Paleontologists Discover First Scratch-Digger Fossils From Early Cretaceous

The fossils of the Early Cretaceous scratch-digger species are the first of these predecessors to mammals to be found in the prehistoric ecosystem.

The dioramic landscape illustrates the Early Cretaceous Jehol Biota, with emphasis on mammaliamorphs. (Credit: Chuang Zhao)

(CN) — Researchers uncovered two fossils of 145 million-year-old scratch-digger species in northeastern China, revealing clues about their evolutionary timelines and the conditions they existed in.

The fossils of the two distantly related species belong to a reptilian, mammal-like creature called a tritylodont and to a eutriconodontan, which is a distant relative of modern marsupials and placental mammals.

The two species — scientific names Fossiomanus sinensis and Jueconodon cheni, respectively — roamed Earth between 100 and 145 million years ago during the Early Cretaceous epoch.

The roughly 12.4 inch long tritylodont fossil was discovered in the Jiufotang formation, a site in the Liaoning province of China which over multiple decades has yielded important fossils of prehistoric birds, amphibians and fish.

The 7-inch eutriconodontan fossil was found in the Yixian formation, another important fossil site in Liaoning.

Researchers Jin Meng, Fangyuan Mao and colleagues set out to analyze how the two creatures developed the burrowing characteristics which are common to other scratch-digger species from that era.

Early Cretaceous scratch-diggers developed certain specialized body features that maximized their digging capabilities, the study said.

“Fossorial animals that dig in soil must be capable of applying great force against the substrate; therefore, unlike cursorial and climbing animals, diggers are constructed such that their legs are short and their relevant bone–muscle systems produce large out-forces,” the study authors wrote.

Both species examined in the study had numerous, more pronounced thoracic vertebrae bones. They also had shorter hind limbs and short tails that enhanced their fossorial lifestyles, and broad forelimbs with powerful, sharp claws.

Despite their distant relationship, Fossiomanus and Jueconodon displayed some of the same features, though the study authors concluded they developed these features independently in the evolutionary process.

The environmental conditions both species encountered — described in the study as “selective pressures” — spurred these specialized developments.

“Viewed within the mammaliamorph phylogeny, the fossorial features shared by these two species can be interpreted as having evolved independently under similar selective pressures for specific morphological traits associated with similar ecological and biomechanical functions,” the researchers wrote. “However, reflecting their distant phylogenetic positions, these animals differ notably in other aspects of the skeleton; for example, Fossiomanus has a postdentary unit whereas Jueconodon does not.”

The scratch-diggers’ fossils represent the first of their kind to be discovered from this ecosystem, according to the study published in the journal Nature.

Researchers did not immediately respond to a request for further comment on the study.

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