Castrocauda lutrasimilis (Ji et al. 2006, Middle Jurassic, 42 cm long) was described a decade ago as a proximal relative to modern mammals provisioned with a broad, beaver-like tail. And in a way, it was…
From the abstract: “A docodontan mammaliaform from the Middle Jurassic of China possesses swimming and burrowing skeletal adaptations and some dental features for aquatic feeding. It is the most primitive taxon in the mammalian lineage known to have fur and has a broad, flattened, partly scaly tail analogous to that of modern beavers. We infer that docodontans were semiaquatic, convergent to the modern platypus and many Cenozoic placentals. This fossil demonstrates that some mammaliaforms, or proximal relatives to modern mammals, developed diverse locomotory and feeding adaptations and were ecomorphologically different from the majority of generalized small terrestrial Mesozoic mammalian insectivores.”
Wikipedia reports: “The discovery of Castorocauda lutrasimilis is the first sign that a close relative of mammals adapted to water before dinosaurs lost dominance 65 million years ago, pushing back the estimated date for mammal relatives adapted to a semi-aquatic lifestyle by 110 million years. Based on fossils known at present, the mammal line would not see another semi-aquatic form evolve until the Eocene.”
Given the present data
the large reptile tree nested Castrocauda between the middle Triassic cynodont, Chiniquodon and the Early Jurassic ictidosaur (trithelodontid) cynodont Pachygenelus. There are no mammaliaforms in the large reptile tree, unless you count Pachygenelus among them. The basalmost mammals remain the monotremes, Ornithorhynchus (platypus) and Akidolestes. As such Castrocauda is one of the last known non-mammalian cynodonts in the Middle Jurassic. Tritylodontids were also late-surviving cynodonts.
It’s not just the posterior mandible bones in Castrocauda
that confirm this nesting. Wikipedia reports, “Pachygenelus had both an articular–quadrate and dentary–squamosal jaw joint characteristic of ictidosaurs. Only mammals possess the dentary-squamosal articulation, while all other tetrapods possess the typical arcticular-quadrate articulation.” Castrocauda also had a a-q/d-s jaw joint. (Fig. 2) and did not have tiny ear bones tucked beneath or behind the jaw. Rather the angular, articular, quadrate and surangular were large and behind the dentary, as in other non-mammalian cynodonts.
If high rez images of Castrocauda become available
I’ll make any changes if called for. At present I have not been impressed by the accuracy of the images in this paper (Fig. 3) — except for the teeth, which are key to mammalogists, but are less important in the present analysis. Ji et al. noted the non-mammalian posterior jaw bones, so it is not a big leap to score the rest of the body following the tenets of phylogenetic bracketing.
Ji Q, Luo Z-X, Yuan C-X, Tabrum AR 2006. A swimming mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals. Science. 311: 1123–1127.