Fossils of joint-footed animals are found in Ediacaran (Upper
Proterozoic) rocks. Trilobites—common Cambrian (early Paleozoic)
fossils having jointed legs and other arthropod features—are
placed in their own phylum and are extinct. Trilobites were the
earliest animals to have eyes. The trilobite also had jointed legs,
one pair of antennae, and a carapace (hard covering) with head,
thorax, and rear segment. (Trilobites are named for their three
body lobes.) Insects arose in the Paleozoic era. At 300 mya, cockroaches
appear in the fossil record. Arthropods—terrestrial mandibulates,
crustaceans (almost all aquatic), and chelicerates—were
traditionally thought to have evolved from annelid ancestors (A-
22), but this view is currently contested. A number of data sets
show phylogenetic grouping of arthropods with Nematoda (A-
11). Phylogenetic relationships between arthropod phyla are not
clear. Affinities of insects, centipedes, and millipedes to crustaceans
and chelicerates are far from certain at present.
Fossils of joint-footed animals are found in Ediacaran (Upper
Proterozoic) rocks. Trilobites—common Cambrian (early Paleozoic)
fossils having jointed legs and other arthropod features—are
placed in their own phylum and are extinct. Trilobites were the
earliest animals to have eyes. The trilobite also had jointed legs,
one pair of antennae, and a carapace (hard covering) with head,
thorax, and rear segment. (Trilobites are named for their three
body lobes.) Insects arose in the Paleozoic era. At 300 mya, cockroaches
appear in the fossil record. Arthropods—terrestrial mandibulates,
crustaceans (almost all aquatic), and chelicerates—were
traditionally thought to have evolved from annelid ancestors (A-
22), but this view is currently contested. A number of data sets
show phylogenetic grouping of arthropods with Nematoda (A-
11). Phylogenetic relationships between arthropod phyla are not
clear. Affinities of insects, centipedes, and millipedes to crustaceans
and chelicerates are far from certain at present.
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