Although unknown before the Middle Jurassic, and therefore not among the oldest known mammals, docodonts are considered to be one of the most archaic mammalian groups. Their remains were first discovered more than a century ago in the Late Jurassic Morrison Formation of Wyoming and Colorado, where they are found together with the bones of giant sauropod dinosaurs. They have subsequently been discovered at several sites in Europe and Asia. A purported docodont (Reigitherium) has been reported from the Late Cretaceous of Patagonia in South America (Pascual et al., 2000), but this attribution is questionable (Kielan-Jaworowska et al., 2004). Rougier, Novacek, et al. (2003) reported new specimens of Reigitherium that show the absence of postdentary bones as well as dental features that suggest that it is a dryolestoid, as originally proposed by Bonaparte (1990).
Most docodonts are known solely from the dentition, which includes complex, broad cheek teeth. In Docodon, the lower molars are rectangular and the buccal cusps are higher than the lingual cusps; the upper molars are hourglass shaped and transversely wider than long (Fig. 4.7B). The teeth of docodonts have been cited as evidence that the group evolved from morganucodonts (e.g., Crompton and Jenkins, 1979). According to this hypothesis, the wide molars of docodonts evolved by expansion of the lingual cingula of typical morganucodont molars. Cusps on the cingula eventually enlarged, and transverse crests formed, joining them to the original (lateral) cusps. The morganucodont Megazostrodon, which has a well-developed lingual cingulum and cingular cusps on the lower molars, represents a plausible morphologic stage from which docodonts might have evolved (Crompton, 1974). As noted above, it is also possible that docodonts evolved in a similar manner from kuehneotheriids. Docodonts also evolved precise molar occlusion in association with their complex molar crowns. These derived conditions are superficially similar to those characterizing therians, but they are different enough to indicate that they arose independently.
Insight on the phylogenetic position of docodonts is afforded by the best-known docodont, Haldanodon, from the Late Jurassic Guimarota lignites (swamp deposits) of Portugal (Lillegraven and Krusat, 1991; Martin and Krebs, 2000). Haldanodon is represented by dozens of jaws, several skulls (Fig. 4.7A), and a skeleton. Based on its robust limb skeleton—especially the scapula with a postscapular fossa, the broad humerus with a prominent deltopectoral crest, an elongate ulnar olecranon process, and short and robust
phalanges—Haldanodon appears to have been fossorial (Krusat, 1991; Martin, 2005). However, its occurrence in lignite deposits suggests that it may also have been semi-aquatic, similar to extant desman moles. Haldanodon resembles cynodonts in several plesiomorphous cranial features that are present in more derived states in morganucodontids. The presence in Haldanodon of a large septomaxilla in the nasal region, retention of larger accessory ("postdentary") jaw bones and a larger stapes than in morganucodontids, and several other cynodont-like features could indicate that this genus was more primitive than Morganucodon and diverged even earlier from the mammalian stem. At the same time, several other cranial characters of Haldanodon are derived, like those of other early mammals. Lillegraven and Krusat suggested that Haldanodon could have acquired many of its "mammalian" traits earlier than, and independently from, morganu-codontids, which would suggest that Mammalia is poly-phyletic. Subsequent phylogenetic analyses (e.g., Rougier et al., 1996a; Luo et al., 2002), however, support a mono-phyletic Mammalia that includes Haldanodon.
A new docodont recently reported from the Middle Jurassic of China provides additional evidence on the relationships and behavior of these archaic mammals (Ji et al., 2006). Based on a partial skeleton, Castorocauda is the largest known docodont, almost half a meter long from the snout to the end of the tail. Its skeleton is adapted for swimming and burrowing, supporting the interpretation that docodonts were semiaquatic. Phylogenetic analysis confirmed that doco-donts are a primitive mammalian clade more derived than morganucodonts but less derived than Hadrocodium. Casto-rocauda is the oldest mammal preserving evidence of fur.
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