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

Cynodonts: Where is the postorbital? or is that the postfrontal?

Comparing therapsid skulls shows that basal forms, like the gorgonpsids (Fig. 1) had a postfrontal and postorbital. Derived forms (like Pachygenelus (Fig. 1) had neither. You can see the jugal rise in certain cynodonts, taking over where the postorbital retreated. You can also see either the fusion of the postfrontal and postorbital, or the disappearance of the postorbital (but workers like to label the remaining bone the postorbital even though it is largely a postfrontal). As this all goes into scoring, it’s important to that end.

Figure 1. Several gorgonopsids and cynodonts along with a single therocephalian documenting the disappearance of the postorbital and postfrontal. Pink is the pre parietal, absent in cynodonts. Yellow = prefrontal. Green = postorbital. Blue = jugal.

Figure 1. Several gorgonopsids and cynodonts along with a single therocephalian documenting the disappearance of the postorbital and postfrontal. Pink is the pre parietal, absent in cynodonts. Yellow = prefrontal. Green = postorbital. Blue = jugal.

Not much else today. Just wanted to share this and invite comments. Does anyone know the transitional taxon that might clarify this issue? Likely a basal cynodont, like Charassognathus (Fig. 1). Aelurognathus might have documented something on this subject, but the parts are missing from the fossil. I think we’re looking for a small gorgonopsid or therocephalian to show us, something like Regisaurus (Fig. 1)but more primitive.

All this and more from a PhD study by Gebauer (2007).

Updated the next day, February 16, 2014.

Figure 1. The therocephalian Annatherapsidus documenting a small postfrontal and postorbital documenting a transition at the base of the Cynodontia.

Figure 1. The therocephalian Annatherapsidus documenting a small postfrontal and postorbital on a flat skull identifying this taxon as close to the transition at the base of the Cynodontia. This gives us a better chance that the postorbital fused to the postfrontal.

I just discovered a therocephalian originally named Anna and later renamed Annatherapsidus (Fig. 2) that had a reduced postfrontal and reduced postorbital along with wide temporal fenestra and a rather flat skull, both as in Procynosuchus (Fig.1). This appears to be a transitional taxon, the proximal outgroup to the Cynodontia. And this taxon appears to duplicate the pre-fused shape of the postfrontal/postorbital. 

References
Gebauer EVI 2007. Phylogeny and Evolution of the Gorgonopsia with a Special Reference to the Skull and Skeleton of GPIT/RE/7113 (‘Aelurognathus?’ parringtoni). PhD Dissertation, Eberhard-Karls University at Tübingen. Online here.