Adelophthalmus

Size

Size comparison of 14 of the species included in Adelophthalmus.

Adelophthalmus ranged in size from 4 to 22 cm (1.6 to 8.7 in), with the largest species being the Carboniferous A. mazonensis and the smallest being the Permian A. douvillei. There does not seem to have been any enormous reductions or increases in size for members of the genus as a whole throughout its long history, with "large" species occurring in the Devonian (A. sievertsi at 18 cm, 7 in, and A. waterstoni at 15 cm, 6 in), the Carboniferous (the aforementioned A. mazonensis, A. wilsoni at 20 cm, 7.9 in, and both A. granosus and A. zadrai at 15 cm, 6 in) and during the Permian (A. luceroensis at 18 cm, 7 in).^[5]

Most of the smaller species are known from the Carboniferous, when Adelophthalmus was the most abundant,^[2] including the "medium-sized" A. irinae (13 cm, 5.1 in) and A. moyseyi (12 cm, 4.7 in) and the smaller A. mansfieldi, A. pennsylvanicus (both at 8 cm, 3.1 in), A. approximatus (7 cm, 2.8 in) and A. dumonti (6 cm, 2.4 in).^[5]

Original description and synonyms

The type specimen of A. granosus, as illustrated in its original description by Jordan and von Meyer in 1854. The apparently eyeless carapace can be seen in the bottom-left illustration.

The first specimen of Adelophthalmus to be found (today held in the arthropod paleontology collections of the Natural History Museum of Berlin under the specimen number MB.A. 890^[3]) was discovered in 1851 by German paleontologist Hermann Jordan in a railway shaft at Jägersfreude, near Saarbrücken in Germany.^[1]

The specimen was described three years later in 1854 in the work Ueber die Crustaceen der Steinkohlenformation von Saarbrücken ("Of the crustaceans of the coal formation of Saarbrücken"), written by Jordan and Hermann von Meyer and featuring descriptions of several other arthropod taxa. The fossil was immediately recognized by Jordan as that of a eurypterid, with both the overall shape and form and the individual parts (particularly the head and the appendages) being vastly similar to those of Eurypterus, which had been described in the United States in 1825, 29 years earlier. Among the differences noted between the specimens were the smaller size and later age of the Saarbrücken fossil and what Jordan and von Meyer perceived to be a complete lack of eyes.^[1]

In the preserved carapace examined, there was no indication what so ever of the presence of eyes, leading Jordan and von Meyer to assume that the animal would have been eyeless and the original description cites several cases in which eyeless examples occur in other arthropod groups that commonly possess eyes, such as in crustaceans and trilobites.^[1] That the specimen lacked eyes and seemingly represented a blind eurypterid prompted the choice of name, Adelophthalmus, which means "no obvious eyes".^[7] Modern researchers tend to treat the assumed eyelessness as a preservational artifact and not a feature that A. granosus would have had in life.^[3]

Fossil of A. imhofi, formerly considered to be the type species of the genus Lepidoderma, exhibited at the Senckenberg Museum of Frankfurt.

The fact that the only known fossil attributed to Adelophthalmus at the time seemingly lacked eyes, also the basis of the name of the genus, made classifying new species confusing and problematic. Lepidoderma imhofi, named in 1855, shows definite eyes. The descriptor, August Emanuel von Reuss, noted that Lepidoderma likely was synonymous with Adelophthalmus, but ignored the rules of taxonomical priority and used his younger name due to it being based on material that he considered to be better preserved.^[8]

Anthraconectes was named as a subgenus of Eurypterus by Fielding Bradford Meek and Amos Henry Worthen in 1868, as Adelophthalmus had been, to refer to a Carboniferous species from Mazon Creek (now classified as A. mazonensis) in the United States.^[9] German researcher Paul Guthörl considered in 1934 that Anthraconectes and Adelophthalmus were so similar that they would have been synonyms had Adelophthalmus possessed eyes.^[10] The name Polyzosternites was coined by German paleontologist Friedrich Goldenberg to replace Adelophthalmus in the belief that the original type specimen of Adelophthalmus represented the fossil remains of a cockroach.^[11] Glyptoscorpius was erected to include some fossils from the Carboniferous of Scotland, including the species G. perornatus (designated as type) and G. kidstoni, by Ben Peach in 1882.^[12] All these genera were subsequently subsumed into Adelophthalmus in studies during the middle twentieth century.^[3][13] Though most authors assign all described species to Adelophthalmus, some, such as Fredrik Herman van Oyen in 1956 have considered Anthraconectes to possibly represent a distinct genus, citing that scorpions with similar dorsal anatomies can be quite different ventrally and that the same can be true for the Carboniferous Adelophthalmus where the ventral morphology is not yet known. A genus Anthraconectes of this nature would thus depend on what the preservational state of the specimen is.^[3]

Notable subsequent discoveries

The type specimen of A. zadrai, MB.A. 889, was collected in the Czech Republic in 1930 or 1931 and first mentioned in a manuscript by French Carboniferous worker Pierre Pruvost, who dubbed it "Eurypterus (Anthraconectes) Zadrai", but he did not formally describe the specimen or taxon. It was first described formally in 1952 as Adelophthalmus zadrai, at a point in time when the type specimen was seemingly lost. The specimen was rediscovered in Berlin under a different species name based on the original collector of the fossil (Dr. Palisa) and without any designation of it representing a type specimen.^[3]

In 2006, the species A. irinae was described, based on a fossil specimens (including the holotype, a prosoma, "head", with the specimen number PIN no. 5109/4) collected by the Krasnoyarsk Geological Expedition near Sakhapta, a village in the Nazarovskii District of the Krasnoyarsk Region of Russia. The fossils, from the Tournaisian Solomennyi Stan Formation, could confidently be assigned to Adelophthalmus based on their scalelike ornamentation, the position of their eyes and the shape of the carapace shortly after their excavation. The species is the first species of Adelophthalmus to be described from Russia, and the first ever Carboniferous eurypterid known from the country. It is also one of few Carboniferous eurypterids found within the territory of the former Soviet Union, the only others being Adelophthalmus carbonarius from Ukraine and Unionopterus from Kazakhstan.^[14]

In 2013, A. piussii became the first eurypterid to be described from Italy. The specimen (specimen number MFSNgp 31681, housed at the Museo Friulano di Storia Naturale in Udine) was collected in the gravel bank of a small creek near the greater Bombaso creek, north of the village of Pontebba and consists of a carapace and seven opisthosomal segments on a large block of sandstone. The name of the species, piussii, honors the collector of the type specimen, Stefano Piussi.^[15]

Devonian

The eurypterids were one of the groups most heavily affected by the Late Devonian extinction event, following a major decline in diversity during the Early Devonian, eurypterids were rare in marine environments by the Late Devonian. Of the 16 eurypterid families that had been alive at the beginning of the Devonian, only three persisted into the Carboniferous. All of these were non-marine groups.^[19] Whilst the suborder Stylonurina was relatively unscathed, adapting new strategies (such as sweep-feeding) to avoid competition, and came to diversify once more in the Carboniferous, the Eurypterina was rendered almost completely extinct, Adelophthalmus being the sole survivor of the suborder.^[20]

The adelophthalmids, the family to which Adelophthalmus belongs, likely first appeared in the waters of the continent Baltica in the Late Silurian, being a part of the rapid diversification of the swimming eurypterids. Baltica would later collide with the continents Avalonia and Laurentia and form the minor supercontinent Euramerica, where most of basal adelophthalmid evolution took place in the Early Devonian.^[16]

The earliest known species of Adelophthalmus is A. sievertsi from Early Devonian (Emsian) deposits of the Klerf Formation in Wilwerath (in the Rhineland-Palatinate), Germany, then part of Avalonia within Euramerica. This species was originally named as a species of Rhenopterus, but can confidently be classified as an adelophthalmid based on it possessing swimming and walking legs similar to those of other species of Adelophthalmus, the carapace shape, the anterior prosomal process and the metastoma shape as well as the general shape of the body. Erecting a new adelophthalmid genus for the species was seen as inappopriate since the features of A. sievertsi either all fell within the known range of later species of the genus, or were unknown in Adelophthalmus.^[21]

Despite its early age, A. sievertsi is morphologically most similar to species known from the Late Carboniferous and Early Permian, such as A. sellardsi and A. luceroensis, and can be distinguished from all other species by its long lateral eyes, a broad carapace, the podomere 7 being short on the swimming leg paddle, the tubercle rows at the posterior margins of the carapace and the segments of the opisthosoma as well as the bifurcate distal joint in the type A genital appendage.^[21]

Two other species from the Middle Devonian, A. kamyshtensis and A. dubius, are the earliest species known of Adelophthalmus outside Europe and North America. The fossils of both, now housed on the Borissiak Paleontological Institute, were collected by Yu. F. Pogonya Stefanovich in 1960, in deposits 3 km southeast of the village of Kamyshta of the Republic of Khakassia, Russia. Despite how poorly preserved the fossils are (in most specimens only the segmentation pattern of the body can be observed), these eurypterids can be placed safely under Adelophthalmus on the basis of morphological characteristics such as the parabolic carapace or the presence of spines in the epimera of the seventh segment. A. kamyshtensis is named after Kamyshta. It was a medium-sized species and remains poorly known. In contrast to most other species, its first segment was narrower relative to the other segments of the mesosoma. The epimera of the last segment of the mesosoma and all the metasomal segments had more or less pronounced spines, while these spines were absent in the rest of the segments. It is similar to A. luceroensis, A. sellardsi, A. imhofi, A. granosus, A. mazonensis, A. wilsoni and A. sievertsi, but differs from all of them in the narrowness of the first segment.^[22]

By the Late Devonian, Adelophthalmus had already become widespread, with the species A. waterstoni having been recovered from deposits of Frasnian (~382.2 to 372.2 million years old) age in the Gogo Formation of Western Australia, the only eurypterid with the exception of Pterygotus known from the continent.^[23] The type, and only, specimen of A. waterstoni, BM(NH) In 60174, was originally referred to Rhenopterus, much like A. sievertsi, partly because there was no solid evidence for the presence of Adelophthalmus as early as the Devonian. The redescription of A. sievertsi as Adelophthalmus and studies noting that A. waterstoni was far more similar to A. sievertsi than to Rhenopterus, especially in the high number of tubercles and their arrangement (a row of smaller tubercles posterior to a row of prominent ones) helped solidify its position as a Late Devonian representative of Adelophthalmus.^[21]

The only other species of Adelophthalmus known from the Devonian is the Famennian (latest Devonian) A. lohesti, known from fossil deposits at Pont de Bonne in Liège, Belgium. Alongside a Famennian species of Cyrtoctenus, C. dewalquei, A. lohesti represents the oldest known eurypterid hitherto discovered in Belgium. A. lohesti is however represented by a single fragmentary specimen whose identification as Adelophthalmus or even eurypterine at large is questionable, with it possibly representing a stylonurid eurypterid instead.^[2] Devonian specimens of Adelophthalmus have allegedly also been recovered from Siberia, which would mean that the range of the genus included water around all then existing continents.^[16]

Carboniferous

Fossil of A. approximatus.

Following the extinction of all other swimming eurypterids in the Devonian,^[20] Adelophthalmus became the most common of all eurypterids of the late Paleozoic, existing in far greater number than the surviving members of the Stylonurina, both in terms of the number of individuals and the number of species.^[2] The Stylonurina saw an increased diversification during the Carboniferous, with the radiation of the Hibbertopteridae and Mycteroptidae families being the last major raditation of eurypterids at the genus level before their extinction.^[24] Adelophthalmus diversified as well, with 22 of its 31 species having been described from the Carboniferous alone,^[18] reaching its peak diversity in the Late Carboniferous.^[15] Unlike the Stylonurina, this diversification did not lead to the evolution of any new genera, and Adelophthalmus remained the only genus of swimming eurypterids throughout the Carboniferous and Permian.^[20]

Already widespread and represented around all major landmasses in the Late Devonian, the amalgamation of Pangaea into a global supercontinent during the Carboniferous and Permian would allow the able swimmer Adelophthalmus to gain an almost worldwide distribution,^[16] with Carboniferous-age fossils of Adelophthalmus having been recovered from the United States, Spain, Belgium, Ukraine, China, Germany, the Czech Republic, Russia, England, Wales, Scotland, France and Italy.^[18]

The Early Carboniferous saw the appearance of a few new species, notably A. approximatus, the earliest record of Adelophthalmus in North America (although this species may have occurred as early as the Famennian stage, the last stage of the Devonian). The genus also spread to modern day Scotland (A. perornatus recovered from fossil deposits of Early Carboniferous age in Glencartholm) and Asia (the Tournaisian-age A. irinae known from fossil deposits near Krasnoyarsk, Russia).^[5] The appearance of A. irinae is particularly notable as it represents the hitherto only known Carboniferous eurypterid in Russia.^[14]

The Late Carboniferous would see the appearance of several more species in various places around the world. During the Bashkirian stage (from 323.2 to 315.2 million years ago), two species appeared in Belgium, A. cambieri from Charleroi and A. corneti from Quaregnon, and a third species, A. zadrai has been reported from deposits of Bashkirian age in Moravo-Silesia of the Czech Republic.^[5] A. zadrai was a slender species with angular epimera (pieces on the segments) on the seventh opisthosomal segment and on the mesosoma. Like other species, A. zadrai possessed scalelike ornamention, in this species taking form of large, round and angular scales covering the mesosomal segments and parts of the carapace. Its eyes were close to the margin of the carapace.^[3]

The Radstockian (Upper Westphalian) species A. brasdorensis from New Campbellton, Canada, was described by the Canadian geologist Walter A. Bell based on a single fossil. The carapace was semiovate, with a posterior margin very slightly convex. Short spines were present in the posterolateral corner. Between the prominent reniform eyes there were a pair of elongated elevations that bordered an ocellar mound, probably the seat of the ocelli. The surface was ornamented by raised semioval scales or mucrones (sharped points). In the posterior half, the mucrones were large and elevated, more pointed and visible to the naked eye, but in the anterior part they were thin and microscopic. A. brasdorensis differs only from A. kidstoni by its slightly lesser proportional length and the presence of a superficial indentation at the posterior edge of the latter. The abundance of the bivalve Anthracomya suggests strong evidence of freshwater deposition in the habitat of A. brasdorensis.^[25]

Fossil of A. mazonensis

The Moscovian stage (from 315.2 to 307 million years ago) saw the appearance of several new species, including the two German species A. raniceps and A. granosus, both from Saarbrücken.^[5] A. granosus was a broad species with large and rounded epimera on the mesasoma, though these were absent from the metasoma. The tergites possessed three distinct zones of ornamentation, with the foremost five segments having no or extremely fine ornament, the middle five segments possessing very fine (less than 0.1 mm) ornaments and the last segments having larger (0.2 mm) and angular scales. The type (and only) specimen of A. granosus lacks eyes completely, which is likely a preservational artifact and not a feature it would have had in life. The apparent absence of eyes in A. granosus, which is the type species of the genus, has caused much confusion with all subsequently named species clearly possessing eyes.^[3]

Further Moscovian-age species include a variety of Adelophthalmus from Europe and North America; A. asturica from d’Ablana, Spain, A. kidstoni from Radstock, England, A. imhofi from Vlkhys, Czech Republic, A. mazonensis from Illinois, USA, A. moyseyi from Blaengarw, Wales, A. pennsylvanicus from Pennsylvania, USA, A. pruvosti from Lens, France, A. wilsoni from Radstock, England and A. dumonti from Mechelen-sur-Meuse, Belgium. The very latest Carboniferous and early Permian would see the appearance of A. mansfieldi in Pennsylvania, USA and A. chinensis from Zhaozezhuang, China.^[18][5]

Furthermore, the species A. piussii has been recovered from Late Carboniferous deposits in the Carnic Alps of Italy, the first and hitherto only eurypterid known from the country. This species had a median furrow (a long and narrow trench) on its carapace, which also lacked posterolateral corners. The first opisthosomal tergite was angled anteriorly and maintained a constant width at its lateral margins, nearly identical to the same tergite in A. wilsoni.^[15]

Permian

The Permian fossil record of Adelophthalmus includes five species, all of which were confined to the Early Permian. The first stage of the period, the Asselian (from 298.9 to 295 million years ago), saw the appearance of A. luceroensis in New Mexico, USA, A. douvillei in Bussaco, Portugal and the continued survival of the Carboniferous Chinese species A. chinensis. A. douvillei lasted until the subsequent stage, the Sakmarian (from 295 to 290.1 million years ago), which also saw the appearance of A. nebraskensis in Nebraska, USA. The youngest described species is A. sellardsi, known from the Artinskian (290.1 to 283.5 million years ago) stage of Elmo in Kansas, USA.^[18][5]

A. luceroensis, named after the Lucero Mesa of New Mexico (which is located near where its fossil was found), was closely related to other species found in the United States, but can be distinguished by its unusually wide prosoma. As with many other species of Adelophthalmus, A. luceroensis appears to have lived in environments of brackish to fresh water on a deltaic plain adjacent to a coastal plain. Climate conditions favorable for the spread and maintenance of such environments were optimal during the Late Carboniferous and Early Permian, with Adelopthalmus being widespread and numerous in these times. In most of the locations Adelophthalmus was present it appears to have been similar ecologically.^[26]

Though habitats of this kind were many, widespread and ecologically stable for a time in the early Permian, they would turn out to be delicate. A changing climate during the Permian altered depositional and vegetation patterns across the northern hemisphere, which drastically affected previously widespread environments such as the signature Carboniferous coal forests as well as brackish and fresh water habitats. As their habitat vanished, Adelophthalmus dwindled in number. Whilst some stylonurine eurypterids that occupied niches outside of these habitats continued to survive for a time, Adelophthalmus, restricted to a rapidly disappearing type of environment, became extinct.^[26]

Diet and predation

As Adelophthalmus in many ways represented the last of its kind, being the final eurypterid to possess swimming appendages, it did not exist in diverse eurypterid faunas such as the ones observed with genera during the Silurian or early Devonian. Instead, the brackish of fresh water environments typically inhabited by Adelophthalmus, such as the Early Permian Madera Formation in New Mexico (where fossils of A. luceroensis have been recovered) preserve other organisms, such as insects, branchiopods, ostracods, millipedes and spirorbid worms. The thin and long paddles of Adelophthalmus indicates that it was a good swimmer, though it is likely that it spent most of its time crawling in the mud. As the chelicerae (frontal appendages) of Adelophthalmus were small, it is most likely that it fed on small organisms, possibly in part the ostracods and branchiopods known from associated fossils. There is a noticeable lack of insects in the fossil beds with dense plant fossils, where they should be more common, and a surprising abundance in fossil beds with few eurypterids, possibly indicating that Adelophthalmus fed on insects that had fallen into the water, hindering these from being preserved as fossils.^[26]

The localities in which Adelophthalmus have been preserved in the Madera Formation are all part of the Red Tanks Member, which does not preserve any known organisms that would have been capable of preying on Adelophthalmus. It is however likely that various predatory fish, amphibians and early reptiles known to have been present at the time would have preyed upon the small eurypterids. Both fish and amphibians are known from similar environments of the same age in the nearby Manzanita Mountains.^[26]

Segregation of adults and juveniles

In the Madera Formation, Adelophthalmus and associated organisms lived in bodies of brackish to fresh water in what is assumed to have been a deltaic plain. The lack of large coal beds suggests that the fossil localities which have yielded Adelophthalmus was a moderately elevated region with less dense vegetation and better drainage than the swamplands that occupied much of the rest of the United States. The discovery of a large assemblage of A. luceroensis, including several adults and juveniles, allowed researchers to determine different habitat preferences for different age groups. Larger individuals (adults) are found associated with large plant fragments, including branches of Walchia and leaves of Cordaites, but smaller individuals (juveniles) are found in fossil beds containing less organic material and mostly smaller plant fragments. The large plant fragments of the adult habitat were deposited in quiet conditions, likely through leaves dropping into enclosed lagoons or standing ponds.^[26]

The juveniles appear to have developed and lived in somewhat different conditions than the adults. In beds were juveniles are more common, insect fossils are more common as well, indicating a lack of adults that were capable of devouring them, and the presence of smaller plant fossils suggest a less prolific vegetation cover, the juvenile environment possibly having been lower areas on the delta plain between the ponds. Periodically, storms would drive marine water into the ponds, where salinity would thus be variable, while juveniles could live in fresher and less variable environments further away from the shoreline. It is possible that the adults mated in the streams that fed the ponds, and then returned to live in the ponds because of a richer food supply being present.^[26]

Age-based segregation of this kind between juveniles and adults of the same population is relatively normal in arthropods, for instance, juveniles of the related and modern Limulus live in different environments and regions than the adults. The advantage of this form of segregation is not only to allow younger individuals to live in conditions more stable from a salinity standpoint, but also to keep juveniles safe from situations in which substantial amounts of marine water decimated the populations in the ponds by altering the living conditions too much. In such a situation, younger populations could after some time recolonize the old habitats.^[26]

Internal phylogeny, monophyly and status as a "wastebasket taxon"

Fossil abdomen and telson of A. mansfieldi.

Adelophthalmus contains a large amount of species (31 as of 2018, the largest amount of any eurypterid^[18][22]), is geographically widespread, named a long time ago (1854) and the nominate form of a higher taxon (lending its name to the family Adelopthalmidae and the superfamily Adelopthalmoidea), meeting every criterion to be dubbed a "wastebasket taxon", a taxon existing for the sole purpose of classifying organisms that do not fit elsewhere. The internal phylogeny and relationships within Adelophthalmus are poorly known, owing to its long history and the large amount of species assigned to the genus, many based on fragmentary remains.^[15]

Victor P. Tollerton suggested in 1989 that some species of Adelophthalmus may be better placed within a new genus in the Slimonidae family of eurypterids, citing their lack of spines, however noted that the then presently available material of Adelophthalmus made it difficult to assess if the legs truly were non-spiniferous.^[28] A new genus for non-spiniferous species could be phylogenetically supported, but transferring the new genus to the Slimonidae based on the loss of a feature which seems to have been lost independently in the two groups is not in line with common practice.^[2]

The cladogram below displays the results of a phylogenetic analysis conducted by O. Erik Tetlie and Markus Poschmann in 2008, featuring seven species of Adelophthalmus and excluding other species on the grounds that they were too incompletely known. All characters were treated as unordered and given equal weight. Orcanopterus, part of a clade that also contains Grossopterus and Waeringopterus, was included in the analysis as an outgroup to polarise the characters.^[27]

The results of the analysis showed that all the genera featured (including Adelophthalmus), with the exception of Nanahughmilleria, where the basal species N. patteni was assigned to the new genus Eysyslopterus, were (or had the potential to be) monophyletic. The monophyly of Adelophthalmus was supported by several synapomorphies, including the presence of an anterior triangle on the carapace (the function of which is uncertain), a central circular area of the carapace being raised, the eyes being further away from the margin of the carapace than from the ocelli, an oval metastoma, a long telson and the presence of epimera on the preabdomen.^[27]

A. sievertsi was recovered as more basal than other species, which fits with it also being the earliest known species in the fossil record, mainly due to the broad swimming appendage being similar to the broad appendages of Parahughmilleria and Nanahughmilleria. All other species of Adelophthalmus where this appendage is known possess one that is thinner.^[27]

The analysis left out many fragmentary species of Adelophthalmus, as their character states could not be confidently taken into account, and Adelophthalmus in terms of all the species it is recognized as containing can thus not be fully confidently stated to be monophyletic, more fragmentary species need to be redescribed and more phylogenetic characters need to be confidently established before the status of the genus can be certain.^[27]

Lamsdell, Simonetto and Selden (2013) stated that the genus as it is currently understood may form a monophyletic, and thus phylogenetically valid, group, but that it likely suffers from an under-splitting at the genus level and over-splitting at the species level. It is possible that the large amount of species form two or more distinct clades that could be split into different genera.^[15] Though most of the species included in the genus appear to form a monophyletic group, some species have been suggested to represent species of other recognized genera, with A. dumonti supposedly being similar to the obscure Unionopterus in its supposed trapezoid carapace (a feature now known to be incorrect and based on an incorrect illustration)^[2] and the large A. perornatus showing ornamentation similar to the one seen in the Hibbertopteridae.^[27] Furthermore, several fragmentary species currently recognized as valid may be synonyms of more well known species. A. imhofi has in particular been suggested to possibly represent a senior synonym of many species, including A. zadrai, A. corneti, A. cambieri, A. pruvosti, A. brasdorensis, A. kidstoni, A. wilsoni and A. sellardsi.^[27]

List of species

Status of names listed below follow a survey by Dunlop, Penney & Jekel, 2018 unless otherwise noted.^[18] Size ranges and temporal ranges follow Lamsdell and Braddy, 2009 unless otherwise stated.^[5]