5
host site to some extant trematodes indicates that the complex life-cycle of trematodes seen in extant taxa have existed since at least the Cretaceous.
Aside from their eggs and larval stages preserved in situ with their host in amber,
larval trematodes may also leave other type of traces in the fossil record. The larval
stages of some trematodes, namely those in the Gymnophallidae family, also infect
bivalves as a part of their life-cycles and their presence can induce pitting, iglooshaped concretions (e.g. Ituarte et al. 2005) and/or pearl formation in the shell
which are more readily fossilised (e.g. Ruiz and Lindberg 1989; Ozanne and Harries
2002; see Huntley and De Baets 2015; Huntley et al. 2021). While shell pitting in
bivalves can also be caused by other non-trematode factors (Leung 2017), the iglooshaped concretions produced by gymnophallid larvae are quite distinct and is considered to be a reliable indicator of that particular trematode lineage (Huntley and
De Baets 2015). A recent study places definitive evidence for the presence of iglooshaped concretions in the Cretaceous (Rogers et al. 2018), and while similar traces
have been found on the shells of Silurian bivalves (Liljedahl 1985), Huntley and De
Baets (2015) cautioned against interpreting those as being caused by gymnophallid
trematodes. Similarly, the presence of “blister pearls” (often represented as pits in
steinkerns) of Early Devonian ammonoids might have been caused by parasites (De
Baets et al. 2011), there is no conclusive evidence that they were caused by trematodes (De Baets et al. 2015). So while trematodes can leave potential trace fossils,
fossilised eggs in coprolites or larval stages preserved in the intermediate hosts
provide more definitive information on the likely lineage or taxonomic identity of
the fossil trematode. Since trematodes are common in extant vertebrates with 18,000
known extant species, fossils can give insight into how and when they became so
diverse and successful (De Baets et al. 2021a).
1.1.3 Nematodes (Roundworms)
Nematodes (commonly known as roundworms) are one of the most diverse and
abundant animal phyla on earth, they inhabit a wide variety of ecological niches
including parasitism (Poinar 1983). For the purpose of this chapter, I will be focusing on nematodes that infected fossil vertebrates, but for an extensive and detailed
overview of parasitic nematodes in the fossil record, readers are referred to (De
Baets et al. 2021a; Poinar 2015).
Parasitism has independently evolved at least 15 times in different nematode
lineages, parasitising invertebrates, vertebrates, and plant hosts (Blaxter and
Koutsovoulos 2015). Based on molecular phylogeny, nematodes have evolved to
parasitise vertebrate animals on four separate occasions, and that they had arisen
from arthropod-infecting taxa (Blaxter et al. 1998). A more recent molecular phylogenetic study proposed that Ascaridoidea—a diverse superfamily of nematodes
found in all major vertebrate groups—has a common ancestor dating back to the
Early Carboniferous (Li et al. 2018). Ascaridoidea is a particularly important group
of vertebrate parasites, containing over 800 known species, many of which are
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…
host site to some extant trematodes indicates that the complex life-cycle of trematodes seen in extant taxa have existed since at least the Cretaceous.
Aside from their eggs and larval stages preserved in situ with their host in amber,
larval trematodes may also leave other type of traces in the fossil record. The larval
stages of some trematodes, namely those in the Gymnophallidae family, also infect
bivalves as a part of their life-cycles and their presence can induce pitting, iglooshaped concretions (e.g. Ituarte et al. 2005) and/or pearl formation in the shell
which are more readily fossilised (e.g. Ruiz and Lindberg 1989; Ozanne and Harries
2002; see Huntley and De Baets 2015; Huntley et al. 2021). While shell pitting in
bivalves can also be caused by other non-trematode factors (Leung 2017), the iglooshaped concretions produced by gymnophallid larvae are quite distinct and is considered to be a reliable indicator of that particular trematode lineage (Huntley and
De Baets 2015). A recent study places definitive evidence for the presence of iglooshaped concretions in the Cretaceous (Rogers et al. 2018), and while similar traces
have been found on the shells of Silurian bivalves (Liljedahl 1985), Huntley and De
Baets (2015) cautioned against interpreting those as being caused by gymnophallid
trematodes. Similarly, the presence of “blister pearls” (often represented as pits in
steinkerns) of Early Devonian ammonoids might have been caused by parasites (De
Baets et al. 2011), there is no conclusive evidence that they were caused by trematodes (De Baets et al. 2015). So while trematodes can leave potential trace fossils,
fossilised eggs in coprolites or larval stages preserved in the intermediate hosts
provide more definitive information on the likely lineage or taxonomic identity of
the fossil trematode. Since trematodes are common in extant vertebrates with 18,000
known extant species, fossils can give insight into how and when they became so
diverse and successful (De Baets et al. 2021a).
1.1.3 Nematodes (Roundworms)
Nematodes (commonly known as roundworms) are one of the most diverse and
abundant animal phyla on earth, they inhabit a wide variety of ecological niches
including parasitism (Poinar 1983). For the purpose of this chapter, I will be focusing on nematodes that infected fossil vertebrates, but for an extensive and detailed
overview of parasitic nematodes in the fossil record, readers are referred to (De
Baets et al. 2021a; Poinar 2015).
Parasitism has independently evolved at least 15 times in different nematode
lineages, parasitising invertebrates, vertebrates, and plant hosts (Blaxter and
Koutsovoulos 2015). Based on molecular phylogeny, nematodes have evolved to
parasitise vertebrate animals on four separate occasions, and that they had arisen
from arthropod-infecting taxa (Blaxter et al. 1998). A more recent molecular phylogenetic study proposed that Ascaridoidea—a diverse superfamily of nematodes
found in all major vertebrate groups—has a common ancestor dating back to the
Early Carboniferous (Li et al. 2018). Ascaridoidea is a particularly important group
of vertebrate parasites, containing over 800 known species, many of which are
1 Parasites of Fossil Vertebrates: What We Know and What Can We Expect…