Scientists have discovered two new species of animals that are digging and that lived in northeast China 120 million years ago.
The skeletal remains of the species, called Fossiomanus sinensis and Jueconodon chenispiky, reveal that they had claws designed for “scratching” – a technique for creating tunnels using the claws of the front limbs.
Experts from the American Museum of Natural History say that the species could benefit from optimal temperatures throughout the year while they were underground.
The two new species have related but independently developed traits that support their mode of digging.
The artist’s impression depicts Fossiomanus sinensis (top right) and Jueconodon cheni in the burrows. They both lived in the early Cretaceous of Jehol Biot (approximately 120 million years ago), in northeast China, and exhibited skeletal traits adapted to a growing lifestyle.
The ancient ecosystem known as Jehol Biota existed in northeast China approximately 120 to 130 million years ago.
They consisted of dinosaurs, mammals, early birds, fish, lizards, and other creatures, while the red areas prominent below cast fossils.
The animals lived among coniferous forests and lakes, in the shadow of volcanoes.
Fossils of these animals are found in rock formations of Yixian and Jiufotang, embedded in layers of volcanic material.
Fossil mammal species (mammalian precursors) were discovered in Jehol Biota in northeast China – a well-known classification of fossils 130 million years old from the Cretaceous period.
They represent the first “scratch diggers” discovered in this ecosystem.
“These two fossils are a very unusual, deeply exemplary example of animals that are not closely related, and yet both have developed highly specialized digger characteristics,” said study lead author Jin Meng, curator at the American Museum of Natural History Paleontology.
‘This is the first convincing evidence for phosphorus life in these two groups.
‘It’s also the first case of scratch diggers we know in Jehol Biota, which has been home to a great variety of life, from dinosaurs to insects to plants.’
Fossiomanus sinensis is a mammal-like reptile called tritylodontid – a predominantly herbivorous family of small to medium-sized mammals.
About one meter long, it was named after the Latin for “digging” (Fossio) and “hand” (manus), as well as “from China” (sinensis).
Jueconodon chenispiky, meanwhile, was named after Jue, meaning “digging” in Chinese pinyin and conodon, often used as a taxonomic suffix of mammals meaning “tooth tooth,” as well as cheni for Y. Chen, who collected fossils.
The seven-inch-long species is eutriconodontan – a distant relative of modern placental mammals and marsupials, which were common in the habitat.
Fossiomanus sinensis holotype specimen. Optical image (left) and composite computational images using ‘laminography’ (type of X-ray tomography)
Remains of Jueconodon chenispiky, which was slightly smaller than the two newly classified species
Mammals that are now adapted to trenches have naturally developed special digging characteristics.
Researchers have found some of these characteristics – including shorter limbs, strong forelimbs with robust arms and a short tail – in both species.
These characteristics especially indicate the type of digging behavior known as “scratching”, which is mainly achieved with the claws of the front limbs.
There are many hypotheses about why animals dig into the ground and live underground – for example to protect themselves from predators.
Other theories suggest that underground dwellers can maintain a temperature that is relatively constant – not too hot in summer and not too cold in winter – or find food sources such as insects and plant roots.
The diorama landscape illustrates the Early Cretaceous Jehol biota – a famous collection of 130 million-year-old Cretaceous fossils
In addition to showing evidence of digging scratches, the two extinct animals share another unusual feature – an elongated spine.
Typically, mammals have 26 vertebrae from neck to hip, but Fossiomanus had 38 vertebrae – 12 more than the usual state – and Jueconodon 28.
To try to determine how these animals got their elongated trunks, paleontologists turned to recent studies in developmental biology.
They believe the variation could be attributed to gene mutations that determine the number and shape of vertebrae at the beginning of animal embryonic development.
Variations in the number of vertebrae can also be found in modern mammals, including elephants, manatees, and hydraxes.
The new species are further described in the work of the research team published in the journal Nature.