497
11.8 Discussion: Trophic Specialization and the Mid
Mesozoic Parasitoid Diversification
Several recent studies have sought to account for the emergence of parasitoids during the mid Mesozoic. One study reconciled the insect fossil record that shows an
upward, rather constant increase in family-level diversity with likely times of considerable evolutionary change resulting from key innovations that should result in a
spikier trajectory of insect diversity (Condamine et al. 2016). Two distinct types of
diversity analyses were used to reconcile both patterns of insect diversity through
time. One approach, using the entire fossil record of family-level data, recorded
distinct bursts of diversification that occurred early in insect evolution and subsequently declined gradually to a modern level, interrupted only by occasional extinction events. The second approach employed molecular phylogenetic data that
contained 82% of extant insect families and identified surges of diversification, but
only for the four, hyperdiverse holometabolous orders. Both approaches did not
detect any effect from the origin of angiosperms on insect diversity, a consequence
that has been borne out previously from several fossil diversity studies (Dmitriev
and Zherikhin 1988; Labandeira and Sepkoski Jr 1993; Jarzembowski and Ross
1996; Labandeira 2014b; but see Wilson et al. 2013). The lack of an effect of angiosperm diversity on insects also is borne out from long-term analyses of major
mouthpart types through time (Labandeira 1997, 2019; also see Nel et al. 2018).
Rather, the authors concluded that clade-specific innovations were responsible for
major diversification events that should be captured by the insect fossil record
(Condamine et al. 2016). Such innovations would have included the highly elongate, valved ovipositor of Hymenoptera; the extensible, telescoped ovipositor of
Diptera; and the host-seeking, mobile, triungulin larva of Coleoptera and planidium
larva of Diptera. Specifically, one of these events involved “… shifts within Diptera
and Hymenoptera [that] may be consistent with the development of trophically specialized habits (i.e. parasitoid) …” (Condamine et al. 2016, p. 8). A specific example may be the combination of small size, koinobiont, endoparasitoid and
superparasitoid life habits that are associated with a high rate of diversification in
particular lineages of wasps such as microgastrine Braconidae (Mardulyn and
Whitfield 1999). Given that Diptera and Hymenoptera constitute about 79% of all
parasitoid taxa in the fossil record (Table 11.2), there is reason to indicate that the
high diversification rate was an indirect reference to the MMPR.
In a separate study (Rainford and Mayhew 2015), a recent phylogeny of
Hexapoda, with age dates, was used to ascertain whether specific patterns existed
between diet and associated patterns of insect diversity such as clade richness. Two
indices of phylogenetic clustering, the net relatedness index and the nearest taxon
index, provided the total phylogenetic distance of an insect family with a particular
diet (for details see Rainford and Mayhew 2015). (The total phylogenetic distance
is the number of all pairwise differences in character states between two phylogenies.) The results of the study indicated that for the diets of detritivory, fungivory,
phytophagy, predation, parasitoidism and ectoparasitism, there were no associations
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution
11.8 Discussion: Trophic Specialization and the Mid
Mesozoic Parasitoid Diversification
Several recent studies have sought to account for the emergence of parasitoids during the mid Mesozoic. One study reconciled the insect fossil record that shows an
upward, rather constant increase in family-level diversity with likely times of considerable evolutionary change resulting from key innovations that should result in a
spikier trajectory of insect diversity (Condamine et al. 2016). Two distinct types of
diversity analyses were used to reconcile both patterns of insect diversity through
time. One approach, using the entire fossil record of family-level data, recorded
distinct bursts of diversification that occurred early in insect evolution and subsequently declined gradually to a modern level, interrupted only by occasional extinction events. The second approach employed molecular phylogenetic data that
contained 82% of extant insect families and identified surges of diversification, but
only for the four, hyperdiverse holometabolous orders. Both approaches did not
detect any effect from the origin of angiosperms on insect diversity, a consequence
that has been borne out previously from several fossil diversity studies (Dmitriev
and Zherikhin 1988; Labandeira and Sepkoski Jr 1993; Jarzembowski and Ross
1996; Labandeira 2014b; but see Wilson et al. 2013). The lack of an effect of angiosperm diversity on insects also is borne out from long-term analyses of major
mouthpart types through time (Labandeira 1997, 2019; also see Nel et al. 2018).
Rather, the authors concluded that clade-specific innovations were responsible for
major diversification events that should be captured by the insect fossil record
(Condamine et al. 2016). Such innovations would have included the highly elongate, valved ovipositor of Hymenoptera; the extensible, telescoped ovipositor of
Diptera; and the host-seeking, mobile, triungulin larva of Coleoptera and planidium
larva of Diptera. Specifically, one of these events involved “… shifts within Diptera
and Hymenoptera [that] may be consistent with the development of trophically specialized habits (i.e. parasitoid) …” (Condamine et al. 2016, p. 8). A specific example may be the combination of small size, koinobiont, endoparasitoid and
superparasitoid life habits that are associated with a high rate of diversification in
particular lineages of wasps such as microgastrine Braconidae (Mardulyn and
Whitfield 1999). Given that Diptera and Hymenoptera constitute about 79% of all
parasitoid taxa in the fossil record (Table 11.2), there is reason to indicate that the
high diversification rate was an indirect reference to the MMPR.
In a separate study (Rainford and Mayhew 2015), a recent phylogeny of
Hexapoda, with age dates, was used to ascertain whether specific patterns existed
between diet and associated patterns of insect diversity such as clade richness. Two
indices of phylogenetic clustering, the net relatedness index and the nearest taxon
index, provided the total phylogenetic distance of an insect family with a particular
diet (for details see Rainford and Mayhew 2015). (The total phylogenetic distance
is the number of all pairwise differences in character states between two phylogenies.) The results of the study indicated that for the diets of detritivory, fungivory,
phytophagy, predation, parasitoidism and ectoparasitism, there were no associations
11 The History of Insect Parasitism and the Mid-Mesozoic Parasitoid Revolution