Updated Sept 08, 2016 with higher resolution images of Henkelotherium.
Today we’ll look at
Zhangheotherium quinquecuspidens (Hu et al. 2009; Late Jurassic/Early Creteacous; dentary length 3 cm; IVPP V7466; Fig. 1). It was originally described as a symmetrodont mammal, an ‘archaic’ taxon typically represented by only tooth and dentary scraps. Here (Fig. 1) a complete skeleton provided new insight to the original authors. They reported Zhangheotherium did not travel in a parasagittal posture and the cochlea (an organ of the inner ear) was not fully coiled.
Keep in mind, for the moment,
that neither bats nor pangolins travel in a parasagittal posture. Pangolins are bipedal (video here). Bats are inverted bipeds. Both fold their fingers posteriorly when not using them. That’s why the forelimbs are lifted here (Fig.1) even though neither Zhangheotherium nor Manis would be reconstructed as a biped if not known from in vivo behavior.
Hu et al. thought Zhangheotherium radiated before the divergence of living marsupials and placentals. Here, in the large reptile tree (LRT) Zhangheotherium nests at the base of the Ernanodon + pangolins clade, close to Chriacus and the bats and not far from the dermopterans. Clearly this was originally or principally an arboreal clade. Ernanodon is the exception that got big as it left the trees for the ground, something other pangolins did, too.
Hu et al note:
“A mobile clavicle–interclavicle joint that allows a wide range of movement of the forelimb has an ancient origin in the mammalian phylogeny.” This is quite visible in the fossil and interesting with regard to Zhangheotherium’s relation to bat ancestors. Bats and Didelphis likewise have a large floating pectoral girdle. Pangolins have a large scapula more closely associated with the rib cage.
In Zhangheotherium, Hu et all note [my remarks follow in brackets]:
- The cervical ribs were unfused.[have not gotten close enough to pangolins to check this out]
- The caudal transverse processes were wide [as in pangolins].
- Three or four sacrals were present [suggesting stress in this area, perhaps for balance].
- The pisiform is very large [as in Ptilocercus and Manis].
- Only the dorsal acetabulum is preserved [open ventrally as in pangolins]
- Zhangheotherium has an external pedal spur, as in Ornithorhynchus [not sure about this disarticulated bone, perhaps not a spur, but a simple spindle-shaped ankle bone similar to one seen in Manis, see Fig. 4]
- The interclavicle is triangular and the sternal manubria are only three in number. [Could not find com parables here]
- It is more primitive than Henkelotherium and Vincelestes in retaining the interclavicle in its pectoral girdle/sternal manubrium [no comparables found]
- These new data suggest that the mobility of the clavicle and scapula has a more ancient origin than the more parasagittal posture of the forelimbs [or… this type of arboreal locomotion loosens the girdles]
- The mobile and pivotal clavicle evolved before the divergence of multituberculates and therians. [in the LRT multituberculates are therians and placentals, and so is Zhangheotherium].
Hu et al. report, “It has been argued that dental characters are as homoplasic as non-dental characters and the reliability of dental characters for inferring the relationships of major lineages of mammals has been questioned. Zhangheotherium has provided more extensive basicranial and postcranial evidence to corroborate the traditional hypothesis that symmetrodonts represent a part of the basal therian radiation.” [Zhangheotherium nests in a basal placental position in the LRT.
Hu et al. link
Zhangheotherium to Henkelotherium (Krebs 1991; Late Jurassic, Kimmeridgian; Figs. 5, 6). So let’s look at Henkelotherium while we’re here.
In the large reptile tree
Henkelotherium and Zhangheotherium do not nest together. Rather Henkelotherium nests with Nambaroo and Plesiadapis + rabbits. Wikipedia considers Henkelotherium a paurodontid dryolestid (formerly considered a eupantothere) and similar in locomotion patterns to tree shrews and opossums. Key to Henkelotherium are the enlarged dentary incisors (premaxilla remains unknown). This represents the first step toward the larger incisors found in plesiadapiformes, Tupaia-like tree shrews, apatemyids and rodents + multituberculates.
Back to Zhangheotherium, you’ll note
the dentary condylar process curves dorsally and no post-dentary bones are present (all had become middle ear bones enclosed within the petrosal). That dorsal curve removes most of the ability to resist jaw dislocation often caused by struggling large prey and or small pieces of even larger prey are working against large canines, which were also not present in Zhangheotherium. These traits point to a tiny prey diet, likely of insects, just like pangolins.
Hu Y-M, Wang YQ, Luo Z and Li CK 1997. A new symmetrodont mammal from China and its implications for mammalian evolution. Nature 390:137-142.
Krebs B 1991. Skelett von Henkelotherium guimarotae gen. et sp. nov. (Eupantotheria, Mammalia) aus dem Oberen Jura von Portugal. Berl Geowiss Abh A.: 133:1–110.