255
Fossil evidence. Microfilaria associated with its female blackfly intermediate host
(see Poinar 2011, 2012).
Host. Female blackfly (Simuliidae)
Phylogenetic Justification. Their dimension, morphology and number speak
for their identification as filarial nematodes. Due to its body size and morphology
with rounded heads and pointed tails, they have been compared with modern
Onchocerca and Spendidofilaria infecting modern mammals and birds, respectively (Poinar 2011).
Minimum age. 34.7 Ma. The age of Baltic amber is still debated. The age of the
richest amber-bearing deposits deriving from the Prussian Formation have been
attributed to the Late Eocene microfaunistically by Kaplan et al. (1977). However,
Ritzkowski (1997) inferred a middle Eocene for the lower layer of the Prussian
Formation based on glauconitic dating of its host sediment. However, this reassignment is not generally accepted (Perkovsky et al. 2007). We hereby conservatively use the 34.7 Ma based on Kaplan et al. (1977) which is younger than the
minimum age estimate of 43.1 Ma by Ritzkowski (1997).
A summary of our exercise (see Fig. 7.5) illustrates that constraints are available
for parasitic lineages or at least free-living relatives assigned to all major nematode
clades (I-IV) proposed by Blaxter et al. (1998). Clade II can possible be traced back
to the Early Devonian, Clade III to the Upper Triassic, Clades I, IV and V at least to
the Cretaceous. Pending reinvestigations and new discoveries, fossil evidence for
additional orders can potentially be traced back to the Mesozoic or even Paleozoic.
7.4.5 Fossil Record of Nematomorphs
Adult nematomorphs are free-living in marine or freshwater, while larvae are parasitic on terrestrial or marine arthropods (Hanelt et al. 2005). The oldest confidently
identified crown-group hairworm derives from Cretaceous amber (Poinar and
Buckley 2006), which is considerably later than the inferred origin based on the
fossil record of their sister-group. The host of the oldest specimen is unknown, but
other finds have been found associated with cockroaches (Poinar 1999). Younger
finds derive from Eocene brown coal deposits (Voigt 1938) and Dominican amber
(Poinar 1999) which is now usually dated to the Miocene (Iturralde-Vinent 2001;
Iturralde-Vinent and MacPhee 1996). All forms so far reported have been attributed
to the Gordioidea—parasites of terrestrial arthropods. They are reminiscent of
mermithid nematodes, which differ in having a smooth outer cuticular layer. No
fossil forms have so far been attributed to the nectonematids, a group which exclusively parasitizes marine invertebrates.
The placement of exclusively parasitic Nematomorpha as sister to Nematoda in
molecular analyses raises the interesting question whether the common ancestor of
nematodes and nematomorph was also parasitic. However, it is clear that although
parasitism arose multiple times in nematodes, the earliest divergent lineages are
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution
Fossil evidence. Microfilaria associated with its female blackfly intermediate host
(see Poinar 2011, 2012).
Host. Female blackfly (Simuliidae)
Phylogenetic Justification. Their dimension, morphology and number speak
for their identification as filarial nematodes. Due to its body size and morphology
with rounded heads and pointed tails, they have been compared with modern
Onchocerca and Spendidofilaria infecting modern mammals and birds, respectively (Poinar 2011).
Minimum age. 34.7 Ma. The age of Baltic amber is still debated. The age of the
richest amber-bearing deposits deriving from the Prussian Formation have been
attributed to the Late Eocene microfaunistically by Kaplan et al. (1977). However,
Ritzkowski (1997) inferred a middle Eocene for the lower layer of the Prussian
Formation based on glauconitic dating of its host sediment. However, this reassignment is not generally accepted (Perkovsky et al. 2007). We hereby conservatively use the 34.7 Ma based on Kaplan et al. (1977) which is younger than the
minimum age estimate of 43.1 Ma by Ritzkowski (1997).
A summary of our exercise (see Fig. 7.5) illustrates that constraints are available
for parasitic lineages or at least free-living relatives assigned to all major nematode
clades (I-IV) proposed by Blaxter et al. (1998). Clade II can possible be traced back
to the Early Devonian, Clade III to the Upper Triassic, Clades I, IV and V at least to
the Cretaceous. Pending reinvestigations and new discoveries, fossil evidence for
additional orders can potentially be traced back to the Mesozoic or even Paleozoic.
7.4.5 Fossil Record of Nematomorphs
Adult nematomorphs are free-living in marine or freshwater, while larvae are parasitic on terrestrial or marine arthropods (Hanelt et al. 2005). The oldest confidently
identified crown-group hairworm derives from Cretaceous amber (Poinar and
Buckley 2006), which is considerably later than the inferred origin based on the
fossil record of their sister-group. The host of the oldest specimen is unknown, but
other finds have been found associated with cockroaches (Poinar 1999). Younger
finds derive from Eocene brown coal deposits (Voigt 1938) and Dominican amber
(Poinar 1999) which is now usually dated to the Miocene (Iturralde-Vinent 2001;
Iturralde-Vinent and MacPhee 1996). All forms so far reported have been attributed
to the Gordioidea—parasites of terrestrial arthropods. They are reminiscent of
mermithid nematodes, which differ in having a smooth outer cuticular layer. No
fossil forms have so far been attributed to the nectonematids, a group which exclusively parasitizes marine invertebrates.
The placement of exclusively parasitic Nematomorpha as sister to Nematoda in
molecular analyses raises the interesting question whether the common ancestor of
nematodes and nematomorph was also parasitic. However, it is clear that although
parasitism arose multiple times in nematodes, the earliest divergent lineages are
7 Fossil Constraints on the Timescale of Parasitic Helminth Evolution