Fossil Representative of Guam
Decapod Crab (common name)
Macrophthalmus guamensis (scientific name)
Macrophthalmus guamensis is a species of decapod crab. Decapods, such as lobsters, crayfish, shrimp, and crabs, are crustaceans with five pairs of walking legs. The species Macrophthalmus guamensiswas named for fossils found on Guam dating from the Miocene epoch to the late Pleistocene epoch, approximately 11,000 to 25 million years ago. This species name is synonymous with the existing Macrophthalmus definitus.
Guam has not designated an official fossil. However,Macrophthalmus guamensis was named for the territory, and its significance to paleontology makes it an excellent symbol of Guam’s geology and history.
The genus Macrophthalmus was named by French zoologist and author Anselme Gaëtan Desmarest (1784–1838) in 1823. The wordMacrophthalmus comes from the Greek words μακρύς (makrus), which means "long," and θάλαμος (thalamos), meaning "chamber."
Crabs belong to the taxonomic infraorder Brachyura, which was named by Swedish botanist and zoologist Carl Linnaeus (1707–1778) in 1758. The word Brachyura comes from the Greek words βραχύ (brachy), which means "short," and ουρά (οura), meaning "tail."
Crabs normally have thick exoskeletons. The shell of the existingMacrophthalmus definitus is strongly arched from front to back. This crab has its greatest width near the middle of the carapace instead of the anterior like other crabs. Macrophthalmus definitus is relatively small, with an average adult width of only 0.6 inch (15 mm). It has four teeth on each side and a pair of claws. Crabs are omnivorous, feeding on algae as well as a mix of plant and animal matter.
On Guam, Macrophthalmus guamensis fossils occur in dark brown mudstone and in Mariana limestone. Macrophthalmus crabs live in varied-species groups and their fossils are found in large numbers. Typhoons, tsunamis, landslides, or other natural disasters on Guam may explain the fossil masses found there.
Macrophthalmus guamensis has several features in common with many living species of the genus. This, however, does not prove that it was an ancestor to any of them.
The earliest known crab fossils date from the Jurassic period, approximately 150 to 200 million years ago. Macrophthalmus guamensis fossils are abundant on Guam, especially in Apra Harbour and on Rizal Beach. These fossils also occur in the coastal waters of Japan, Indonesia, China, and Malaysia. Today Macrophthalmus crabs are widespread, existing in the seas bordering Indonesia, the Philippines, Japan, Australia, the Solomon Islands, India, and eastern and southern Africa.
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|Author: World Trade Press|