By Kevin Jiang

Two 160 million-year-old fossils suggest widespread ecological diversity among the earliest mammals, hint at genetic parallels to modern mammals

The fossils of two interrelated ancestral mammals, newly discovered in China, suggest that the wide-ranging ecological diversity of modern mammals had a precedent more than 160 million years ago.

With claws for climbing and teeth adapted for a tree sap diet, Agilodocodon scansorius is the earliest-known tree-dwelling mammaliaform (long-extinct relatives of modern mammals). The other fossil, Docofossor brachydactylus, is the earliest-known subterranean mammaliaform, possessing multiple adaptations similar to African golden moles such as shovel-like paws. Docofossor also has distinct skeletal features that resemble patterns shaped by genes identified in living mammals, suggesting these genetic mechanisms operated long before the rise of modern mammals.

These discoveries are reported by international teams of scientists from the University of Chicago and Beijing Museum of Natural History in two separate papers published Feb. 13 in Science.

“We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals,” said Zhe-Xi Luo, PhD, professor of organismal biology and anatomy at the University of Chicago and an author on both papers. “The groundwork for mammalian success today appears to have been laid long ago.”

Agilodocodon and Docofossor provide strong evidence that arboreal and subterranean lifestyles evolved early in mammalian evolution, convergent to those of true mammals. These two shrew-sized creatures – members of the mammaliaform order Docodonta – have unique adaptations tailored for their respective ecological habitats.

Agilodocodon, which lived roughly 165 million years ago, had hands and feet with curved horny claws and limb proportions that are typical for mammals that live in trees or bushes. It is adapted for feeding on the gum or sap of trees, with spade-like front teeth to gnaw into bark. This adaptation is similar to the teeth of some modern New World monkeys, and is the earliest-known evidence of gumnivorous feeding in mammaliaforms. Agilodocodon also had well-developed, flexible elbow and ankle joint that allowed for much greater mobility, all characteristics of climbing mammals.

“The finger and limb bone dimensions of Agilodocodon match up with those of modern tree-dwellers, and its incisors are evidence it fed on plant sap,” said study co-author David Grossnickle, graduate student at the University of Chicago. “It’s amazing that these arboreal adaptions occurred so early in the history of mammals and shows that at least some extinct mammalian relatives exploited evolutionarily significant herbivorous niches, long before true mammals.”

Docofossor, which lived around 160 million years ago, had a skeletal structure and body proportions strikingly similar to the modern day African golden mole. It had shovel-like fingers for digging, short and wide upper molars typical of mammals that forage underground, and a sprawling posture indicative of subterranean movement.

Docofossor had reduced bone segments in its fingers, leading to shortened but wide digits. African golden moles possess almost the exactly same adaptation, which provides an evolutionary advantage for digging mammals. This characteristic is due to the fusion of bone joints during development – a process influenced by the genes BMP and GDF-5. Because of the many anatomical similarities, the researchers hypothesize that this genetic mechanism may have played a comparable role in early mammal evolution, as in the case of Docofossor.

The spines and ribs of both Agilodocodon and Docofossor also show evidence for the influence of genes seen in modern mammals. Agilodocodon has a sharp boundary between the thoracic ribcage to lumbar vertebrae that have no ribs. However, Docofossor shows a gradual thoracic to lumber transition.  These shifting patterns of thoracic-lumbar transition have been seen in modern mammals and are known to be regulated by the genes Hox 9-10 and Myf 5-6. That these ancient mammaliaforms had similar developmental patterns is an evidence that these gene networks could have functioned in a similar way long before true mammals evolved.

“We believe the shortened digits of Docofossor, which is a dead ringer for modern golden moles, could very well have been caused by BMP and GDF,” Luo said. “We can now provide fossil evidence that gene patterning that causes variation in modern mammalian skeletal development also operated in basal mammals all the way back in the Jurassic.”

Early mammals were once thought to have limited ecological opportunities to diversify during the dinosaur-dominated Mesozoic era. However, Agilodocodon, Docofossor and numerous other fossils – including Castorocauda, a swimming, fish-eating mammaliaform described by Luo and colleagues in 2006 – provide strong evidence that ancestral mammals adapted to wide-ranging environments despite competition from dinosaurs.

“We know that modern mammals are spectacularly diverse, but it was unknown whether early mammals managed to diversify in the same way,” Luo said. “These new fossils help demonstrate that early mammals did indeed have a wide range of ecological diversity. It appears dinosaurs did not dominate the Mesozoic landscape as much as previously thought.”

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The study, “Evolutionary development in basal mammaliaforms as revealed by a docodontan,” was supported by the Beijing Science and Technology Commission, the Ministry of Science and Technology of China and the University of Chicago. Additional authors include Qing-Jin Meng, Qiang Ji, Di Liu, Yu-Guang Zhang and April I. Neander.

The study “An arboreal docodont from the Jurassic and mammaliaform ecological diversification,” was supported by the Beijing Science and Technology Commission, the Ministry of Science and Technology of China, the Chinese Academy of Geological Science and the University of Chicago. Additional authors include Qing-Jin Meng, Qiang Ji,Yu-Guang Zhang and Di Liu.

Authors and Contacts

Qing Megn Jin
Beijing Museum of Natural History
Beijing, China
+86-10-6702-0959
mengqingjin@bmnh.org.cn

Zhe-Xi Luo
Department of Organismal Biology and Anatomy, University of Chicago
Chicago, IL, USA
773.702.7816
zxluo@uchicago.edu

Attention: Artwork and photographs downloaded from here are for media use only. High resolution images available from April Neander at aisch@uchicago.edu.

	Agilodocodon Fossil The Fossil of Agilodocodon scansorius. The specimen is preserved in two rock slabs. The main part (BMNH-PM001138A, left) and counter part (BMNH-PM001138B, right) of Agilodocodon scansorius type specimen of Beijing Museum of Natural History (BMNH).  Found in lake sediments of the 165 million years old Daohugou Fossil Site of Inner Mongolia of China, Agilodocodon scansorius is preserved with a halo of dense, carbonized furs and hair impressions.  The horny claws on hands and feet are also preserved (Photo by Zhe-Xi Luo, the University of Chicago). 攀援灵巧柱齿兽－北京自然博物馆馆藏正型标本（BMNH－PM00138A-正化石板）照片。 9KB, 150px X 120px 28KB, 288px X 231px 401KB, 1200px X 962px
	Agilodocodon Reconstruction Skeletal and life style reconstructions of Agilodocodon scansorius, a docodont mammaliaform.  Agilodocodon has a skull length of 2.7 cm and body length about 13 cm (about 5 inches) and likely weighed between 27 grams and 40 grams (1 to 1.4 ounces).  The skeletal features suggest that it was an agile and active animal living on the tree.  Its incisors were specialized to feed on tree sap (exudate feeding) and its molars suggest omnivorous feeding (Illustration by April I. Neander, the University of Chicago). 攀援灵巧柱齿兽（Agilodocodon scansorius）骨骼复原和生活方式重建 （美国芝加哥大学April I. Neander） 9KB, 150px X 150px 21KB, 288px X 288px 168KB, 1200px X 1200px
	Docofossor fossil Photos of the type specimen of Docofossor brachydactylus Beijing Museum of Natural History BMNH-PM131735A (main part, left) and BMNH-PM131735B (counter part, right), as preserved in split rock slabs in part and counter-part.  Docofossor was found in lake sediments of the Jurassic Ganggou fossil site in Hebei Province of China.  The fossil of Docofossor is preserved with dense and carbonized furs around its skeleton (Photo by Zhe-Xi Luo, the University of Chicago). 短指挖掘柱齿兽－北京自然博物馆馆藏正型标本（BMNH－PM131735A正化石板和BMNH-PM131735B对化石板）照片。 1.1MB, 105px X 106px 17KB, 288px X 204px 220KB, 1200px X 849px
	Docofossor Reconstruction Skeletal and life style reconstructions of Docofossor brachydactylus.  Docofossor has a mandible length of 1.7 cm and an estimated skull-to-rump length of 7 cm (about 2.8 inches), and the animal weighed up to 16 grams (about half of an ounce).  Docofossor has robust and short finger bones, but greatly enlarged terminal phalanges (claw-bones) and massive limb bones for its tiny body size, all for digging. Docofossor lived in burrows on the lakeshore and fed on the worms and insects in the soil.  Docofossor is also unique in that it has one segment fewer of finger bone segments, than most other mammals, suggesting that it had a unique embryonic development of its hands and feet (Illustration by April I. Neander of the University of Chicago). 短指挖掘柱齿兽（Docofossor brachydactylus）骨骼复原和生活方式重建 （美国芝加哥大学April I. Neander 9KB, 150px X 150px 23KB, 288px X 288px 207KB, 1200px X 1200px
	Map of Fossil Sites Geographical locations of the new fossil discoveries. The specimen of Agilodocodon scansorius is from the Middle Jurassic Daohugou fossil site in Ningcheng County of Inner Mongolia Region of China。  The Daohugou Site is directly dated to be 165 million years.  The specimen of Docofossor brachydactylus is from the Late Jurassic Ganggou Fossil Site in Qinglong County of Hebei Province of China.  Although not directly dated, the Ganggou Site is estimated to be 160 million years old by geological correlation (Illustration by April I. Neander of the University of Chicago). 柱齿兽类新化石的野外化石地点。攀援灵巧柱齿兽发现于内蒙古宁城的中侏罗系道虎沟化石产地。短指挖掘柱齿兽发现于 河北省青龙县晚侏罗系干沟村化石产地。 5KB, 150px X 118px 11KB, 288px X 226 70KB, 1200px X 940px
	Evolutionary History of Stem and Extant Mammals Stem mammaliaforms (also known as “stem mammals”) are long-extinct relatives to the extant mammals (crown Mammalia).  Docodonts are such a lineage of stem mammaliaforms.  Their morphologies provide evidence for the ancestral mammalian condition.  Their functional adaptations provide new insight on the ecological diversification of the earliest mammals.  The newly discovered arboreal specialization and fossorial and subterranean specialization of fossil docodonts indicate that the stem mammaliaforms had very diverse feeding and locomotor functions, and a much wider ecological divergence as a whole, despite the dominance of dinosaurs in the Mesozoic (Illustration by April I. Neander of the University of Chicago). 柱齿兽哺乳型动物与现代哺乳动物的演化历史。哺乳型动物是现生哺乳动物的已绝灭的近亲支系。柱齿兽类是哺乳型动物多个支系中的一个广泛分布的支系。柱齿兽哺乳型动物的形态特征代表了哺乳动物起源演化过程中原始的形态。新发现的前所未知的柱齿兽动物的树栖和挖掘穴居生态适应类型，表明最早的原始哺乳动物更为多样，分异范围更宽 。它们多元的生态适应为哺乳动物最早期生态分异的演化机制提供了新证据和新认识。 7KB, 50px X 133px 37KB, 500px X 443px 129KB, 1200px X 1062px
	Docodont Reconstruction Movie Image reconstruction from CT scanning of the jaws of the docodont mammaliaforms Agilodocodon and Docofossor (visuals available from  April I. Neander of the University of Chicago: aisch@uchicago.edu).