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Cretaceous Mammal

Fossiomanus sinensis and Jueconodon cheni, two distantly related species of mammaliamorphs that lived some 120 million years ago (Early Cretaceous epoch), were well adapted for fossorial (burrowing) life, and are the first ‘scratch-diggers’ known from the Jehol Biota, which is distributed mainly in western Liaoning Province and neighboring areas in northeastern China.

The dioramic landscape illustrates the Early Cretaceous Jehol Biota with emphasis on mammaliamorphs. Image credit: Chuang Zhao.

The dioramic landscape illustrates the Early Cretaceous Jehol Biota with emphasis on mammaliamorphs. Image credit: Chuang Zhao.

Jueconodon cheni is a eutriconodontan, a distant cousin of modern placental mammals and marsupials, and was 17.8 cm (7 inches) long.

Fossiomanus sinensis is an herbivorous mammal-like animal called a tritylodontid, and was 31.6 cm (12.2 inches) in length. It is the first of its kind to be identified in the Jehol Biota.

“The Early Cretaceous Jehol Biota has generated many well-preserved fossils that have furnished a great deal of information on the morphology and evolution of early mammals,” said Dr. Jin Meng from the American Museum of Natural History and colleagues.

“The two new species expand the diversity of the mammaliamorph assemblage and increase its morphological disparity, as they show unequivocal evidence of convergent adaptation for a fossorial lifestyle.”

“The two specimens also provide an opportunity to learn more about the biology (such as axial skeletal development) of these extinct forms.”

Fossiomanus sinensis (upper right) and Jueconodon cheni in their burrows. Image credit: Chuang Zhao.

Fossiomanus sinensis (upper right) and Jueconodon cheni in their burrows. Image credit: Chuang Zhao.

Mammals that are adapted to burrowing have specialized traits for digging. Dr. Meng and co-authors found some of these hallmark features — such as shorter limbs, strong forelimbs with robust hands, and a short tail — in both Fossiomanus sinensis and Jueconodon cheni.

In particular, these characteristics point to a type of digging behavior known as ‘scratch digging,’ accomplished mainly by the claws of the forelimbs.

“There are many hypotheses about why animals dig into the soil and live underground,” Dr. Meng said.

“For protection against predators, to maintain a temperature that’s relatively constant or to find food sources like insects and plant roots.”

“These two fossils are a very unusual, deep-time example of animals that are not closely related and yet both evolved the highly specialized characteristics of a digger.”

Holotypes of Fossiomanus sinensis (left) and Jueconodon cheni (right). Abbreviations: cl – clavicle, cor - coronoid process of the dentary, den – dentary, ds - dentary symphysis, fe – femur, fi – fibula, hu – humerus, il – ilium, man – manus, nap - nasal anterior process, nuc - nuchal crest, oco - occipital condyle, r - rib (with number), ra – radius, sc – scapula, ti – tibia, ul – ulna, upc - upper postcanines, za - zygomatic arch; the (r) and (l) in parentheses denote right and left, respectively. Scale bars - 10 mm. Image credit: Mao et al., doi: 10.1038/s41586-021-03433-2.

Holotypes of Fossiomanus sinensis (left) and Jueconodon cheni (right). Abbreviations: cl – clavicle, cor – coronoid process of the dentary, den – dentary, ds – dentary symphysis, fe – femur, fi – fibula, hu – humerus, il – ilium, man – manus, nap – nasal anterior process, nuc – nuchal crest, oco – occipital condyle, r – rib (with number), ra – radius, sc – scapula, ti – tibia, ul – ulna, upc – upper postcanines, za – zygomatic arch; the (r) and (l) in parentheses denote right and left, respectively. Scale bars – 10 mm. Image credit: Mao et al., doi: 10.1038/s41586-021-03433-2.

Fossiomanus sinensis and Jueconodon cheni also share another unusual feature: an elongated vertebral column.

Typically, mammals have 26 vertebrae from the neck to the hip. However, Fossiomanus sinensis had 38 vertebrae, while Jueconodon cheni had 28.

To try to determine how these animals got their elongated trunks, the paleontologists turned to recent studies in developmental biology.

They found that the variation could be attributed to gene mutations that determine the number and shape of the vertebrae in the beginning of the animals’ embryotic development.

“These fossils shed light on the evolutionary development of the axial skeleton in mammaliamorphs, which has been the focus of numerous studies in vertebrate evolution and developmental biology,” they said.

The discovery of Fossiomanus sinensis and Jueconodon cheni is reported in the journal Nature.

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F. Mao et al. Fossoriality and evolutionary development in two Cretaceous mammaliamorphs. Nature, published online April 7, 2021; doi: 10.1038/s41586-021-03433-2

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