Castrocauda is a cynodont, not a ‘mammaliaform.’

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…

Figure 1. Castrocauda insitu. Note the scale bars don't match. The pelvis shape is not mammalian. Five sacrals are present.

Figure 1. Castrocauda insitu. Note the scale bars don’t match by about 10 percent. The pelvis shape is not mammalian. Five sacrals are present.

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.”

Figure 2. Castrocauda had a complete postorbital ring, a cynodont trait, along with the presence of post-dentary jaw bones that mark it as a non-mammalian cynodont.

Figure 2. Castrocauda had a complete postorbital ring, a cynodont trait, along with the presence of post-dentary jaw bones that mark it as a non-mammalian cynodont. The nares are anterolateral with a short premaxillary ascending process.

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.”

Figure 3. Original reconstruction of Castrocauda in vivo along with corrections and deletions following a DGS tracing of the specimen.

Figure 3. Original reconstruction of Castrocauda in vivo along with some corrections and deletions following a DGS tracing of the specimen. The pectoral girdle is unknown. The skull has a postorbital bar. The pelvis is typically cynodont, not mammalian. The femur is much shorter.

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.

Figure 3. Castrocauda nests outside the Mammalia, between Chiniquodon and Pachygenelus, two non-mammalian cynodonts.

Figure 3. Castrocauda nests outside the Mammalia, between Chiniquodon and Pachygenelus, two non-mammalian cynodonts.

It’s not just the posterior mandible bones in Castrocauda
that confirm this nesting. Wikipedia reports, “Pachygenelus had both an articularquadrate and dentarysquamosal 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.

Figure 3. Chiniquodon in situ, plate, counter plate, with selected bones colorized and manus + plate reconstructed. There are only 3 sacrals here, not 5. Note the parafibular sesamoid.

Figure 4. Chiniquodon in situ, plate, counter plate, with selected bones colorized and manus + plate reconstructed. There are only 3 sacrals here, not 5. Note the parafibular sesamoid.

This is one more nail in the coffin
of the ‘Mammaliaforms,’ a clade without any taxa in the large reptile tree. The term has no utility, like the “Ornithodira‘, the “Amniota‘ and the ‘Parareptilia.’

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.

References
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.

wiki/Castrocauda
wiki/Pachygenelus

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3 thoughts on “Castrocauda is a cynodont, not a ‘mammaliaform.’

  1. Wikipedia reports, “Mammaliaformes (“mammal-shaped”) is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts.” So, if you think Pachygenelus is a mammaliaform, then Castrocauda is not in that clade. If you think Castrocauda is a mammaliaform, then it is one. Probainognathus has not been tested yet. Chiniquodon is the next proximal outgroup at present.

    • Rowe (1988) defined Mammaliaformes as the least inclusive clade containing Morganucodon and extant mammals. Morganucodon doesn’t seem to have been tested here yet, so nothing can be said about the utility of the term Mammaliaformes yet.

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