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Arthropod taxonomy and systematics

Arthropods are the best represented group of organisms in Spanish amber, as occurs in all the other amber-bearing deposits worldwide. Arthropods preserved in amber have been studied unevenly depending on the groups and outcrops. Whereas the study of the amber from Peñacerrada I began in the mid 90s, the outcrop of San Just was discovered in 2005, its first paleontological excavation taking place in 2007. Work on El Soplao outcrop began later, in 2008. Even though the main arthropod groups have been recognized from all these three outcrops, the best known fauna is still the one from Peñacerrada I. To date, all known public collections with Spanish amber pieces have been examined.

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Their inclusions have been assigned to general groups to obtain data on presence and abundance in the different outcrops and to estimate the work volume necessary for studying minoritary groups. The relative abundance of some groups found in other amber localities reflects the unique characteristics of the Iberian Plate, resulting from the climatic regime and insularity during the Albian. As during the Jurassic and Cretaceous the Iberian Plate was an island, several arthropod groups show endemism, as occurs today in insular areas such as Macaronesia.

Several groups of arachnids, crustaceans, and insects have been recognized. Arachnids include mites, pseudoscorpions, and spiders. Mites are composed by the families Ametroproctidae, Archaeorchestidae, Cepheidae, Erythraeidae, and Trombididae. Spiders are represented by the families †Lagonomegopidae, Oonopidae, and Araneidae. The latter record is considered the oldest of the group (orb-web weaving spiders); orb-web fragments have also been found within the amber, some with trapped insects. This fact points out that one of the spider web types better adapted to catch flying insects most likely appeared at the same time than modern insect groups, linked with the radiation of the angiosperms. On the other hand, several crustaceans belonging to Tanaidacea (Alavatanaidae) and Isopoda have been found.

Seventeen orders of Hexapoda have been recognized, all insects except to the Collembola: Archaeognatha, Blattaria, Isoptera, Orthoptera, Mantodea, Dermaptera, Psocoptera, Thysanoptera, Hemiptera, Raphidioptera, Neuroptera, Coleop¬tera, Trichoptera, Lepidoptera, Diptera, and Hymenoptera. The most abundant group in Peñacerrada I amber is Diptera, followed by Hymenoptera. On the contrary, San Just and El Soplao ambers have the inverse relationship, first hymenopterans and then dipterans. The ensemble of biota preserved in Spanish amber includes aerial, litter, and arboreal fauna, all of them associated with different microecosystems in/on/around the ancient resin-producing trees.

Up to date, several new arthropod families have been erected based on Spanish amber material:

- Acariformes, Oribatida: Archaeorchestidae,
- Crustacea, Tanaidacea: Alavatanaidae,
- Hexapoda, Psocoptera: Archaeatropidae,
- Hymenoptera: Alavarommatidae, Radiophronidae, and Spathiopterygidae
- Diptera: Chimeromyiidae.

A total of 52 genera and 57 species of insects have been described from Spanish amber so far. A lot of specimens are waiting to be studied, and orders of insects have to be yet started to examine. Significant differences have been detected between the arthropod assemblages from the nine Spanish outcrops in which bioinclusions have been detected, although there are some similarities. The use of synchrotron imaging has opened new possibilities in the systematic and phylogenetic studies of some groups.

 

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