Evolutionary History

The earliest beetlelike insects are known from Lower Permian (280 mya) fossil deposits in Moravia, Czech Republic, and the Ural Mountains of Russia. These insects, classified in the family Tshekardocoleidae, order Protocoleoptera, resemble present-day species of the archostematan families Ommatidae and Cupedidae. They differ from true beetles in having 13-segmented antennae, elytra with more well-developed venation and more irregular longitudinal ribbing, and an abdomen and ovipositor extending beyond the apex of the elytra (Fig. 1).

We now date the origin of true Coleoptera as Triassic, about 240 mya. These fossils exhibit the coleopteran 11-segmented antennae, have more regular longitudinal ribbing on the elytra, and possess internal genitalia (Fig. 2). The earliest fossil beetle faunas have been described from Queensland in Australia, South Africa, and central Asia. The four lineages now recognized as suborders appear to have been extant at this time. The Archostemata were represented by species assignable to Ommatidae and Cupedidae, plus others belonging to families not lasting past the Mesozoic. The Adephaga included species sharing enlarged hind coxal plates such as are seen in present-day Haliplidae, plus other ground beetle-like species of Trachypachidae. Myxophagan ancestors included a variety of genera in the extinct families Catiniidae and Schizophoridae. The currently dominant suborder Polyphaga was represented in these faunas by members of the Elateroidea and Curculionoidea. These earliest beetles inhabited a world made up of early forked-leaved pteridosperms, lycopods, cycads, gingkos, and early conifers. The large animals of these communities included therapsid reptiles and dinosaurs; however, neither birds nor true mammals had yet evolved.

During the Jurassic period (210-145 mya), known family-level beetle diversity increased dramatically. Among the Adephaga, first appearances are documented for the whirlygig beetle family Gyrinidae, the ground beetle family Carabidae, and the predaceous diving beetle family

FIGURE 3 Strict-consensus estimate of the phylogeny of Chrysomeloidea and outgroups, with host groups mapped onto the cladogram. Numbers of synapomorphies/bootstrap values exceeding 50% shown along branches. Colors indicate major host group attributable to common ancestor of each group (green, Coniferae; mustard, Cycadales; red, dicotyledonous angiosperms; blue, monocotyledonous angiosperms; black, do not feed on living plants). Approximate ages of Mesozoic and early Tertiary fossils only are indicated where known, since almost all subfamily groups are present in the mid-Tertiary fossil record. [Redrawn with permission from Farrell, B. D. (1998). "Inordinate fondness" explained: Why are there so many beetles? Science 281, 555-559. © 1998 American Association for the Advancement of Science.]

FIGURE 3 Strict-consensus estimate of the phylogeny of Chrysomeloidea and outgroups, with host groups mapped onto the cladogram. Numbers of synapomorphies/bootstrap values exceeding 50% shown along branches. Colors indicate major host group attributable to common ancestor of each group (green, Coniferae; mustard, Cycadales; red, dicotyledonous angiosperms; blue, monocotyledonous angiosperms; black, do not feed on living plants). Approximate ages of Mesozoic and early Tertiary fossils only are indicated where known, since almost all subfamily groups are present in the mid-Tertiary fossil record. [Redrawn with permission from Farrell, B. D. (1998). "Inordinate fondness" explained: Why are there so many beetles? Science 281, 555-559. © 1998 American Association for the Advancement of Science.]

Dytiscidae. In all three families, the predaceous habit would be considered to be the ancestral condition. Among Polyphaga, the major families Staphylinidae, Scarabaeidae, Tenebrionidae, and Chrysomelidae are first documented. Other earliest occurrences include members of the scavenging water beetles (Hydrophilidae), carrion beetles (Silphidae), ovoid bark-gnawing beetles (Trogossitidae), tumbling flower beetles (Mordellidae), sap beetles (Nitidulidae), and false blister beetles (Oedemeridae). Of these, Scarabaeidae, Chrysomelidae, Oedemeridae, Mordellidae, plus the Triassic-aged Curculionoidea are strictly phytophagous or saprophagous. Members of the large, diverse present-day assemblage of Chrysomelidae use a broad diversity of plant hosts, ranging from cycads to conifers to angiosperms. Based on a phylogenetic hypothesis derived from extant species, the basal chrysomelid lineages are associated with primitive conifers (Araucaria spp.) and cycads (Fig. 3). The

FIGURE 4 World distribution of Derodontidae. Areas supporting species include North America, Europe, Siberia, Japan, the Valdivian forest of Chile, and the South Island of New Zealand. [From Crowson, R. W. (1981). "The Biology of Coleoptera," p. 349, Fig. 2. Academic Press, London.]

Curculionoidea, the sister group to chrysomeloids, also exhibits this ancestral association with conifers and cycads. Third, the larvae of present-day Oedemeridae are borers in conifers. Thus it appears that at least several lineages of phytophagous Coleoptera were in place before the evolutionary advent of the angiosperms.

The Cretaceous witnessed initiation of the most recent round of southern landmass fragmentation, via the opening of the southern Atlantic Ocean and the isolation of New Zealand. South America and Antarctica plus Australia became progressively isolated from Africa, although they maintained contact with one another. Beetle families responded to this pattern of vicariance, with relictual distributions of several extant taxa supporting their origin during this time (Fig. 4). Continuing vicariance of the southern portions of Gondwana continued into early Tertiary, with progressive isolation of Australia, and finally the separation of Antarctica and South America at the start of the Oligocene (38 mya). This last event permitted formation of the circum-Antarctic current, helping plunge the world into a latitudinally zonated climate similar to that of today.

Preservation of beetles in amber has provided unparalleled levels of information about extinct taxa. The deposits of Baltic amber dated at 35 to 50 mya, and Dominican amber dated 15 to 40 mya, open windows onto the transition from the tropical world of the Eocene to the climatically zonated world of today. Most often, amber fossils (Fig. 5) indicate historically broader distributions for taxa presently known from only one continent (Fig. 6). This range contraction, continuing from the Eocene until the present day, suggests one explanation for the current latitudinal pattern of biodiversity. Many of the tropically adapted groups of organisms, of which beetles count significantly, have been progressively excluded from higher latitudes through the advent of cool to cold higher latitude climes, followed by the dramatic climatic perturbations associated with Pleistocene glaciation. G. Russell Coope goes so far as to argue that Pleistocene glaciation has put a halt to speciation of beetles in the temperate zones most influenced by the glaciation. His argument is based a simple fact: as he and his students studied subfossil beetle bits interred in wetland peats throughout various portions of Europe and North America, they found that all species taken from deposits younger than Pliocene could be identified as currently extant. These findings contrast starkly with those from tropical island systems, where speciation may have occurred in far younger areas. In Hawaii, for example, cave-adapted carabid beetles with reduced eyes and elongate legs have evolved from fully eyed, short-legged, epigean ancestors on the younger volcanoes of East Maui and Hawaii Island, which respectively broke the ocean surface no earlier than 750,000 and 430,000 years ago. Numerous Hawaiian beetle radiations in the Carabidae, Anobiidae, Nitidulidae, Cerambycidae, and Curculionidae demonstrate the many rapid and extensive bouts of speciation that occur in newly evolving tropical island communities.

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