In Pursuit of Early Mammals

7. Docodontans

107 7 7.1. A. Docodon victor, the first described docodontan, represented by a dentary with dentition. B. Tegotherium gubini, a lower molariform tooth, in labial (B1 ), lingual (B2 ), and occlusal (B3 ) views. C. Reconstruction of the skull of Haldanodon exspectatus, in dorsal (C1 ), ventral (C2 ), and lateral (C3 ) views. D. Tom Martin . E. Alexander Averianov. F. Alexy Lopatin. A. From Kielan-Jaworowska et al. (2004: figure 5.3.A). B. Courtesy of the late Leonid P. Tatarinov, who sent me the original of his drawing. C. From Kielan-Jaworowska et al. (2004: figure 5.1.A–D), modified from Lillegraven and Krusat (1991). Docodontans The docodontans are a group of very small mammaliaforms, mostly mole size or even smaller, of Middle to Late Jurassic age. An exception with respect to size is the Middle Jurassic Castorocauda lutrasimilis , with a skull that measures 6 cm and a body about 42 cm long (Ji, Luo, Yuan, and Tabrum 2006). Reigitherium from the Late Cretaceous of the Los Alamitos and La Colonia Formations of Patagonia, South America, has been assigned to the docodontans, but I regard it as a dryolestoidean. The characteristic feature of the docodontans is the presence of an extensive posterolingual trough on the dentary, housing the postdentary bones, as well as the presence of the double jaw joint. In a complete lower jaw of Reigitherium found by Rougier and his colleagues, no posterolingual trough is present, and the jaw joint is single. Rougier, Novacek, Ortiz-Jaureguizar, and Purerta in their abstract (2003, not cited herein) convincingly argued that Reigitherium, which originally was assigned to the Dryolestoidea by Bonaparte (1990), is better allocated there than to the docodontans. The first docodontan was discovered in 1881 by Othniel Charles Marsh, who described a nearly complete lower jaw, dubbed Docodon. It occurs in the Jurassic Morrison Formation of Colorado and Wyoming . Characteristic of docodontans (figure 7.1A) are wide upper molars, rectangular or figure-eight-shaped in occlusal view, resembling those of advanced placental mammals. The lower molars are longitudinally elongated. Strong and prominent crests joining the molar cusps are also characteristic of docodontans. When in 1959 Walter G. Kühne and the young geologist Wolfgang Frey discovered the skull of a multituberculate mammal in the Guimarota Coal Mine (Kimmeridgian age) in central Portugal, they also found isolated teeth of various mammals (especially multituberculates; see chapter 10), and among these was the best-preserved skull of a docodontan, named Haldanodon exspectatus by Kühne and Krusat in 1972. The skull of Haldanodon was first described in English by the German paleontologist Georg Krusat in a monograph with a Portuguese title, published in the series Memórias dos Serviços Geológicos de Portugal in 1980. In 1991, after Jason A. Lillegraven had spent a sabbatical year in Berlin (1988–1989), he and Georg Krusat published a more detailed account of the skull and lower jaw of Haldanodon. Their study (Lillegraven and Krusat 1991) included details of the lower jaw suspensorium showing that there were numerous postdentary bones in a wide posterolingual trough of the dentary and a double jaw joint (figure 7.1). In Pursuit of Early Mammals 108 The Guimarota locality yielded several specimens of Haldanodon, among which there were fragments of skull associated with a large part of the postcranial skeleton, figured and briefly described by Georg Krusat (1991). Although the postcranial skeleton was not complete, the preserved bones, especially the femur and humerus, were found to be very robust, indicative of a fossorial (or burrowing) habit. Martin and Novotny, in the book edited by Martin and Krebs (2000), offered a general review of Haldanodon, with a life reconstruction painted by E. Gröning (Martin and Novotny 2000: figure 14.3). Martin (2005) re-described the skeleton of Haldanodon in great detail, together with several isolated bones found at the same locality. Among the genera assigned to Docodonta, most of which are known from the dentition, of special interest is the highly specialized Middle Jurassic genus Castorocauda (figure 7.2). Castorocauda lutrasimilis was described by Ji, Luo, Yuan, and Tabrum in 2006, in an article published in Science. The beautiful reconstruction from this article (artwork by Marc A. Klingler) was used for an attractive cover of the journal. The only specimen of C. lutrasimilis (the holotype) was recovered from the Middle Jurassic Jiulongshan Formation of Inner Mongolia (Chinese People’s Republic). There is no doubt that Castorocauda possessed swimming and burrowing skeletal specializations and a dentition adapted for aquatic feeding...