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Terropterus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Terropterus, T. xiushanensis, is known from deposits of Early Silurian age in China.
Terropterus was the earliest known and largest Mixopteridae eurypterid. Fossil specimens referred to T. xiushanensis are estimated to have reached up to 40 centimeters (15.7 in) in length, but other fossils, either representing older T. xiushanensis or a second species of Terropterus, demonstrate that members of the genus could reach upwards of at least 100 centimeters (3.3 ft) in length. Terropterus is the only mixopterid known from the ancient southern continent of Gondwana, with the other two mixopterid genera, Mixopterus and Lanarkopterus, only being known from what was once the northern continent of Laurussia. The discovery of Terropterus significantly expanded the known geographical and temporal ranges of the Mixopteridae.
The deposits in which fossils of Terropterus were discovered were once a marine environment. Fossils of several other groups are known from the same deposits, including Trilobite, Brachiopod, Gastropoda and other eurypterids. Due to its large size and its predatory adaptations, including the characteristic mixopterid large and spiny forward-facing appendages, Terropterus was likely the apex predator of its marine ecosystem.
Terropterus can be distinguished from other mixopterid genera mainly by the unique arrangement and the relative length of the spines on its third pair of appendages. The fourth podomere (leg segment) of this appendage had two spines, with the succeeding podomeres each having three long (every spine being of more or less the same length) spines arranged in an even manner. These three long spines on each podomere are also interlaced with additional shorter to moderately long spines. The preceding second pair of appendages were short and had several pairs of spines at their distal (furthest from the body) ends. The fourth and fifth pair of appendages had long and thin spines at the end of the podomeres, smaller than, but similar to, the third pair of appendages. In these limbs, the spines ran almost in parallel to the direction of the limbs. In addition to the features of the third appendage, additional features that distinguish Terropterus from other mixopterids are the shape of its Arthropod coxa (base segment of the legs) and the proportional length of its podomeres.
In addition to the fossils referred to T. xiushanensis, Wang and colleagues also described two larger, but more incomplete fossil specimens from Llandovery-age deposits (nearly of the exact same age as the Xiushan deposits) in the Fentou Formation in Wuhan, Central China. These fossils included a portion of the preabdomen (portion of the body preceding the abdomen), a carapace (head plate) and parts of the great third appendage. Because the spine patterns of the appendage and the proportions of the joints in the limb are more or less the same as in Xiushan fossils, they were referred, tentatively, to ‘ Terropterus sp.’. Wang and colleagues noted that it was difficult to determine whether these fossils represent a different second species of Terropterus, or (since they were larger) an older individual of T. xiushanensis. Differences to the other fossils were noted to be much larger size as well as the spines on the appendage being proportionally longer and arranged more sparsely.
Prior to the discovery of Terropterus, only two other genera of mixopterids were known: Mixopterus itself (known from Europe and North America) and Lanarkopterus (known from Europe). Both genera are Silurian in age and exclusively known from locations that in their time was part of the northern continent Laurussia. This meant that the morphological variety, evolutionary history and the geographical range of the mixopterids were relatively poorly known. The discovery of Terropterus, the only mixopterid known from the southern continent of Gondwana, as well as the oldest representative of the family, not only expanded knowledge on the morphological variety within the Mixopteridae but also extended the known temporal and geographical range of the group. In particular, the discovery of Terropterus, and the possibility of finding more fossil material in the future, was noted by Wang and colleagues as indicating that the Mixopteridae may have had a more cosmopolitan (worldwide) distribution than previously assumed.
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