Jeholodens and Spinolestes: two new tritylodontids

Revised October 4, 2016 with a shifting of Jeholodens and Spinolestes to the Tritylodontidae, which are pre-mammals arising from Pachygenelus. Tritylodontids replace their molars, something mammals do not do.

The Early Cretaceoous
included the first radiation of basal mammals. The vast majority of these, so far, have been multituberculates, small rodent-like taxa that actually nest with rodents in the large reptile tree. Traditional paleontologists nest multituberculates much more primitively, prior to the Theria (live-bearing mammals) despite their many rodent-like traits, like enlarged incisors followed by a diastema (toothless region) and flat cranial region.

With so many multituberculates
in the Cretaceous, I’m always looking for non-multituberculate mammals from the era.  Repenomamus was a tritylodont, pre-mammal. Vincelestes was a marsupial. Maotherium was an Early Cretaceous basal primate. Liaoconodon, was a pre-tritylodont.  I had high hopes that the next two Early Cretaceous mammals were, as advertised (i.e. something other than multituberculates.)

Another traditional clade of Early Cretaceous mammals
are the eutriconodonts. These include taxa with traditional tooth arcades, but lacking deep canines. Repenomamus is one traditional eutriconodont, but nests in the large reptile tree with Pachygenelus in the tritylodontids. Spinolestes (Fig. 1) and Jeholodens (Fig. 2) are also listed at eutriconodonts, but they nest together in the large reptile tree both as sisters and as derived tritylodontids.

Figure 1. Spinolestes with bones colorized in DGS and both manus and skull reconstructed.

Figure 1. Spinolestes with bones colorized in DGS and both manus and skull reconstructed. Note the tooth pattern recovered here is different than as originally described. If your screen is 72 dpi, then this image is about half again as large as life size.

Spinolestes
has accessory neural articulations, like some shrews do. It also appears to have two sacrals and is otherwise robust overall. The entire foot is present, but scattered. I attempted a reconstruction of the lateral view of the skull based on published clues (Fig. 1).

Figure 2. Jeholodens holotype. Note the tip of the snout is missing, and so are the large anterior premaxillary teeth that characterize this clade.

Figure 2. Jeholodens holotype. Note the tip of the snout is missing, and so are the  anterior premaxillary teeth. If your screen is 72 dpi, then this image is almost twice as large as life size.

Jeholodens jenkinsi
(Ji et al. 1999) was also considered a triconodont, but the tip of the snout is missing. Not as robust as Spinolestes, Jeholodens (Fig. 2) was nevertheless a flat-bodied specimen able to slip into rock cracks. Wikipedia reports the eye was 5 cm across. The true figure is 5 mm.

Ji et al. 1999 reported, “The postcranial skeleton of this new triconodont shows a mosaic of characters, including a primitive pelvic girdle and hindlimb but a very derived pectoral girdle that is closely comparable to those of derived therians. Given the basal position of this taxon in mammalian phylogeny, its derived pectoral girdle indicates that homoplasies (similarities resulting from independent evolution among unrelated lineages) are as common in the postcranial skeleton as they are in the skull and dentition in the evolution of Mesozoic mammals.”

The present analysis indicates
that Jeholodens actually nested with pre-mammal tritylodontids. The naris and small premaxillary teeth provided in the original reconstruction are imagined because that part is broken off the matrix (Fig. 2).

Be wary when you see
the terms ‘mosaic’ and ‘modular’. As we’ve seen before, that usually means the phylogeny is off. Evolution works by a gradual accumulation of traits all over the body, not in a modular or mosaic fashion. Trityodontids look like mammals because they are the proximal outgroup taxon. Multituberculates nest with rodents, with whom they share so many traits.

Figure 3. The tarsus of Jeholodens compared to that of a cynodont, mutituberculate and Didelphis, a marsupial (metatherian).

Figure 3. The tarsus of Jeholodens compared to that of a cynodont, mutituberculate and Didelphis, a marsupial (metatherian). Note that metatarsal 5 is missing in all taxa. Metatarsal 4 becomes a dual metatarsal in Didelphis and most eutherians, but not in Vulpavus, Onychonycteris,

 

Figure 4. Rattus pes. Note distal tarsal 4 only backs up pedal digit 4 and digit 5 rides alongside.

Figure 4. Rattus pes. Note distal tarsal 4 only backs up pedal digit 4 and digit 5 rides alongside.

 

 

 

Using Didelphis for a derived tarsus is a little misleading…
From the Ji et al. 1999 paper (Fig. 3) it looks like Jeholodens has a basal tarsus because distal tarsal 4 is not wide enough to double as a distal tarsal 5, as it does in the marsupial, Didelphis. A quick peek at Rattus (Fig. 4), Vulpavus and Onychonycteris shows that these placental taxa likewise do not widen distal tarsal 4 to back up pedal digit 5. And it is not clear how the Jeholodens tarsus actually stacks up. (Fig. 5). If the tarsus was loose, as it is in bats like Onychonycteris or Pteropus, then it doesn’t necessarily mean the tarsus was primitive, like that of a cynodont. That’s why the overall scores are more important than individual character scores.

Just because something is published in Nature or Science, doesn’t mean it’s necessarily right, as we’ve seen before with Yi, Cartorhynchus, Sclerocormus, Chilesaurus, dinosaur origins. pterosaur origins and turtle origins.

Figure 5. Tarsus of Jeholodens with elements colorized as in other taxa.

Figure 5. Tarsus of Jeholodens with elements colorized as in other taxa.

And while I’m thinking about it,
it may be that clades like “Triconodonta” and “Eutriconodonta” may be junior synonyms for long established taxa, as we looked at earlier here.

References
Ji Q, Luo Z and Ji S. 1999. A Chinese triconodont mammal and mosaic evolution of the mammalian skeleton. Nature 398:326-330. online.

Martin T et al. 2015. A Cretaceous eutriconodont and integument evolution of early mammals. Nature 526:380-384. online.

 

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