Spiders represent the most diverse group of obligate predators (that is, predators that feed solely on other animals) in terrestrial ecosystems today, with almost 48,000 extant species in 118 families described to date. The number increases annually by approximately 500 species as a result of new discoveries and it has been estimated that the true diversity may number around 160,000 extant species. This great diversity is no doubt at least in part due to their geological longevity, with the oldest known fossil spider dating back to the Carboniferous. In addition, spiders appear to have co-radiated along with their insect prey over geological time and they also appear to have been relatively resistant to extinction during the major events that eliminated many other terrestrial animal groups, such as the dinosaurs (Penney and Selden, 2011).
Most people seem to presume that spiders do not have a very good fossil record on account of their very small size and their lack of a mineralised, bony skeleton. However, spiders actually have a very good fossil record, with 1,347 fossil species currently recognised. Fossil spiders occur in rocks of various different types, but the vast majority and best-preserved spiders are found as inclusions in amber from various localities dating back to the Cretaceous, although preservation tends to be better in the younger (for example, Miocene and Eocene) ambers. The best known of these deposits is Baltic amber, with more than 650 fossil spider species recognised (Penney et al., 2012), representing close to half of the known fossil spider species described to date.
The Baltic amber forest
Baltic amber is by far the most famous and richly-endowed fossiliferous amber deposit anywhere in the world, with more than 3,500 described arthropod species to date. It is often dated as mid-Eocene (that is, Lutetian, about 44 to 49 million years old) and is thought to have been produced by an umbrella pine (Sciadopitys sp.), although the identity of the Baltic amber tree is still somewhat of an enigma. In many respects, the fossil assemblage is indicative of a tropical-subtropical forest, with lightly wooded areas and plenty of freshwater habitats (Weitschat and Wichard, 2010). However, more recent work has proposed a late Eocene age and that the palaeoenvironment consisted of warm temperate humid conditions (Sadowski et al., 2017). The Baltic amber forest covered a vast area of Northern and Central Europe, so no doubt included a wide range of different habitat types and palaeoenvironmental settings.
History of Baltic amber spider research
Research on Baltic amber spiders has spanned almost two centuries, with major monographic contributions by Koch and Berendt (1854), Petrunkevitch (1942, 1958) and Wunderlich (2004). All of the spider species in Baltic amber are extinct, but the majority are assigned to extant families. There are also a few strictly fossil families (for example, Baltsuccinidae, Praetheridiidae, Protheridiidae and Succinomidae), the papers on which were self-published in non-peer-reviewed journals by Wunderlich, so these would benefit from independent scrutiny to confirm their taxonomic validity. Extinct families described by earlier workers, such as Koch and Berendt, and also Petrunkevitch, have subsequently been synonymised with extant families, apart from Insecutoridae, Spatiatoridae (Fig. 1) and Ephalmatoridae, which are still considered valid for the time being.
As a very crude estimate, spiders represent around 20% of all described Baltic amber species, yet they only represent around 4% to 6% of inclusions in a ‘random’ sample. However, the Baltic amber fauna is poorly known, with a mere 3,500 species described to date out of a predicted 193,000 species in total (Penney and Preziosi, 2014). Hence, the relative spider quotient (in terms of described fossil species) of Baltic amber is exceptionally high compared to other orders that we would expect to greatly exceed them in terms of palaeodiversity, for example, Diptera (flies, with about 800 species), Coleoptera (beetles, with 130 species), Hymenoptera (ants, bees and wasps, with 448 species) and Hemiptera (true bugs, with 111 species) (Baltic amber insect data from Weitschat and Wichard, 2010). Described Baltic amber Diptera may be closer to 1,000 species. In extant arthropod samples from trees in both temperate and tropical forests, species from these insect orders far outnumber those of spiders. Thus, the described palaeodiversity of this deposit is still rather low in terms of the remaining taxonomic groups. Of course, there may be some unknown preferential entrapment bias operating for positive selection of spiders, but this is highly unlikely (Penney, 2016). The simplest explanation is that the high species richness of spiders in Baltic amber is significantly skewed as a result of the monographic studies mentioned above, which have preferentially chosen to cover spiders, particularly those of Wunderlich, who has described many of the species.