Distribution of Jacobsoniidae
Extant members of the family Jacobsoniidae are distributed worldwide, although they are particularly well represented in tropical areas and on oceanic islands (Fig. 1). Interestingly, they are not known from the South American landmass. Moreover, Europe and the African landmass are, until today, only represented by inclusions in Cretaceous and Eocene amber and in Holocene copal, respectively. Of the 24 current species in the family, 18 are found in isolated countries such as Papua New Guinea, New Zealand, or New Caledonia. On the continental mainland, only Sarothrias hygrophilus Pal 1998 and S. indicus Dajoz, 1978 are found in India, S. lawrencei Löbl and Burckhardt, 1988 in Australia, S. sinicus Bi and Chen, 2015 and S. songi Yin and Bi, 2018 in China, and Derolathrus cavernicolus in Florida (USA)2. Saphophagus is only know from the monotypic species S. minutus Sharp, 1886 restricted to New Zealand2. Only two species in the family show a wider distribution compared with other species in the family. Derolathrus atomus is reported from the Hawaii islands and the West Indies2, and Derolathrus cavernicolus represents the most widespread species in the family, reported from the USA (Hawaii and Florida), Barbados, Japan, and now in Holocene copal from Tanzania (Fig. 1c).
How such small beetles can show a worldwide, yet highly endemic distribution is an intriguing question. Their particular distribution is understood as the result of long-distance dispersal, which is a natural dispersal mechanism in the family. It is facilitated by air currents, aided by the beetles’ feather-like hind wings and small body size16,21. Since some species appear to feed on fungi in soil and would naturally occur in association with soil on plant roots and in leaf litter, it has been suggested that their distribution pattern may be associated with anthropogenic causes, such as navigation or the introduction of plant species from many parts of the tropical world21. However, this explanation remains unsatisfactory and raises the question of why their long-distance dispersal by means of natural or anthropogenic mechanisms would be restricted to islands or isolated land areas rather than entire continents. On the other hand, it is possible that the beetles had a much wider distribution in the past, and the current endemism is due to regional extinctions, combined with new colonisations6,16. A divergence time analysis, calibrated using fossil data, revealed an Early Jurassic origin of Jacobsoniidae, at which time they split from their sister families Ptiliidae and Hydraenidae14. The land distribution at that time indicates that the family was already widespread when the supercontinent Pangea began to separate significantly for the first time. The fossil record of the family encompasses the Tethyan and Austral realms in the Cretaceous, and the Afrotropical realm in the Holocene. This fossil distribution suggests that Jacobsoniidae is a very ancient group, and that the group was more diverse in the past than it is in the present4. It appears that the family currently has a limited distribution, as it was unknown from the African continent until the description of the specimen here. The geographically closest relative is the species D. anophthalmus, which has been reported from the Canary Islands. However, Cai et al.16 cited unstudied material and undescribed species from Africa, Australia, the Neotropics, New Caledonia, and the Solomon Islands. Such new information would help to understand the distribution of this exceptional family and complement the conclusions obtained here.
Ecology of Jacobsoniidae
There are now four fossil species known in Jacobsoniidae, all of them described from amber inclusions: one species from the Albian of France6, two species from the Cenomanian of Myanmar4,5, and one species from the Eocene of Poland16. All of these organisms were found in ambers of different gymnosperm origin. The specimen identified here as Derolathrus cavernicolus is preserved in Holocene copal from Tanzania, which has an angiosperm origin. It is the fifth species preserved in resin of a different maturity, age, and botanical origin.
The answer to the question why all of the known fossil specimens in the family were found in amber or copal deposits may be found in the fossilisation process. In this regard, the characteristics of the resins themselves, the small size of this type of beetles, and what we consider the most relevant aspect, their biology, played an essential role. Many of the specimens preserved in ambers are small. Historically, it was thought that arthropod fossilisation in amber was biased with regard to body size. However, the size distribution of arthropods preserved in diverse ambers is similar to the general body size distribution of living insects in similar environments, and the size bias is qualitatively independent of the kind of trap for non-extreme values25. For organisms such as spiders or ants, it has been demonstrated that the size of specimens enclosed in amber depends more on the complexity of the forest structure and the biology of the organism rather than resin entrapment-related biases26,27. More specifically, this means that selected taxa trapped in resins represent the fauna living in and around the resin-producing trees and appear in resins because of their ecology and behaviour, usually closely related to a tree-inhabiting life27. Still, without the size bias, the high-quality preservation of specimens in amber offers a rich record of fragile and hard-to-preserve fossils. This includes specimens that would otherwise go unnoticed in compression deposits, since it is challenging to detect fossils of such a small size. Nevertheless, it also implies that species of Jacobsoniidae had some degree of relationship with the resin-producing plants, both gymnosperms and angiosperms, since the Cretaceous.
The biology of jacobsoniids is poorly known. They have been collected from leaf litter and rotten wood7, but they are also associated with fungi and bat guano28. Sarothrias may be myrmecophilous based on an overall morphology that possibly suggests an inquilinous lifestyle28, and several specimens of S. lawrencei were collected from ants’ nests (H. Escalona, pers. obs. in4). The morphology of the group appears to have changed little since the Cretaceous5,6. Moreover, this morphological stasis may result from a stabile microhabitat throughout geological times, providing ‘refuge niches’29,30. Thus, some taxa are protected from strong selective pressures and extinction, something similarly described in different beetle species before5,18.
Biodiversity loss
Amber is a fossil material well known for its capacity to preserve biological inclusions from ancient ecosystems with remarkable fidelity31. The organisms preserved in copal and Defaunation resin are also of scientific interest32,33, because these samples document the impact, principally on arthropod biodiversity, of Recent climatic events and ecosystem changes on a shorter timescale, such as anthropogenic effects33. Nature is declining globally at unprecedented rates in human history and this loss is a direct result of human activity34. As a result, modern species no longer occur in a geographic area because the destruction of their habitat is directly or indirectly caused by human activity. In this regard, endemic species or those with a very narrow niche are particularly affected. The small family of jacobsoniid beetles, particularly found on tropical oceanic islands (Fig. 1), is unknow from the African continent, with the only exception of the specimen of Derolathrus cavernicolus from the Tanzanian Holocene copal described here (Fig. 1c). With a more widespread distribution of the species on other continents, D. cavernicolus may yet be undiscovered on the African continent, or it may already be one more example of extinction in the region.
Many modern species are well known from copal and Defaunation resin, and some species were first documented in such resins and were later discovered to be living in the same environs today. However, other species from copal and Defaunation resin remain the only known representatives of their taxon (see examples in Solórzano-Kraemer et al.33). Coastal forests of Eastern Africa are ranked among the world’s priority biodiversity hotspots35, with a high incidence of forest-obligate endemism or threatened species with narrow geographic ranges that are often endemic to a single site or forest patch36. However, at the same time it is a region of increased anthropogenic impact, inducing climate change and loss of biodiversity, which is projected to increase in magnitude37. Thus, it is likely that species in copal and resin from this region have recently become extinct due to human activity33. This emphasises the importance of investigating copal and Defaunation resin biotas as a source of data for exploring recent impacts of habitat and climate change. To confidently assess that D. cavernicolus is a recently extinct species from this region, and not a species present in the modern African fauna albeit as yet unrecognised, extensive efforts to search for individuals of this taxon should be made in these still preserved coastal forests. However, the absence of the family from some of the large landmasses on Earth (South America, Europe, and Africa) and its almost complete absence from North America, together with the highly endemic distribution of the different species, suggests that the regional extinction hypothesis is the most likely. In fact, a more widespread distribution of jacobsoniid beetles during the Cretaceous in Europe and Asia is suggested based on amber4,6. Despite their more or less widespread distribution among islands worldwide2 and their facility to spread via air currents16, it has been suggested that, at least to some extent, their current range is the result of anthropogenic introductions21. Moreover, the current biogeographical range of Jacobsoniidae may be the remnant of a more widespread distribution in the past due to extinction in large regions on Earth, with the main refugia on oceanic islands in tropical areas. The analysis of the reasons for such diversity loss and limitation of geographic distribution is a line of research that could be developed in future publications.
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