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ECDYSOZOA: ARTHROPODS 381
35
30
25
Number of orders 20
15
10
5
0
S4 D1 D2 D3 C1 C2 P1 P2 Tr1 Tr2 Tr3 J1 J2 J3 K1 K2 Pal E O M Pli Q R
Origination Extinction Diversity
Figure 14.19 Ranges of selected insect orders. Geological period abbreviations are standard, running
from Silurian (S) to Recent (R). (Based on Jarzembowski, E.A. & Ross, A.J. 1996. Geol. Soc. Spec.
Publ. 102.)
Carboniferous a very diverse insect fauna had
SUBPHYLUM HEXAPODA
evolved, with forms such as the dragonfl ies
The Hexapoda, essentially the insects, can be and mayflies capable of powered fl ight (Box
divided into pterygotes (with wings) and 14.7). By the end of the Permian, most of the
apterygotes (without wings) and include the familiar insect orders had appeared. During
springtails, dragonfl ies, cockroaches and the later Mesozoic and Cenozoic, signifi cant
locusts. The group may prove to have as many coevolutionary relationships were established
as 10 million living species when the rich between plants and insects, particularly
faunas of the tropics have been completely between flowering plants and insect pollina-
described. The subphylum also includes the tors, and possibly even between spiders and
onychophorans, with fl exible segmented flies (see Box 14.6). Moreover, by the Miocene,
bodies and unjointed limbs propelled by fossil hair trapped in amber together with the
changes in blood pressure analogous to the sand fl y Lutzomyia suggests that these blood
water vascular system of the sea urchins. The suckers were already feeding on mammals
hexapods have unbranched or uniramous in arboreal nests during the Mid Tertiary
appendages, a simple gut, a single pair of (Peñalver & Grimaldi 2006).
antennae and a pair of mandibles, together
with a toughened head capsule. Insects have
six limbs.
The oldest insect is probably the springtail SUBPHYLUM CRUSTACEA
Rhyniella praecursor from the Lower Devo- As the name suggests, the crustaceans have a
nian Rhynie Chert of the Orcadian Basin hard, crusty carapace. The group, which fi rst
of northeast Scotland (Fig. 14.19). Conrad appeared in the Cambrian, is aquatic, mainly
Labandeira (Smithsonian Institution, Wash- marine, with gills, mandibles, two pairs of
ington) and his colleagues have shown that antennae and stalked compound eyes. The
insects diversified earlier than had been heavily armored crabs and lobsters typify this
thought (Labandeira 2006), and the group diverse subphylum; but the barnacles and
probably originated in freshwater during the ostracodes are also crustaceans with a notable
Late Silurian, which may account for the poor geological record.
fossil record of the group before the Devonian There are at least eight main classes of crus-
(Glenner et al. 2006). Early and Mid Devo- tacean, but with the exception of the ostra-
nian faunas are now well known from Rhynie, codes, which are usually considered part of
Gaspé, Québec and Gilboa, New York State the microfauna, only two groups, the Cirripe-
and these probably coincided with the diver- dia and the Malacostraca have signifi cant
sification of land plants. And by the Late geological records.
