Mammals are a diverse and successful group of vertebrates that have colonized almost every habitat on Earth.
However, the origin and early evolution of mammals are still poorly understood, as fossil evidence is scarce and fragmentary.
Yanliao Biota: a Jurassic treasure trove
JOSEPH EID/AFP via Getty Images
The Yanliao Biota is a collection of fossils from the Middle to Late Jurassic (174 to 145 million years ago) of northeastern China, mainly from the Liaoning and Hebei provinces, as per Phys.org.
The fossils are preserved in fine-grained sedimentary rocks that were deposited in lacustrine (lake) and fluvial (river) environments.
The fossils include plants, insects, crustaceans, fish, amphibians, reptiles, dinosaurs, pterosaurs, birds, and mammals.
The Yanliao Biota is known as a Lagerstätte, a term that refers to a fossil site with exceptional preservation of soft tissues and anatomical details. The fossils are often preserved in three dimensions, with skin, feathers, fur, scales, and internal organs visible. The fossils also show evidence of behavior, such as feeding, mating, nesting, and flying.
The Yanliao Biota is one of the most important sources of information on Jurassic life in Asia.
It provides insights into the biodiversity, ecology, and evolution of various groups of organisms during this period. It also helps to fill the gap between the older fossil assemblages from the Triassic (252 to 201 million years ago) and the younger ones from the Cretaceous (145 to 66 million years ago).
Among the fossils from the Yanliao Biota, one of the most intriguing and significant groups is the early mammals and their close relatives. These include:
Haramiyidans: small rodent-like animals with long tails and specialized teeth for feeding on seeds and nuts. They are considered to be either basal mammaliaforms (stem mammals) or allotherians (a group of extinct mammals related to multituberculates).
Triconodonts: small shrew-like animals with three-cusped teeth for feeding on insects and small vertebrates. They are considered to be either basal mammaliaforms or eutriconodontans (a group of extinct mammals related to therians).
Symmetrodonts: small mole-like animals with symmetrical teeth for feeding on insects and worms. They are considered to be either basal mammaliaforms or symmetrodontans (a group of extinct mammals related to therians).
Euharamiyidans: small squirrel-like animals with long tails and complex teeth for feeding on seeds and fruits. They are considered to be either basal mammaliaforms or allotherians.
Multituberculates: small rodent-like animals with multiple tubercles on their teeth for feeding on plants. They are considered to be either basal mammaliaforms or allotherians.
Eutriconodontans: small to medium-sized carnivorous animals with three-cusped teeth for feeding on vertebrates. They are considered to be either basal therians (crown mammals) or eutriconodontans.
Trechnotherians: small to medium-sized herbivorous or omnivorous animals with diverse teeth for feeding on plants or animals. They include dryolestoids (related to marsupials) and spalacotheriids (related to placentals).
The early mammals from the Yanliao Biota show a remarkable diversity of forms and functions, indicating that they occupied various ecological niches during the Jurassic, as per WN.com.
They also show a mosaic of primitive and derived features, indicating that they represent transitional stages in the evolution of mammals from their reptilian ancestors.
The early mammals from the Yanliao Biota also provide clues about the phylogeny (evolutionary relationships) of mammals and their relatives. They help to test different hypotheses about the origin and diversification of mammals, such as:
When did mammals first appear? The oldest known mammal is Adelobasileus from the Late Triassic of Texas, dating to about 225 million years ago. However, some molecular studies suggest that mammals may have originated earlier, in the Middle Triassic or even earlier.
How are mammals related to other vertebrates? Mammals belong to a larger group of vertebrates called synapsids, which include extinct forms such as pelycosaurs and therapsids. Synapsids are distinguished by having a single opening in the skull behind the eye, called the temporal fenestra. Synapsids are closely related to another group of vertebrates called diapsids, which include reptiles and birds. Diapsids have two openings in the skull behind the eye, called the upper and lower temporal fenestrae.
How are mammals classified? Mammals are divided into three main groups: monotremes (egg-laying mammals such as platypus and echidna), marsupials (pouched mammals such as kangaroo and koala), and placentals (live-bearing mammals such as humans and elephants). These groups are based on their reproductive modes and anatomy. However, many other groups of extinct mammals do not fit into these categories, such as multituberculates, triconodonts, and symmetrodonts. These groups are classified based on their dental and skeletal features.
How did mammals diversify? Mammals diversified in response to various factors, such as environmental changes, ecological opportunities, and evolutionary innovations. Mammals evolved various adaptations, such as endothermy (warm-bloodedness), lactation (milk production), hair (insulation), heterodonty (different types of teeth), enlarged brain (intelligence), and social behavior (communication and cooperation).
