Docodonta

Docodonta
Castorocauda , living reconstruction
Temporal occurrence
Middle Jura to Upper Cretaceous ?
165 to 70 million years
Locations
Eurasia, North America, South America
Systematics
Synapsids (Synapsida) Therapsids (Therapsida) Cynodontia Eucynodontia Mammaliaformes Docodonta
Scientific name
Docodonta
Kretzoi , 1946

The docodonta are a group of extinct mammalian ancestors ( Mammaliaformes ) that lived in the Jurassic and Cretaceous periods . The best-known representative of this group is probably the genus Castorocauda, which was discovered in 2004 and described for the first time in 2006 and adapted to life in water .

description

The most prominent features of the docodonta were the widened molars (molars) with complex chewing surfaces. These teeth made it possible to chew more strongly than other mammals. The teeth are usually only around 0.5 to 1.5 millimeters in size. It is controversial for which food these teeth were optimized, a herbivorous or omnivorous diet is discussed. Castorocauda, on the other hand, could have fed on fish.

Apart from teeth or jaw parts, nothing is known of many species. Of Haldanodon, however, a nearly complete skeleton was discovered, the animal had very stocky and horizontally oriented limbs. This construction has led to speculation about an underground burrowing or swimming way of life, the second possibility having become more probable since the discovery of Castorocauda with its clear adaptation to aquatic life.

Development history

Almost all finds of the Docodonta date from the period between the Middle and Upper Jurassic or the earliest Cretaceous and are known almost exclusively from Europe, Asia and North America, which suggests that this group emerged from Pangea via the former continent Laurasia , the northern one Large continent, was common. However, there are finds of teeth and jaw parts from the Upper Cretaceous Period from South America. The genus in question was called Reigitherium and could be an indication that the docodonta may have survived longer in isolated regions. However, the finds are very poorly preserved, which makes a clear systematic assignment difficult.

External system

Due to the primeval structure of the temporomandibular joint , the docodonta are counted among a series of animals that have advanced mammal-like characteristics, but still differ in details from today's mammals and are therefore summarized as Mammaliaformes (mammalian-like) or as Mammalia sensu lato (in a broader sense) become. Whether they are already referred to as mammals or still as mammalian ancestors is a question of definition. Cladistic analyzes classify them as more developed than the Morganucodonta or Sinoconodon , but more primitive than Hadrocodium or the actual mammals.

The Castorocauda finds were surprising in that conspicuous specializations in mammals were much older than previously thought.

Known genera

• Haldanodon is a comparatively original representative of the Docodonta. A nearly complete skeleton was discovered in the Guimarota coal minein Portugal in the 1970s, along with numerous skulls and teeth. The genus is dated to an age of around 152 million years and was the first known mammal from the Upper Jurassic to be more than scarce. As mentioned above, the stocky, horizontally attached limbs could indicate a water-dwelling way of life.
• Docodon also lived in the Upper Jurassic, finds (by Othniel Charles Marsh ) are known from North America and possibly England. The genus shows particularly widened molars, but little else is known about them.
• Castorocauda from the Upper Jurassic was discovered in China in 2004 and is remarkable for its adaptations to aquatic life and its size.
• Reigitherium is known for a jaw part with three teeth that wasfoundin Patagonia and comes from the Upper Cretaceous (around 70 million years ago). Both the lower age and the location contradict the other representatives of this group. The heavy wear of the teeth and the poor condition of the finds do not allow an unequivocal systematic classification. As already mentioned, it is conceivable that the find could be an indication that the group survived longer in some regions.
• Docofossor was described in 2015 using an almost complete skeleton from the Upper Jurassic of China . It showed clear adaptations to a burrowing ( fossorial ) way of life, as shown by the elongated upper articular process of the ulna (the olecranon ) and the broad, shovel-like claws. It measured 9 cm from snout to pelvis and weighed between 13 and 17 g. The reduced number of finger and toe links is reminiscent of the gold mole in Africa, which also lives underground.
• Agilodocodon was a tree climber, which can be recognized, among other things, by the elongated and laterally narrowed claws, the mobile elbow and possibly also by the very agile tail. In addition, the structure of the teeth shows an adaptation to a plant-based diet, which has not yet been observed within the docodonta. The genus was also described in 2015 using a well-preserved skeleton. This comes from the Middle Jurassic of China. The animals were 14 cm in total length (including tail) and weighed an estimated 27 to 40 g.
• Microdocodon was first described in 2019 and lived in the late Middle Jurassic between 166 and 164 million years ago. The find of an almost complete skeleton comes from Daohugou in northeast China. The animals weighed only 5 to 9 g, had a slender body with an extremely long tail and, according to the proportion of their limbs, presumably lived in trees. The fully developed hyoid bone was U-shaped and articulated, as in modern mammals. It thus indicates that even the primeval, mammal-like forms could chew their food and swallow it in dosed portions, unlike most other vertebrates, which usually devour their food completely or in large portions. Compared to the hyoid bone, which appears modern, the bones of the middle ear were still primitively designed and, as in other early mammal-like forms, connected to the lower jaw.

Individual evidence

1. ↑ University of Tübingen ( Memento of the original from September 15, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. : Scanning electron microscope images of docodontic teeth  
2. ↑ University of Tübingen  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. : Model for determining the chewing movements of Docoodonten  
3. ↑ Zhe-Xi Luo, Qing-Jin Meng, Qiang Ji, Di Liu, Yu-Guang Zhang and April I. Neander: Evolutionary development in basal mammaliaforms as revealed by a docodontan. Science 347, 2015, pp. 760-764
4. ↑ Qing-Jin Meng, Qiang Ji, Yu-Guang Zhang, Di Liu, David M. Grossnickle and Zhe-Xi Luo: An arboreal docodont from the Jurassic and mammaliaform ecological diversification. Science 347, 2015, pp. 764-768
5. ↑ Chang-Fu Zhou, Bhart-Anjan S. Bhullar, April I. Neander, Thomas Martin and Zhe-Xi Luo: New Jurassic mammaliaform sheds light on early evolution of mammal-like hyoid bones. Science 365, 2019, pp. 276–279, doi: 10.1126 / science.aau9345

literature

• Thomas S. Kemp: The Origin & Evolution of Mammals. Oxford University Press, Oxford 2005, ISBN 0-19-850761-5 .

Web links

• Docodontic cladogram