have been discovered in the JurassicTiaojishan Formation ofChina. Chinese docodontans includeotter-like,[4]mole-like,[5] andsquirrel-like species,[6][7] hinting at impressive ecological diversity within the group. Many docodontans have muscular limbs and broad tail vertebrae, adaptations for burrowing or swimming. Like true mammals, docodontans have hair,[4] a saddle-shapedhyoid apparatus,[7] and reduced postdentary jaw bones which are beginning to develop intomiddle 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 thespine'sthoracic-lumbar transition is rather subdued.[5][6]
Docodontans have a long and lowmandible (lower jaw), formed primarily by the tooth-bearingdentary bone. The dentary connects to the cranium via a joint with thesquamosal, 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 (thepostdentary 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. Theectotympanic bone, also known as theangular, fits into a deep slot on the dentary which opens backwards, a characteristic unique to docodontans. Themalleus (also known as thearticular) sends down a particularly well-developed prong known as the manubrium, which is sensitive to vibrations. Theincus (also known as thequadrate) 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 theossicles of the middle ear and embracing a circulareardrum.[8][4][6][7]
Docodontan skulls are generally fairly low, and in general form are similar to other early mammaliforms such asmorganucodonts. The snout is long and has severalplesiomorphic traits: thenares (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 largeseptomaxilla, a bone which is no longer present in mammals. Thenasal bones expand at the back and overlook thicklacrimals. Thefrontal andparietal bones of the skull roof are flat and broad, and there is nopostorbital process forming the rear rim of theorbit (eye socket).[8][5][9]
Docodontans also see the first occurrence of a mammalian-style saddle-shaped complex ofhyoids (throat bones).Microdocodon has a straight, sideways-orientedbasihyal 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 posteriorthyrohyals (which link to thethyroid cartilage). This hyoid system affords greater strength and flexibility than the simple, U-shaped hyoids of earliercynodonts. It allows for a narrower and more muscular throat and tongue, which are correlated with uniquely mammalian behaviors such assuckling.[7][10]
The oldest unambiguous fossil evidence ofhair is found in a well-preserved specimen of the docodontanCastorocauda, though hair likely evolved much earlier insynapsids.[4] The structure of thevertebral column is variable between docodontans, as with many other mammaliaforms. The components of theatlas are unfused, attaching to the large and porousoccipital condyles of the braincase.[11] Vertebrae at the base of the tail often have expandedtransverse processes (rib pedestals), supporting powerful tail musculature.[4][6][11] Most docodontans have gradually shrinking ribs, forming a subdued transition between thethoracic andlumbar 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 theolecranon process of theulna is flexed inwards.[12][5][11] All limb bones except thetibia lackepiphyses, plate-like ossified cartilage caps which terminate bone growth in adulthood. This suggests that docodontan bones continued growing throughout their lifetime, like some other mammaliaforms and early mammals.[12][11] The ankle is distinctive, with a downturnedcalcaneum and a stoutastragalus which connects to the tibia via a trochlea (pulley-like joint).[5][6][7][11] The only known specimen ofCastorocauda has a pointedspur on its ankle, similar to defensive structures observed in malemonotremes and several other early-branching mammals.[4][13]
Like other mammaliaforms, docodontan teeth include peg-likeincisors, fang-likecanines, and numerous interlockingpremolars andmolars. Most mammaliaforms have fairly simple molars primarily suited for shearing and slicing food. Docodontans, on the other hand, have developed specialized molars with crushing surfaces. The shape of each molar is defined by a characteristic pattern of conicalcusps, with sharp, concave crests connecting the center of each cusp to adjacent cusps.[1]
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 thetribosphenic teeth found in truetherian mammals, like modernmarsupials andplacentals. However, there is little consensus forhomologizing docodontan 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 docodontans, is positioned directly behind cusp X. Many docodontans 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 docodontans, 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 docodontans 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 teethocclude (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 thetalonid-and-protocone crushing complex which characterizetribosphenic teeth. Tribosphenic teeth show up in the oldest fossils oftherians, the mammalian subgroup containingmarsupials andplacentals. This is a case ofconvergent evolution, as therian talonids lie at the back of the lower molar rather than the front. The opposite is true for docodontan teeth, which have been described as "pseudotribosphenic".[1]
Pseudotribosphenic teeth are also found inshuotheriids, an unusual collection of Jurassic mammals with tall pointed cusps. Relative to docodontans, 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 modernmonotremes.[3] Docodontan and shuotheriid teeth are so similar that some genera, namelyItatodon andParitatodon, have been considered members of either group.[14][15] A 2024 study, describing the new shuotheriidFeredocodon, even proposed that shuotheriids and docodontans were most closely related to each other among mammaliaforms. The study named a new clade, Docodontiformes, to encompass the two groups.[2]
Docodontans and otherMesozoic mammals were traditionally thought to have been primarily ground dwelling andinsectivorous, but recent more complete fossils from China have shown this is not the case.[16]Castorocauda[4] from the Middle Jurassic of China, and possiblyHaldanodon[17][18] from the Upper Jurassic of Portugal, were specialised for a semi-aquatic lifestyle.Castorocauda had a flattened tail, similar to that of abeaver, and recurved molar cusps, which suggests a possible diet of fish or aquatic invertebrates.[4] It was thought possible that docodontans 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 speciesDocofossor,[5] and specialised tree-dwellerAgilodocodon[6] suggest Docodonta were more ecologically diverse than previously suspected.Docofossor shows many of the same physical traits as the modern daygolden mole, such as wide, shortened digits in the hands for digging.[5]
A 2024 study on adult and juvenileKrusatodon specimens found that docodontans 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 itsdeciduous teeth withpermanent (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 andhyraxes, thoughKrusatodon 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, docodontans contradict earlier assumptions that high metabolism evolved in sync with ecological diversity, since their diversity far outpaces their metabolism.[20]
The lineage of Docodonta evolved prior to the origin of living mammals:monotremes,marsupials, andplacentals. In other words, docodontans are outside of the mammaliancrown group, which only includes animals descended from thelast common ancestor of living mammals. Previously, docodontans were sometimes regarded as belonging toMammalia, owing to the complexity of their molars and the fact that they possess adentary-squamosal jaw joint. However, modern authors usually limit the term "Mammalia" to the crown group, excluding earliermammaliaforms like the docodontans. Nevertheless, docodontans are still closely related to crown-Mammalia, to a greater extent than many other early mammaliaform groups such asMorganucodonta andSinoconodon. Some authors also consider docodontans to lie crownward of the orderHaramiyida,[5] though most others consider haramiyidans to be closer to mammals than docodontans are.[4][21][7] Docodontans may lie crownward of haramiyidans inphylogenetic analyses based onmaximum parsimony, but shift stemward relative to haramiyidans when the same data is put through aBayesian analysis.[22]
Cladogram based on a phylogenetic analysis of Zhou et al. (2019) focusing on a wide range of mammaliamorphs:[7]
Docodontan fossils have been recognized since the 1880s, but their relationships and diversity have only recently been well-established.Monographs byGeorge 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 byBryan Patterson instead argued that docodontan teeth were impossible to homologize with modern mammals. He drew comparisons to the teeth ofMorganucodon 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 docodontans 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 asDelsatia andWoutersia (from theNorian-Rhaetian ofFrance).[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 docodontans, which have a similar size and position. The lingual cusp (cusp X) is prominent inWoutersia.[1] Another proposed docodontan relative,Tikitherium fromIndia, was originally considered to have been a very early mammaliaform which lived during theCarnian stage of the Triassic. Later investigation found thatTikitherium was likely a misidentification ofNeogeneshrew teeth, completely unrelated to docodontans or any Mesozoic mammaliaforms.[29]
Unambiguous docodontans are restricted to theNorthern Hemisphere, abruptly appearing in the fossil record in theMiddle Jurassic. Very few docodontans survived into theCretaceous Period; the youngest known members of the group areSibirotherium andKhorotherium, from theEarly Cretaceous ofSiberia.[30][31] One disputed docodont,Gondtherium, has been described from India, which was previously part of theSouthern Hemisphere continent ofGondwana.[32][1] However, this identification is not certain, and in recent analyses,Gondtherium falls outside the docodontan family tree, albeit as a close relative to the group.[6][7]Reigitherium, from theLate Cretaceous ofArgentina, has previously been described as a docodont,[33] though it is now considered ameridiolestidan mammal.[34] Some authors have suggested splitting Docodonta into two families (Simpsonodontidae and Tegotheriidae),[35][14][36] but themonophyly 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 docodontan relationships:
| Topology of Zhou et al. (2019), based on tooth, cranial, and postcranial traits:[7] | Topology of Panciroli et al. (2021), based on dentary and tooth traits:[9]
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