Updated July 06 2022 with a revised nesting for Zhangheotherium.
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.
Hu et al. reported\,
Zhangheotherium radiated before the divergence of living marsupials and placentals. Here, in the large reptile tree (LRT) Zhangheotherium nests with Maotherium, prior to the origin of mammals.
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.
In Zhangheotherium, Hu et all note
- The cervical ribs were unfused.
- The caudal transverse processes were wide
- Three or four sacrals were present [suggesting stress in this area, perhaps for balance].
- The pisiform is very large
- Only the dorsal acetabulum is preserved
- 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.
- It is more primitive than Henkelotherium and Vincelestes in retaining the interclavicle in its pectoral girdle/sternal manubrium
- 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
- The mobile and pivotal clavicle evolved before the divergence of multituberculates and therians. [in the LRT multituberculates are therians and placentals]
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 has five or more molars, as in Maotherium].
Hu et al. link
Zhangheotherium to Henkelotherium (Krebs 1991; Late Jurassic, Kimmeridgian; Figs. 5, 6).
In the large reptile tree
Henkelotherium and Zhangheotherium do not nest together. Rather Henkelotherium nests with 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. 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.
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.