Interesting Facts

Paleontologists specified the time of appearance of the first arthropods. Part one.

The International Panel of Paleontologists has published a fresh survey of data relating to the appearance in the paleontological chronicle of the first representatives of the largest type of animals – arthropods. Several disciplines, including molecular phylogenetics and tafonomy (the science of the regularities of burial of fossil remains), lead to the conclusion that the real time of occurrence of arthropods was the boundary of the Ediacaran and Cambrian periods when the so-called “Cambrian explosion” occurred. The survey authors reject the competing hypothesis of “a long hidden Precambrian evolution,” according to which the main branches of animals, including arthropods, arose much earlier.

Fig. 1. Evolutionary tree of Pan-Carts, accompanied by images of some members of this group. Detailed explanations are in the text. The detailed structure of the reduced tree in this case is unimportant, it’s just one of the versions. Among the animals depicted there are two modern ones (a slow-moving and a horseshoe) and three fossils, of which hallucigenia is close to onychophores, and Kerygmachela, Anomalocaris and Diania belong to the “stem euarropods” discussed below. It is worth mentioning that the kerygmakhel was named in honor of the great Danish philosopher Søren Kierkegaard – Kerygmachela kierkegaardi. Illustration from the article D. A Legg et al., 2013. Lobopodian phylogeny reanalised, with changes

Owners of the Earth

It is said that the famous English biologist John Burden Sanderson Haldane once found himself in the company of theologians, and they asked him: what features of the Creator are found in studying the nature of creation? Haldane, known for his materialistic views, growled: “An inordinate fondness for beetles”, quoted in the article: GE Hutchinson, 1959. Homage to Santa Rosalia or why are there so many kinds of animals?). This comic response, however, was based on biological facts. It is widely known that beetles, or coleoptera, are the most numerous detachment in the entire animal kingdom. If, for example, beetles and mammals are compared, it turns out that the number of beetles known at the moment is more than 72 times the number of mammal species. And this is despite the fact that mammals are not a detachment, but a class, that is, a group of a much higher level.

What has been said about beetles can be extended to their evolutionary relatives, from close to distant ones. Beetles are insects, which, in turn, are included in the type of arthropods. The variety of this type is truly grandiose. According to modern estimates, more than 80% of all types of multicellular animals are arthropods. Another so successful evolutionary branch is simply not in nature. Edward Osborne Wilson’s remarkable book, much of which talks about the origins of social systems in arthropods, is not without reason called “The Earth’s Owners.”

It is not surprising that the evolution of arthropods occupies many researchers. The matter is facilitated by the fact that the paleontological chronicle of arthropods is comparatively rich. Moreover, her “reading” – the work is grateful enough. According to the principle of actuality, the key to understanding the past is the present. In particular, any conclusions concerning fossil animals should ultimately necessarily be based on data on modern animals – otherwise the researcher will simply have nothing to rely on. In the case of arthropods, this principle works perfectly because at the disposal of scientists there is a huge amount of both modern and fossil forms, very diverse, but at the same time arranged more or less according to one plan. This is an ideal situation for a paleontologist, and it is natural that researchers of fossil arthropods use it in full (see,Paleoentomology in Russia ).

Arthropods are a group of organisms very suitable for testing all possible hypotheses concerning the course of evolution. For example, did most modern large groups of animals originate during the Cambrian explosion, or did their sudden appearance at the beginning of the Cambrian period involve the formation of solid skeletons, and the true evolutionary “roots” lie much deeper? Attempts to find out this, based on the material on arthropods, have already been made (see Arthropods confirm the reality of the Cambrian explosion, “Elements”, 17.11.2013). But the evolution of arthropods, especially the early ones, is extremely interesting, in and of itself, as a story whose heroes are many living beings, quite unusual from our modern point of view. Paleontology of the XXI century managed to make many discoveries in this area. However, as always, there are still unresolved issues.

Antennae, mandibles and chelicera

According to modern data, the type of arthropod consists of two large evolutionary branches.

One branch – Chelicerata (Chelicerata), which includes arachnids, horseshoes and sea spiders. They do not have antennae, but there are chelicerae – perioral appendages that end in claws, hooks or stilettos. Most Helicer, at any rate large, are predators, because for other ways of feeding their mouthpiece is not suitable.

The second branch of arthropods is called zhalovalnymi (Mandibulata). Their mouthpiece includes jaws – including mandibles, which are otherwise called mandibles – representing nothing more than strongly altered walking legs. These mandibles are fundamentally different from chelicers, which have nothing to do with their legs. The mouth cavity apparatus, equipped with a set of jaws, allows us to master the most diverse methods of nutrition, which we see on the example of members of this group – millipedes, crustaceans and especially insects. In addition, for zhuvalonosnyh very characteristic antennae, or antennas – one or two pairs. As already mentioned, Helicercal antennae do not.

Strictly speaking, a type consisting of Zhvanotnos and Heliceria is called Euarthropoda (euarropods, “real arthropods”). This name, more accurate than just “arthropods,” was proposed in 1904 by the famous English zoologist, Ray Edward Lankester. Among the signs of euartropod – a clear division of the body into divisions (tagma) and the presence of articulate limbs, inside which there are joints.

Animals, in which the articulate limbs are not and never were, do not belong to euarropods, even if on many other grounds they are close to them. There are two groups of such animals: onychophores are terrestrial creatures, sometimes called velvet worms, and slow-moving animals, the smallest water inhabitants (see, for example: The shortening of the body is quiet due to the loss of Hox genes , Elements, 03/04/2016). Both in the onychophorion, and in the throat, the body is segmented almost in the same way as in arthropods, but the limbs are not segmented. Therefore, the diagnosis of the type Euarthropoda, as well as the literal meaning of the word “arthropods”, they do not correspond.

Nevertheless, onychophores and slow-moving dogs, of course, are close relatives of euartropods. At the end of the 20th century, the euartropods, the onychophora and the thistles were formally united in the supertype Panarthropoda (lit. “all arthropods”). Panartropods are a huge evolutionary branch, which, in turn, is part of a group of molting animals (Ecdysozoa; see, for example: Chinese paleontologists have found the oldest head-worm , Elements, June 4, 2014). This is the position of arthropods in the animal world.

“Stew euarropods”

So far, speaking of arthropods and their relatives, we mentioned only modern groups. However, for a complete understanding of the evolutionary events that formed the most numerous type of animal kingdom, it is simply necessary to take into account the paleontological record, and above all the chronicle of the Cambrian period, when almost all the major evolutionary branches of animals were formed. In the Cambrian there are interesting living creatures, quite unlike their modern relatives (many of them can not be attributed to any modern type – at least without obvious stretch). The difficulty is that “soft-bodied” creatures that do not have mineral skeletons or shells are relatively rare in the fossil state, and alas, this also applies to arthropods. For their burial, campstems are necessary – clay sedimentary rocks,The early history of Metazoa is the view of a paleontologist ). Especially famous as the source of Cambrian fossils, encased in campsite, are the Canadian shales Burgess (Burgess Shale), where excavations have been going on for more than a hundred years. The shrewd fauna of Burgess is so famous that locations with such a type of conservation are designated in scientific articles by the BST abbreviation Burgess Shale-type, the “type of the Burgess slates”. Paleontology is developing rapidly now, and many BST locations are known, and in some of them the degree of preservation is simply astounding. This allowed a good study of the ancient relatives of arthropods and, as they say, shed light on their origin. True, the opening picture can not be called simple. But the more interesting it is.

Let’s take a look at the evolutionary tree (Fig. 2). According to molecular data, which in this case agree well with paleontological data, the nearest “external” relatives of euartropods, who have survived to the present day, are onychophores. In the paleontological chronicle, the onychophores – or, at least, the forms close to them – are also present. True, in the Cambrian period they were not yet terrestrial, but sea ones (see Aysheaia). These are caterpillar creatures that moved along the bottom on a set of short legs, ending with claws. It is likely that the common ancestors of the euartropod and onychophora were similar to them.

Fig. 2. Simplified evolutionary tree of arthropods and their relatives. Tihodoki, whose position is still controversial and which in any case represent an extremely specialized side branch, is not included here. Orange arrow indicates the representative of the “stem euartropod”. He can see swimming outgrowths on the trunk, faceted eyes and long, powerful segmented perioral appendages (see below in the text for these signs)

However, they are hardly the direct ancestors of euartropods (in any case, euartropods in the strict and traditional sense of the word). This is where the fun begins. On the real evolutionary tree between the branch of the onychophor and the branch of the euartropod, there is a whole set of very original completely extinct branches, whose members possessed separate “euartropos” signs, but have not yet acquired their full complex, which we see in a spider, centipede or fly. These branches could be called transitive, but we must remember that for their environment and their time, each animal related to them was a completely completed organism, perfectly inscribed in a certain ecological niche (otherwise it would not have fallen into the paleontological chronicle). Although, of course,

In modern scientific literature, the aforementioned set of evolutionary branches is commonly called the “stem group of euartropods”, or simply “stem euarthropods” (stem-Euarthropoda). To be honest, this designation can be pretty confusing: it’s the fact that the animals on which it is distributed do not belong to euarropods in the narrow sense of the word (see J. Ortega-Hernandez, 2014. Making sense of ‘lower ‘and’ upper ‘stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848). However – here we will inevitably have to use a sentence saturated with terms – cladistic systematics, which prevails in modern biology almost completely, categorically prohibits the introduction of paraphyletic taxa. To put it simply, this means that a group of organisms, embracing not a single evolutionary branch, but an entire evolutionary level, can not be considered a unit of classification and have its own scientific name (in particular, Latin). Since in this case such a group is present and somehow call it still necessary, the researchers use a temporary designation that was considered correct for formal reasons. We too – where to go – will use this designation, leaving it in quotes and remembering that from the point of view of classical zoology, “stem euarropods” are not euarropods.

The position of the “stem euartropod” on the evolutionary tree is shown in Fig. 2. If for a while to leave aside strict terminology, we can say that “stem euarropods” is an evolutionary level that covers part of the tree above the onychophora and below the euartropod in the narrow sense. What happened at this evolutionary level?

A lot of interesting things happened there. “Stem euartropody” can impress anyone’s imagination – so unusual and diverse is their appearance (see Figures 1 and 3). Their articulate body was often decorated with rows of gill-like, wing-like or blade-like outgrowths that could serve as fins. Chitinous head shields appeared, which in some groups became bivalves. The walking limbs varied throughout the entire range from primitive “papillae” (or lack thereof) to complex long legs, segmented, and sometimes bifurcated, like crustaceans. An incredible variety was found in the circumoral appendages: articular or non-segmented, rudimentary or very powerful, short or long, grasping or filtering, sometimes branching, and sometimes with claws, hooks, thorns or tentacles. Finally, in a number of forms, huge complex facetted eyes appeared, approximately the same as those of modern insects, and sometimes sitting on stems. And all these signs entered into many combinations, often unexpected for modern zoologists.

Fig. 3. Panartropoda from the shores of Burgess. A – close to modern onychophores Aysheaia. B – Opabinia, the owner of fin outgrowths, faceted eyes and a segmented trunk, in this case bent under the trunk. C – Hurdia, from which only the oral apparatus with a concentric arrangement of chitinous teeth is photographed (in real arthropods nothing of the kind happens). D-Anomalocaris. Long, segmented perioral appendages are well visible. E is another Anomalocaris. White arrows indicate segmental muscles, black arrows indicate glandular outgrowth of the intestine. F – Leanchoilia. Peri-lateral appendages are long and branched, these branches serve as feelers. G – Perspicaris, owner of a double-winged head shield. H, I, J – Helmetia, Sidneyia and Emeraldella, probable relatives of trilobites. The length of the scale ruler is 5 mm on A,

Source: Allison C. Daley, Jonathan B. Antcliffe, Harriet B. Drage, and Stephen Pates. Early fossil record of Euarthropoda and the Cambrian Explosion // Proceedings of the National Academy of Sciences . 2018. V. 115. No. 21. P. 5323-5331. DOI: 10.1073 / pnas.1719962115.

Sergey Yastrebov http: // …

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