Markuelia
| Markuelia | ||||||||||||
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| Middle Cambrian | ||||||||||||
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| Scientific name | ||||||||||||
| Markuelia | ||||||||||||
| Valkov , 1983 |
Markuelia is a genus of fossil worms preservedfrom the geological period of Tommot in the middle Cambrian . There are two known species that probably the closest known relatives of the three modern animal phyla of loricifera (Loricifera) kinorhyncha (Kinorhyncha) and priapulida represent (Priapulida). Embryos of Markuelia hunanensis were foundin the Hunan Province of southern China, andthose of Markuelia secunda in eastern Siberia . The former have been preserved from almost all developmental stages from the first cell division to the time of hatching and give a unique insight not only into the embryonic development of these animals, but also into the tribal history of the entiregroup knownas Scalidophora .
The fossils
The preservation of soft-bodied animals as fossils is already extremely unusual, but the possibility of preserving embryonic life stages has been practically ruled out for a long time. For this reason, the present finds are of great scientific value.
In both species, preservation in limestone was made possible by fine-grain calcium phosphate , which preserved the soft tissues from decay either through complete replacement or through a fine film coating. Since this process must have happened very quickly after death, it is assumed that the action of bacteria played a role in it.
Sedimentary phosphate deposits and the associated extraordinary conservation conditions are found comparatively frequently in the transition period from the Ediacarian to the Cambrian - why this was the case in these early geological epochs is still almost not understood - it is possible that this early period of the ancient world was due to a completely different ocean chemistry than shaped today.
Markuelia hunanensis
The fossil embryos of Markuelia hunanensis come from the Bitiao Formation Wangcuns in the Hunan Province of southern China - the second component, the epithet, of the scientific name is also derived from the latter. The degree of conservation of the fossils varies greatly; in the best case, however, structures on the order of about 0.3 micrometers can still be made out.
The animals are at different stages of development, the dimensions of which are between approximately 240 and 410 micrometers. The earliest stage shows the embryo in the blastula stage, the dividing furrows between the individual cells, the blastomeres , the number of which is estimated at 485, are clearly visible. In later, but still spherical stages, differentiated embryo tissue and undifferentiated surface parts can be distinguished, which presumably represent yolks. The embryo now has a worm-like appearance and is curled up on the surface of the presumed yolk ball so that the head and tail come to lie next to each other. The fuselage connects the two with a double loop in the shape of an inverted S. It is covered on the outside by more than a hundred rings around 180 to 190 micrometers wide and 20 to 25 micrometers long, which, as can be seen in broken specimens, continue inwards , i.e. do not represent any surface features. The body is drawn perpendicular to the rings by fine bands that are only 0.3 to 0.5 micrometers wide.
In the tail region the rings are much less pronounced; Instead, there are six curved, 50 to 90 micrometer long spines, which in living animals were probably arranged in a ring around a trough closing off the tail, which can be interpreted as a body opening, for example an anus.
The head is in a meaningful state of preservation in only one specimen. As on the tail, the rings are very inconspicuous here; instead, as there, there are numerous spines, although somewhat shorter at around 30 micrometers. They are arranged in three annular, overlapping rows and point in the direction away from the mouth. Flattened and with a smooth surface, they were probably surrounded by an outer skin ( cuticula ), but according to the discovering scientists, they cannot be viewed as rigid extensions of the body wall. At the top of the head is the final mouth, which was measured to be 46 micrometers in diameter.
The discoverers estimate that the embryo had reached a length of about three millimeters at the time of fossilization - it was already larger than most adult animals of the modern Scalidophora. It can be considered very likely that the animals were directly developing, i.e. that there was no intermediate larval stage.
The holotype of the species is now in the Geological Museum of Peking University .
Markuelia secunda
The fossils of Markuelia secunda were first found in the 1980s by Russian paleontologists in the Eastern Siberian Pestrotsvet Formation on the Aldan River in southern Yakutia , but were only recognized as embryos late. They also date from the geological period of the Tommotium.
The spherical fossils with a diameter of about half a millimeter are very similar to Markuelia hunanensis , but are in a poorer state of preservation. Here, too, the embryos are worm-shaped, wrinkled on the outside and wrapped in loops around the spherical surface. As with Marcuelia hunanensis , the trunk is arranged in the shape of an inverted S, so that the somewhat widened head and tail regions come to lie next to each other on one hemisphere. The total length of the embryo is given as 3, the width as 0.2 to 0.4 millimeters, while the length of the approximately 75 rings is estimated to be around 30 to 50 micrometers.
The head end of Markuelia secunda is not well preserved; a total of four spike-like structures arranged in two mirror-symmetrical pairs can be made out in the tail region. At the same time, on every third segment, conical, tapering projections repeat in approximately the same position.
Inside the body of broken specimens, fine rod-like objects can be recognized, which can each be assigned to exactly one ring, but are probably not part of the body wall - this would suggest an internal segmentation of the animals. However, it is very difficult to decide whether these are internal organs, and if so, which ones, as the phosphate deposits could have falsified the original dimensions of the structures.
Also with Markuelia secunda the development was presumably direct and the embryo already reflects the basic body shape of the adult animal.
Markuelia secunda is differentiated from Markuelia hunanensis because of the mirror-symmetrical, pincer-like rather than radial arrangement of the spines ; However, their similarities seemed sufficient to the discoverers of the latter species to place both in the same genus. Markuelia secunda specimens are now in the Swedish Museum of Natural History in Stockholm.
Kinship in tribal history
For Markuelia secunda , until the discovery of Markuelia hunanensis, various modern and extinct animal groups were considered as closer relatives, including in particular the stumpworms (Onychophora), the annelids (Annelida) and the segmented Halkieriids (Halkieriida). The latter are a paraphyletic group of extinct species that are systematically believed to be on the lineage of annelids and mollusks (Mollusca).
Since it can be assumed that most of the characteristics of Markuelia hunanensis are characteristic of the genus, this made a revision of the older ideas cited above necessary: The absence of any form of body appendages, even in the late developmental stages of the embryos, is difficult to relate to reconcile with annelids or columbus. Already in the scientific literature on Marcuelia secunda , i.e. before the discovery of the second, better preserved species, attention is drawn to the lack of transverse walls ( septa ) arranged transversely to the body axis and an evolutionary correspondence between the spiky structures found and the body appendages ( Parapodia ) of annelid worms called into question.
In contrast, the orientation of the spines is different in the Halkierids and the mouth is positioned differently, not closing the body towards the front ( terminal ), as in Markuelia , but behind it ( subterminal ). The direct development of Markuelia , i.e. without an intervening larval stage , contradicts an all too close relationship with the animal groups mentioned above.
Instead, the structure, arrangement and orientation of the spines suggest a close relationship with the Scalidophora , a natural family group of hookweed (Kinorhyncha), priapworms (Priapulida) and corset animals (Loricifera); the position of the mouth is characteristic of a broader taxon called Cyclioneuralia which additionally thread (Nematoda) and hairworm comprises (Nematomorpha). A cladistic analysis resulted in the following systematic classification of the two Markuelia species:
| Cycloneuralia |
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Effects on embryology and systematics
On the one hand, the fossil finds confirm predictions made by comparative embryology and thus underline the importance embryological studies have for an understanding of the phylogenetic relationships of taxa (systematically named groups of living beings). On the other hand, they increase knowledge of the embryonic development of the Scalidophora itself - the early life history of the Markuelia species, which is more than 500 million years old, is ironically better known than that of most of their modern relatives. They also have direct relevance to modern systematics and also allow for the first time a historical comparison and thus an insight into the evolutionary development of ontogenetic , i.e. life-history processes.
The prediction of comparative embryology that the first Scalidophora developed directly into an adult animal, i.e. without going through a plankton living larval stage, was recognized as correct by the Markuelia findings; it seems to be possible to draw comparative conclusions from the life history of the modern taxa about the original course of their extinct predecessors. A detailed study of the biographical development of the Markuelia species also enables hypotheses put forward by comparative embryology to be scientifically tested for the first time on the evolution of individual characteristics or combinations of characteristics.
An important systematic consequence of the findings is that the larval stages of priapworms and corset animals have to be interpreted as derived characteristics ( apomorphies ) and perhaps even can be interpreted as common derived characteristics ( synapomorphies ). In the latter case, the controversy about the phylogenetic relationships between the three Scalidophora taxa would be resolved in the sense of a sister taxon relationship between priap worms and corset animals. A convergent development , i.e. the independent occurrence of structurally similar ( analog ), but not ( homologous ) features that cannot be traced back to a common predecessor structure , cannot yet be ruled out.
The Markuelia finds also shed new light on the phylogenetic relationships of the Scalidophora themselves , as numerous suspected homologies between the Scalidophora and their presumed sister group, the Nematoida (roundworms and string worms), are already known from the Scalidophora lineage, that is do not represent derived characteristics of the modern taxa. At the same time, this leads to the question of which of the characters of the Scalidophora are derived and which are to be regarded as original, or in other words, how much the Scalidophora have deviated from the common basic plan of all Cycloneuralia (the combination of Scalidophora and Nematoida). Since numerous characteristics can possibly also be found as evolutionary homologies in the outer group of the abdominal curls (Gastrotricha), the Scalidophora can probably be viewed as a comparatively conservative group of the Cycloneuralia and possibly even the more comprehensive taxon of molting animals (Ecdysozoa), so that one of their early Representatives like Markuelia could give an idea of the characteristics of the parent species.
Connected with this is the question of evolutionary history, whether the segmentation of the body, which is clearly recognizable, for example, in the arthropods , which include insects (Insecta), spiders (Araneae) or crustaceans (Crustacea) represents an original or a derived characteristic of molting animals. For a long time, for example, the body sections of worm-like hookweed called zonites were considered to have developed convergent. If the paleontological findings regarding Markuelia's rings that extend deep into the body are confirmed, this view may need to be reconsidered.
Paleontologically, Markuelia proves the existence of the Scalidophora group in the middle Cambrian , a fact that could previously be indirectly inferred from the knowledge of Cambrian arthropods, which form the other branch of molting animals, but which has now been confirmed independently. It also reinforces the prediction that the Nematoida lineage was also established at this point in time, but paleontological evidence from Cambrian strata in 2004 is still pending.
The findings of Markuelia thus provide a good example of how the interlinking of paleontological, embryological and paleoembryological research results in interaction with the knowledge of modern systematics can lead to a deeper understanding of evolutionary processes - both at the level of the individual organism and its ontogenesis , i.e. life-history development , as well as on the level of the community of descent and its phylogenesis , i.e. phylogenetic development.
literature
- S. Bengtson, Z. Yue: Fossilized metazoan embryos from the earliest Cambrian. In: Science . Washington DC 277.1997, p. 1645. ISSN 0096-3771
- S. Conway-Morris: Eggs and Embryos of the Cambrian. In: Bioessays. Cambridge University Press, Cambridge 20.1998, p. 678. ISSN 0265-9247
- XP. Dong, PCJ Donoguhe, H. Cheng, JB Liu: Fossil embryos from the Middle and Late Cambrian period of Hunan, south China. In: Nature . Macmillan Journals, London 427.2004, p. 237. ISSN 0028-0836
