What Is an Arthropod?

Transcription

1 What Is an Arthropod? 8 Iagine: you turn over a rock in your backyard, and little squiry, crawly creatures race around trying to escape. In the flurry of jointed legs and shiny, arored bodies, you can see ants, spiders, centipedes, a daddy longlegs, and ore. Do you run screaing back into the house, or do you grab the nearest agnifying glass for a closer look? Many peo are grossed out by bugs of any kind, but the truth is, insects and their relatives are aazing and necessary creatures that perfor any iportant (and soeties yucky!) jobs on our planet. They are extreely flexible, adapting not only to eet drastic changes on Earth but even to life in and around huan cities and towns. Scientifically speaking, insects are arthropods (ARTH-rowpods), which eans jointed leg. One look at an ant or a centipede, and you can see why. Other arthropods are related to insects, like scorpions, crabs, and lobsters. About 570 illion years ago, during the beginning of the Paleozoic era, any strange and aazing arthropods were evolving in the oceans. (Nothing lived on land yet!) Paleontologists refer to this tie as the Cabrian explosion because new creatures appeared rapidly. Soe were arthropods, but scientists are puzzled by others. Many of those ancient arthropods were unique, eaning that they are not related to present-day arthropods. Over tie the creatures spread through the oceans and onto land. Soe evolved wings and were the first anials on Earth to fly. Arthropods today live in soe of the ost hostile conditions on our planet. Aazingly, soe, like the horseshoe crab and the lowly cockroach, haven t changed uch since they first developed hundreds of illions of years ago. The ability to adapt and survive in a changing environent has ade insects and other arthropods soe of the ost successful creatures ever to live on Earth. Scientists classify arthropods into five ajor groups, based on the special characteristics each group has: yriapods (MEER-ee-ah-pods) Includes illipedes and centipedes Bodies are divided into the head and the trunk, which is ade up of lots of identical segents Each segent has the sae set of organs and either one pair of legs (for centipedes) or two pairs of legs (for illipedes) Heads have one pair of antennae chelicerates (kuh-liss er-uts) Includes spiders, ites, and scorpions Bodies are divided into two parts: the head or cephalothorax (seff-a-low- THOR-ax) and the abdoen Heads have no antennae, but have two sets of libs called chelicerae (kuh-lisser-ee) and pedipalps (PED-ih-palps) used for sensing the environent or catching food crustaceans (crus-tay-shuns) Includes crabs, lobsters, shrip, and isopods (EYE-so-pods), like pill bugs Bodies are divided into two parts: the cephalothorax and the abdoen Heads have two pairs of antennae, and three pairs of libs around their ouths insects Includes beetles, ants, bees, and wasps Bodies are divided into three parts: head, thorax, and abdoen Heads have one pair of antennae Thoraxes have three pairs of libs and usually one or two pairs of wings trilobites (TRY-low-bites) Extinct, crawled and swa through the Paleozoic oceans for 300 illion years Bodies had three parts: the cephalon (SEFF-uh-lon), or head; the thorax; and the pygidiu (pie-jid-ee-u), or tail Bodies also divided into three parts along their length (trilobite eans three lobes ) One pair of antennae on their heads Many pairs of walking libs with gills on their thoraxes

2 The Characteristics of Arthropods Arthropods, both extinct and living, have a few things in coon: Arthropods are bilaterally syetrical, which eans that the left side of their body is the irror iage of the right side. Arthropods are invertebrates (in-vert-uh-brits), which eans they do not have backbones. Their hard exoskeleton provides a frae to which their uscles are attached. Arthropods bodies are divided into segents. The segents often contain identical sets of internal organs. Insects have three body sections, while spiders have two. Arthropods have an exoskeleton (ex-ohskell-uh-tin), a hard shell, or casing, on the outside of their bodies that provides structure and protection. This is very different fro aals, reptiles, aphibians, and fish, which have skeletons inside their bodies. To grow, an arthropod ust olt, or shed, its exoskeleton and replace it with a larger one. Arthropods have flexible, jointed legs. Muscles attached to either side of the joint ake the leg ove. Arthropods have any pairs of libs. In soe arthropods like spiders, crabs, and insects, soe of these libs have evolved into special tools for eating, grasping, and sensing the environent. Different arthropods have different nubers of libs; insects have six libs for oveent, while spiders have eight. K070026_p /30/07 9:30:36 PM

3 Paleozoic Arthropods and Their Relatives The Shape of Things to Coe Suddenly, spiny legs grab unsuspecting Opabinia. nctacaris, a 4-inch-long swiing arthropod another hunter in these Cabrian seas has just caught its lunch. nctacaris waves its paddleshaped libs softly to hold its position. nctacaris has a large head shield, overlapping aror plates along its back, and a tail that looks soething like that of a lobster. nctacaris is considered an early eber of the chelicerate (kuh-liss-er-ut) group of arthropods, which includes our odern-day spiders and scorpions. nctacaris spiny grasping ars feed the struggling Opabinia into its outh. It starts to crunch away at Opabinia s hard outer skeleton and, within a few short inutes, nctacaris is ready to start searching for its next eal. years ago, in an area of Earth we now call Canada, a very strange creature is poking along the sea floor next to a steep underwater wall. The creature has five doed eyes, a nozzle reaching out fro underneath the front of its body with spiny jaws at the end, and a body covered with plates. This is Opabinia, a bizarre 21/2-inch-long creature that swa in the seas during the Cabrian period. It is looking for tiny crawling things that live on the sandy sea botto. Predatory wors, called Ottoia, poke their toothy ouths out of the sand as Opabinia paddles around the sponges growing on the botto. It coes across an abandoned bit of trilobite shell and then another. A very young trilobite has olted; while its new exoskeleton is still soft, it will be easy prey. Iagine: about 530 illion 10 K070026_p /30/07 9:30:37 PM

4 Just then, strong vibrations shoot through the water. An undersea hillside collapses and blocks out the light fro the surface. A wall of sedient swiftly rains down and buries nctacaris and the other creatures living here on the sea floor under a thick layer of ud. Hundreds of illions of years later, scientists discover the fossils of the creatures buried under that ancient udslide. After soe study they recognize soe of the creatures as true arthropods, but any do not fit neatly into any arthropod group. Scientists decide they ay have been evolutionary experients that didn t survive past the Cabrian period. The arthropods of the Paleozoic era include: nctacaris (a chelicerate), Marella (a unique arthropod), Calyene (a trilobite), Pterygotus (a sea scorpion), Liuloides (an ancient horseshoe crab), Myriacantherpestes (a spiky illipede), Rhyniognatha (an early insect), Meganeura (a giant dragonfly), Arthroura (a huge arored illipede), and Aphthoroblattina (an ancient cockroach). K070026_p /30/07 9:30:43 PM

5 SANCTACARIS (sank-tah-care-iss) 530 illion years ago S anctacaris, a 4-inch-long arored arthro- pod, first appeared about 30 illion years into the Paleozoic era. Scientists believe that nctacaris was an ancient chelicerate related to the spiders and scorpions we see today. It had six pairs of libs on its wide head, which is a characteristic of chelicerates. Along its sides, it had a row of paddle-shaped libs that also held gills for breathing. These double-sectioned libs are biraous (BYE-ra-us), eaning they have two parts. On its last segent, nctacaris had a wide, flat tail, called a telson, which would have helped it ove quickly through the water in search of prey. nctacaris was probably an efficient predator of the Cabrian seas, but it had to avoid an even bigger carnivore: the huge Anoalocaris, which reached 2 feet in length. 12 K070026_p /30/07 9:30:45 PM

6 sanctacaris MARELLA (ah-rell-uh) 530 illion years ago siracatcnas yeldarb.j yhtoit 7002 arella alleram yeldarb.j yhtoit 7002 lived at the sae tie as explorer nctacaris. It was less than an inch long, and had a large head shield with four large spines curving gracefully back over its body. Scientists believe that Marella crawled along the ocean botto, feeling about for any particles of food with its antennae. Fossils of Marella show that its body contained fro 24 to 26 segents. Each segent had a pair of biraous libs. The lower section was for walking, and the upper section featured feathery gills for breathing. The last body segent, the telson, was very sall. While Marella is considered an arthropod, it ay not be directly related to the kind of arthropods living today. M arella strange creatures fro the burgess shale The fossil foration where nctacaris and Marella were found is called the Burgess Shale, located in Yoho National Park in Canada. About 530 illion years ago, an underwater udslide buried any strange creatures here, aazingly preserving any of the soft body parts that do not usually becoe fossilized. It is possible that the udslide swept the creatures into a lower underwater trench where the lack of oxygen in the water kept the creatures fro decoposing. Fossils fro the Burgess Shale were originally found in the early 1900s, but were reexained in the 1970s with odern techniques and instruents, with baffling results. After extensive research, paleontologists feel that soe of the creatures found here, like Opabinia, Nectocaris, and Anoalocaris, ight have been unique, eaning they do not have any surviving relatives beyond the Cabrian period. These creatures show that there is uch still to learn about the life of the distant past. opabinia is alocar Ano anoalocaris nectocaris K070026_p /30/07 9:30:52 PM

7 CALYMENE 430 illion years ago L ike all arthropods, Calyene was a sall 4-inch-long trilobite predator that crawled across the sea botto during the Silurian (sih-lure-ee-an) period. It had sall eyes on its cephalon. Its body, or thorax, was ade up of 13 segents, and its sall tail, or pygidiu, ade up of 7 segents fused, or stuck, together. In soe fossils, Calyene is rolled up in a ball, uch like a odern-day pill-bug when threatened. This ay have been how Calyene defended itself fro attack. trilobites were divided into any segents. But unlike ost arthropods, trilobites were also divided lengthwise into three sections, or lobes. The two outer lobes are called ura (PLOOR-ah), and the center lobe is called the axis. Typically, only a trilobite s hard outer shell is preserved as a fossil because the rest of its body was too soft to survive. But scientists have found trilobite fossils that show their legs were biraous, with both walking and gill sections. Trilobites also had antennae to help the sense the world around the. (CAL-ee-een) 14 K070026_p /30/07 9:30:55 PM

8 PTERYGOTUS (terr-ee-goat-us) 430 illion years ago explorer 2007 Tiothy J. Bradley Calyene Pterygotus was a ferocious 9-foot-long ocean creature that belonged to a group of arthropods called eurypterids (yoo-rip-ter-ids) or sea scorpions. Pterygotus looked a little like a flattened odern-day scorpion, but it had soe aazing adaptations that allowed it to live in the ocean. One set of its forward libs had grabbing claws, which would have helped it catch its prey. Its flat body shape and paddle-shaped libs ay have helped it aneuver in the water. During the Silurian period, sea scorpions like Pterygotus were probably the top predators in the oceans. pterygotus radiaspis, a super-spiny trilobite calyene great white shark Soe trilobites had an aazing array of spines sticking out of their bodies. Fossils of these creatures have been discovered in the part of the world we now call Morocco. Radiaspis (raydee-as-pis) is one of these super-spiny trilobites. A pair of spines grew fro its head shield, and several ore grew fro the back of its head and arched over its thorax. Another pair of long spines grew fro each segent of Radiaspis thorax, and a series of spines grew fro its tail. These spines ay have protected Radiaspis fro predators. Judging by the size of the spines, Radiaspis ust have lived in a very dangerous neighborhood! explorer diver Radiaspis 2007 Tiothy J. Bradley