Castorocauda is a cynodont, not a ‘mammaliaform.’

Castorocauda 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. Castorocauda 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. Castorocauda 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.

Figure 2. Castorocauda 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. Here it is shown next to its LRT sister Probainognathus.

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 Castorocauda 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 Castorocauda 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. Castorocauda nests outside the Mammalia, between Chiniquodon and Pachygenelus, two non-mammalian cynodonts. Many more taxa are known presently. 

It’s not just the posterior mandible bones in Castorocauda
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.” Castorocauda 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 Castorocauda 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/Castorocauda
wiki/Pachygenelus

Advertisements

3 thoughts on “Castorocauda 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 Castorocauda is not in that clade. If you think Castorocauda 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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.