Fig. 1. Three Jurassic animal – like a squirrel, tree shrews or lemur Agilodocodon, digging krotopodobny Docofossor and waterfowl, like an otter or beaver Castorocauda – demonstrate the diversity of morphology and ways of life of early mammals. Drawing one of the authors of one of the items discussed, April Isch Neander from the University of Chicago, from a site sci-news.com
Fossil remains of two primitive mammals found in the mid- and late Jurassic deposits of northeastern China, showed that the Jurassic beasts were more diverse than previously thought. One of the new species led a burrowing lifestyle and remotely resembled a mole; the second lived in the trees and looked like a modern squirrel or tree shrews. Both species belong to dokodontam – a dead-end branch of the basal mammals consisting in a more distant relationship with placental and marsupial than the platypus and echidna. Apparently, many adaptations, such as the reduction of the lumbar ribs, and phalanges have evolved independently in the different groups of mammals based on the same genetic mechanisms.
It was once thought that almost all Mesozoic mammals were small, inconspicuous and monotonous, like shrews. However, the findings of the last decades, many of which were made in China, denied this view. “Elements” talked about some of these discoveries, including the amazing waterfowl “bobrohvoste» Castorocauda from the group dokodontov (Docodonta), which resembled an otter Borba and both could eat fish (see ref. At the end of the news).
Chinese and American paleontologists reported in the latest issue of the journal Science just two remarkable finds made in the north-east of China. The findings showed that the ecological and morphological diversity of Jurassic mammals were even higher than previously thought.
Both new kind as mentioned above bobrohvost refer to dokodontam. This group, according to many experts, is one of the deadlock (who left no living descendants) basal branches, separated from the common trunk mammal before, the latter divided into the ancestors of modern monotremes (platypus, echidna) and terievyh (marsupials and placental) . However, there are others, including radically different versions of the classification of mammals and their ancestors (see .: MF Ivakhnenko. The problem of transition Theromorpha – Mammalia).
The first of two newly discovered animals called Docofossor brachydactylus, meaning “digging shirokopaly dokodont.” His bones were found in Late Jurassic deposits, Hebei Province (Fig. 2).
Fig. 2. Docofossor brachydactylus: general view of the findings (A), the portrayal of what remains of the skull (B), and the reconstruction of the skeleton (S). The image of the article under discussion Zhe-Xi Luo et al. in Science
Dokofossor – mammal a length of about 9 cm (from nose to base of tail) and weighing 13-17 In the structure of the skull and teeth are, on the one hand, primitive features characteristic of other mammalian basal (mammaliaform, mammaliaformes), on the other – specific features characteristic of modern burrowing animals such as marsupial moles and golden mole. The most convincing evidence in favor of digging lifestyle revealed in the structure of the limbs. Paws dokofossora – a specialized foot underground dweller. In particular, his fingers are made up of only two phalanges (instead of three, as in most ancient and modern mammals), and the terminal phalanx enlarged, expanded like a shovel, and a special projection at the base prevents the terminal phalanges folded back. All this – typical signs of burrowing animals. For example, reducing the number of phalanges is typical for modern golden mole, fingers that look like fingers dokofossora (Fig. 3). This is a typical example of a parallel evolution, i.e. forming an independent similar adaptations in different groups of animals, in this case – at basal mammals (dokodontov) and placenta.
Fig. 3. Evolutionary Tree dokodontov (A), the reconstruction of wood dokodonta Agilodocodon (B), its paw (C), floating dokodont Castorocauda (D), newfound digging dokodont Docofossor (E). Right (F) – comparison of the proportions of the phalanges in mammals living in trees (arboreal), do not live in trees all the time, but it’s good climbing (scansorial), land-based (terrestrial), burrowing (fossorial) and underground inhabitants (subterranean). Shown fingers following animals (top to bottom): flying lemurs (Cynocephalus), lemur (Lemur), Agilodocodon, opossum (Didelphis), hedgehog (Erinaceus), a viper (Tachyglossus), Docofossor (BMNH131735), golden mole (Chrysochloris). The image of the article under discussion Qing-Jin Meng et al. in Science
The authors suggest that the basis for such convergence are common genetic mechanisms responsible for regulation of the limb development in embryos of terrestrial vertebrates. At the golden mole in the course of embryonic development in the future fingers formed the beginnings of the three phalanges, but then two of them (the proximal and middle) merge. In mice and humans the same phenotype (merger phalanges) sometimes occurs because of the disruption of the gene regulatory cascades that control tab of the joint between the phalanges. Among the regulators involved in this process include proteins BMP2 (bone morphogenetic protein 2), GDF5 (growth and differentiation factor 5) and several others. These regulators are likely to manage the development of fingers and Jurassic mammals, and similar variations in their work caused by mutations that lead to a similar phenotype changes – for example, to merge the phalanges.
Likewise, it explains the parallel evolution of another important feature – lumbar ribs and lumbar spine. Modern mammals lumbar vertebrae ribs there, and lumbar spine morphology clearly different from the breast-carrying ribs. Loss of lumbar ribs and separation of the lumbar considered an adaptation associated with increased mobility and flexibility of the back of the body and the intensification of respiration. According to the fossil record, in different groups of early mammals reduction of lumbar ribs (partial or complete) occurred in parallel and independently.
New findings have shown that this is true for dokodontov. In dokofossora, Castorocauda and other dokodontov has lumbar ribs, the size of which gradually decreases as it approaches the sacrum. It seems, the original (ancestral) condition tag for dokodontov and all mammals. However, the second newly discovered dokodonta, agilodokodona (See below.) Lumbar ribs No lumbar clearly distinguished from breast. This implies that the evolutionary lineage dokodontov, as in the other groups of early mammalian occurred independent reduction lumbar ribs.
The reason lumbar ribs easily lost, and sometimes may appear again, presumably lies in the fact that the genetic regulatory cascade manage bookmarks of ribs in embryogenesis, the same for all mammals. A key role is played by genes Hox9, Hox10, Myf5, Myf6 and others. Changes in the work of a small number of genes may be sufficient for a radical change in phenotype. For example, you can get the mouse embryos developed lumbar ribs, if you turn off all three copies of the gene Hox10 (see .: New fossil discovery sheds light on the early evolution of mammals, “Elements”, 17.03.2007).
The second ancient mammals, called Agilodocodon scansorius («agile dokodont climbing”), found in the Middle Jurassic of Inner Mongolia (Fig. 4). This dokodont was also small (14 cm from nose to tail, the weight of 27-40 g). Judging by the structure of the spine and extremities, agilodokodon was well adapted to climbing trees. In particular, the proportion of his fingers point to the arboreal (Fig. 3). All other known dokodonty were either ground or floating (bobrohvost), or, as we now know, digging. Thus, the finding broadens understanding about the range of environmental adaptations dokodontov.
Fig. 4. Agilodocodon scansorius: reconstruction (A), drawing the skeleton (B), and a general view of the findings (C). The image of the article under discussion Qing-Jin Meng et al. in Science
The structure of the teeth agilodokodona indicates a mixed diet, which included not only insects and other small animals, but plant foods. On grounds such as the shape of the molars and the relief of the surface, similar to agilodokodon some lemurs, lorises and Galago – primates eat insects, fruits, gums, and plant juices. The similarity with primates can also be seen in the structure of the paws (Fig. 3), which is not so surprising when you consider that primates – originally a group of wood. Other dokodontov in the structure of molars is not so clear-cut signs of adapting to plant foods.
Cutters agilodokodona have an unusual shape: they look like a wide, pointed at the end of a shovel or a spoon, strongly convex on the outside (from the mouth) and concave on the inside (by the language). The authors note that the incisors are similar in some American monkeys: marmosets, spider monkeys, howler. These monkeys are used for gnawing incisors bark to get to the juice and sweet selections. Apparently agilodokodon also earn their living in this way.
New findings have shown that the ability to convergent evolution of similar systems of life forms and the parallel development of the same sets of ecological niches is typical not only for the “higher” terievyh mammals (marsupials and placental). This ability has had basal mammaliaformy who had to “invent” its lemurs, otters, moles and long before the heyday terievyh.