Читать книгу Parasitology - Alan Gunn - Страница 32

Most parasites are soft‐bodied organisms, and they lack the hard structural features that facilitate preservation in the fossil record. It is therefore impossible to ascertain whether parasitism has always been a common ‘lifestyle’ – although this is highly likely. Conway Morris (1981) suggested surveying the commensals, symbionts and parasites of those organisms that have remained apparently unchanged for millions of years (the so‐called living fossils) might reveal unusual organisms and provide insights into animal associations. For example, horseshoe crabs (Phylum Chelicerata, Subclass Merostomata) have existed almost unchanged for hundreds of millions of years. There is little published information on their parasites although flatworms of the family Bdellouridae only form associations with them (Riesgo et al. 2017). Despite the paucity of the fossil record, studies to date suggest that many parasite–host relationships persist for millions of years, and that parasite life cycles and morphology remain remarkably constant (Leung 2017)

Copepod ectoparasites that were morphologically similar to those in existence today have been identified attached to fossil teleost fish dating to the Lower Cretaceous Period (145–100.5 million years ago) (Cressey and Boxshall 1989). Evidence of nematode parasites is largely restricted to those infecting insects that became trapped in amber (Poinar 1984). Helminth eggs can be identified in coprolites (fossilised faeces), but while there have been extensive studies on animal and human faeces found in archaeological sites (Camacho et al. 2018), there is less data on coprolites dating back millions of years. As with any faecal analysis, one must not assume that presence indicates parasitism. An organism’s presence may result from passage through the gut following accidental consumption (e.g., eggs of a parasite of another animal) or invasion of faeces after its deposition (e.g., eggs of a detritivore). Preservation of animals following rapid mummification under desiccating conditions or freezing in tundra enables the identification of soft‐bodied parasites with greater accuracy. For example, nematodes and botfly larvae can be identified from woolly mammoths that died thousands of years ago on the Siberian tundra (Grunin 1973; Kosintsev et al. 2010).

Sometimes one can infer the presence of parasites in fossilised remains from the pathology they cause (Donovan 2015). For example, the pearls found in mussels and oysters often form because of infection by trematode parasites. Pearls thought have been caused by trematode parasites have been identified in fossil mussels dating back to the Triassic era (250–200 million years ago) (Newton 1908). Dinosaurs almost certainly had their full complement of parasites although their evidence is sadly lacking from the fossil record. However, marks found on the bones of the dinosaur Tyrannosaurus rex are thought to resemble the pathology caused by the protozoan parasite of birds Trichomonas gallinae (Wolff et al. 2009). Similarly, Tweet et al. (2016) found sufficient evidence in the fossilised gut contents of a hadrosaurid dinosaur to describe a vermiform organism that they called Parvitubilites striatus that may have been parasitic. Poinar and Poinar (2008) have even suggested that parasites were a major factor in the ultimate extinction of the dinosaurs – although this is not a widely accepted view amongst palaeontologists.