Telmatrechus is an extinct heteropteran genus in the water strider family Gerridae which is solely known from Early Eocene sediments exposed in western North America. The genus contains three described species, the type species Telmatrechus parallelus, plus Telmatrechus defunctus, and Telmatrechus stali. The genus has been considered closest to remigis-group species of the living Aquarius, which are all confined to the Nearctic.
Distribution and age Fossils of Telmatrechus are found in early Eocene, Ypresian age lacustrine deposits in two areas of Western North America. Both the type species, Telmatrechus stali and the third species to be described Telmatrechus defunctus come from sites in the Eocene Okanagan Highlands. T. stali has only been reported from an outcrop of the Allenby Formation on the North Fork of the Similkameen River near Princeton, British Columbia. T. defunctus was collected nearly 30 years later by the Lawrence M. Lambe expedition of 1906 as it passed through the Quilchena, British Columbia area and collected from outcrops of the Coldwater Beds.
Early estimates of the highlands sites ranged from Miocene to Eocene in age. The age of the Allenby Formation was debated for many years, with fish and insect fossils hinting at an Eocene age, while mammal and plant fossils suggested a Late Oligocene or Early Miocene age. The lake sediments at Princeton were radiometrically dated using the K-Ar method in the 1960s based on ash samples exposed in the lake bed. These samples yielded an age of ~; however, dating published in 2005 provided a 40Ar-39Ar radiometric date placing some Princeton sites at . A report using dating of detrital zircon crystals from several of the southern highlands lake beds consistently reaffirmed ages in the Late Ypresian, with dates oldest likely ages between .
The remaining species, Telmatrechus parallelus was first described from the "Twin Buttes" locality of Fossil Lake in 1890. Later in 1984, additional specimens were reported from the Laney Members "G-1, Fontenelle Reservoir" outcrop which is included in Lake Uinta. Both the Fossil Butte and Laney members are included in the Green River Formation.
The Fossil Butte Member of the Green River Formation represents the smallest and shortest-lived of the three prehistoric lake systems that make up the Green River Formation, the other two paleolakes being Lake Unita and Lake Gosiute. The lake sediments include a volcanic ash dating to 51.66 million years ago, during the Early Eocene. Fossils are abundant in the rocks of Fossil Lake, and illustrate a diverse assemblage of plants, bivalves, snails, crustaceans, insects, rays, bony fish, salamanders, turtles, lizards, snakes, crocodilians, birds, and mammals. The ages of the Fossil Lake and Lake Unita are Eocene, but are encompassing different sections. The older Fossil Butte sediments are considered Early Eocene, and overlap the Okanagan Highlands Ypresian dating. In contrast the Laney member is placed as early to early middle Eocene and in the Bridgerian.
History and classification The first species to be described was named by Samuel H. Scudder (1879) from material recovered by the George M. Dawson expedition through British Columbia in the 1870s. Scudder placed the new species into the genus "Hygrotrechus" and coined the specific epithet stali as a patronym to honor Swedish entomologist Carl Stål, a prolific studier of hemipterans and friend of Scudders who died that same year. The genus Telmatrechus was first defined and named by Scudder in his 1890 monograph The Tertiary insects of North America which he had been compiling for several years. Scudder noted the newly coined genus name as a combination of the Greek language words τέλμα (télma), meaning "quagmire", and τρέχω (trécho), meaning "run". Scudder did not designate either of the two species described at the time as the type species, choosing to move Hygrotrechus stali into the genus and name another species without comment. Based on a group of four fossils from the recently discovered Green River Formation outcrops at Twin Creek, Wyoming. Unlike both the first species and the genus, Scudder did not opt to provide a specified reasoning for choosing the species name "parallelus", though he specifically noted how remarkably almost "perfectly parallel" the majority of the abdomen sides are. The final species was not named until 1910 when Austrian paleontologist Anton Handlirsch was loaned a group of British Columbian fossils for description. While working on the fossils, he formed the opinion that Scudders genus Telmatrechus was "not well founded" and indicating that he considered T. stali as the type species. He chose to instead place T. stali into Gerris but did not address at all the placement of T. parallelus. With a specimen from Quilchena deemed distinct enough from T. stali, Handlirsch described the new species G. defuncta though he did not provide and an etymology.
The inclusion of the British Columbian species in Gerris while the Green River species was left Telmatrechus remained unchallenged for close to 90 years. A comprehensive review of the know fossil water-striders and their close relatives was conducted by Nils Møller Andersen (1998), in which he addressed the placements of all three species in question. Anderson upheld the validity of Telmatechus as a genus based on the differences that Scudder also noted. As such he moved T. stali back into the genus, and also deemed Handlirschs species as part of the genus, pacing it as Telmatrechus defuncta. The validity of the genus was again questioned a decade later in Jakob Damgaards 2008 evaluation of Gerromorpha evolution and fossil history. Taking a middle of the road approach between Scudder and Handlirsch, he noted that the three species shared a number of characters with the remigis-species group in genus Aquarius. Species of the genus, which Scudder's referenced Hygrotrechus is a junior synonym of, were called notably similar to the extinct species, and he raised the possibility they were congeners of the remigis-species group. He did not, however, take the full steps to formally synonymize the genera or move the species.
Description cudder erected the genus based on characters of the antennae, head, leg proportions, and abdomen. The genus diagnosis was updated by Andersen (1998) described the genus as being of large adult striders which have a body length between long and having both apterous and macropterous forms. The bodies are elongated and yet are more robustly built than species of Limnoporus or Palaeogerris. While Scudder only suggested one of the abdominal connexivum was modified into a spine, likened to that of "Limnotrechus", Andersens definition included all of the connexiva as modified spines. The antennae are at most equal to half the body length, often shorter and composed of three segments. The individual segments of the antennae are elongated, though segments two and three are less in combined length to segment one, which is 1.3 times longer than the head. Segment three is additionally shorter than segment two. This supports Scudders original diagnosis of the first antennal segment being longer than the second. The middle and hind femorae are nearly equal to each other in length while the front femora is described as "robust". The middle tibiae are just about the same length as, or a little shorter than the middle femorae. The pronotum is an elongated lobe with a rear marginal area thats broadly rounded. Scudder also deemed the eyes to be not very prominent on the head, and the thorax to be generally shorter than in "Hygrotrechus". Andersen noted that if the points held as consistent, the antennal proportions separate Telmatrechus from Aquarius, while the leg proportions distinguish it from Limnoporus. Telmatrechus defunctus
The two reported specimens, 1910 and 1988, for Telmatrechus defunctus both have an approximately long body, and are both wingless adults. The head is an equilateral triangle in shape, with only slightly protruding eyes and a length of . The antennae are at least long, with the 1910 specimen being reported with the first and third segments of similar length. The 1988 specimen has possibly incomplete antennae as the third segment is as less than half the length of either other segments. The head and thorax are proportionally longer than the abdomen, and have a pronotum that is about and is slightly wider than the abdomen. The long abdomen is just under 2 1/2 times longer than its widest point near its base. On the sides of at least the 7th segment are spine like corners formed by the connexival join. The middle legs are the longest, with a femora of , and a tibia over long. The front legs are shorter, having femurs and tibiae of only and .
Telmatrechus parallelus Telmatrechus parallelus is larger than T. defunctus, but of similar length to T. stali, with an estimated length from the two 1890 specimens of . Scudder called out the nearly parallel sides of the abdomen in T. parallelus as the most notable distinguishing feature of the species. This feature was questioned by Andersen, who suggested the profile of the abdomens was a preservational artifact, and the laterotergites on the sides were simply missing. As preserved the abdomens are comparable to the thorax width until the last two abdominal segments where they narrow further. The individual middle abdomen sections are equally wide as long, in contrast to those of T. stali, where the segments are half as long as wide. The head is described as shorter than that of T. stali and notably smaller than the wide thorax. Scudder described the coloration of the specimens as fairly uniform "brick" tone, with a course or rough exoskeleton texture on both head and thorax.
Telmatrechus stali