''Venustulus'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of synziphosurine, a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group of fossil

chelicerate The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mite ...

arthropods Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

. ''Venustulus'' was regarded as part of the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Prosomapoda Prosomapoda is a clade of euchelicerates including the groups Xiphosura (horseshoe crabs) and Planaterga (a group comprising bunodids, pseudoniscids, chasmataspidids, eurypterids and arachnids), as well as several basal synziphosurid genera ...

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s of the single and

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...

, ''V. waukeshaensis'', have been discovered in

deposits A deposit account is a bank account maintained by a financial institution in which a customer can deposit and withdraw money. Deposit accounts can be savings accounts, Transaction account#Current accounts, current accounts or any of several othe ...

of the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

Wisconsin Wisconsin () is a state in the upper Midwestern United States. Wisconsin is the 25th-largest state by total area and the 20th-most populous. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake M ...

, in the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. ''Venustulus'' is one of the few synziphosurine genera with fossil showing evidence of appendages, the other ones being '' Weinbergina, Anderella'' and ''

Camanchia ''Camanchia'' is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. ''Camanchia'' was regarded as part of the clade Prosomapoda. Fossils of the single and type species, ''C. grovensis'', have been discovered in dep ...

''. Despite often being aligned close to

Horseshoe crabs Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to arachn ...

, it has been found that ''Venustulus'' and its relatives form a group made up of various basal

euchelicerate The subphylum Chelicerata (from New Latin, , ) constitutes one of the major subdivisions of the phylum Arthropoda. It contains the sea spiders, horseshoe crabs, and arachnids (including harvestmen, scorpions, spiders, solifuges, ticks, and mit ...

arthropods more distant to the xiphosurans.

Description

''Venustulus'' is the third largest member of its grouping just behind ''

Willwerathia ''Willwerathia'' is a genus of synziphosurine, a paraphyletic group of horseshoe crab-like fossil chelicerate arthropods. ''Willwerathia'' known only by one species, ''Willwerathia laticeps'', discovered in deposits of the Devonian period from ...

'' and ''Weinbergina'' respectively. This arthropod appears to be blind due to the lack of eyes on its

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

, suggesting it lived either in deep water or buried in the sediments. Unlike modern horseshoe crabs, the prosomal appendages of ''Venustulus'' covered less of the total carapace length. The animals carapace is semi-circular in shape and has a procurved posterior area. The genal areas (sides of the head) of the arthropod are angular in shape, but are not developed into the genal spines seen on some

trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

and xiphosurans. The opisthosomal region of the arthropod is mainly composed of ten articulating segments. This creature was once considered the earliest known member of its grouping, however more recently synziphosuran fossils have been found in earlier

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

deposits. This arthropods five-six pair of appendages contrasts the seven usually seen in other synziphosurans, possibly suggesting that this creature represents a more derived form than other members of its grouping.

History of research

The genus name ''Venustulus'' derives from

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...

and means "charming". The species name ''waukeshaensis'' is derived from the town of Waukesha where the biota containg the fossils is found. This chelicerate, along with many of the other animals of the biota (like ''

Parioscorpio ''Parioscorpio'' is an extinct genus of arthropod containing the species ''P. venator'' known from the Silurian-aged Waukesha Biota of the Brandon Bridge Formation near Waukesha, Wisconsin. Taxonomy The fossils were originally discovered in 19 ...

'' and '' Thylacares'') were discovered in 1985, but were not given scientific names until decades later. ''Venustulus'' was formally described in 2005, and was based of eight specimens that are currently held in the

UW–Madison Geology Museum The UW–Madison Geology Museum (UWGM) is a geology and paleontology museum housed in Weeks Hall, in the southwest part of the University of Wisconsin–Madison campus. The museum's main undertakings are exhibits, outreach to the public, and resea ...

. These specimens vary in quality with three being almost complete and the other five only being the

prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

and

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to a ...

of the arthropod. Fossil specimen of Venustulus (Synziphosurina) from the Waukesha lagerstätte

Classification

The synziphosuran grouping is divided into multiple families including the Bunodidae, the

Pseudoniscidae Pseudoniscidae is an extinct Family (biology), family of synziphosurine chelicerates that lived in the Silurian. Pseudoniscidae is classified inside the clade Planaterga, alongside Bunodidae and Dekatriata (Chasmataspidida, chasmataspidids, Euryp ...

, and the Weinberginidae. Although the taxonomic relationships of ''Venustulus'' are still up for debate, the paper that described it noted that the arthropod has shared characteristics more similar to the weinberginid synziphosurans ''Weinbergina'', ''

Legrandella ''Legrandella'' is a genus of synziphosurine, a paraphyletic group of fossil chelicerate arthropods. ''Legrandella'' was regarded as part of the clade Prosomapoda. Fossils of the single and type species, ''L. lombardii'', have been discovered in ...

'' and ''Willwerathia.'' As a group the synziphosurans were first regarded as basal members of the xiphosuran lineage, however more recently they have been found to be more closely related to more primitive chelicerate arthropods and only distantly related to the xiphosurans.

Paleoecology

The oceans that covered Waukesha were a diverse and bustling ecosystem that housed a variety of strange fauna. Arthropods like ''Venustulus'' were abundant and diverse with many species of

, a possible

cheloniellid Cheloniellida is a taxon (usually referred to as an order) of extinct Paleozoic arthropods. As of 2018,Wendruff, Andrew James, et al. "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA." BioRxiv (2018) ...

, a thylacocephalan, multiple

myriapods Myriapods () are the members of subphylum Myriapoda, containing arthropods such as millipedes and centipedes. The group contains about 13,000 species, all of them terrestrial. The fossil record of myriapods reaches back into the late Silurian, a ...

, phyllocarids, and several arthropods like ''Parioscorpio'' and the " butterfly animal" being enigmatic in their taxonomy. Also inhabiting the area were multiple soft-bodied animals like

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

worms and lobopodians. Multiple

chordates A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...

also lived in the area evidenced by fossils including an extremely rare body fossil of the

conodont Conodonts (Greek ''kōnos'', "cone", + ''odont'', "tooth") are an extinct group of agnathan (jawless) vertebrates resembling eels, classified in the class Conodonta. For many years, they were known only from their tooth-like oral elements, which ...

Panderodus ''Panderodus'' Is an extinct genus of jawless fish belonging to the order Conodonta. This genus had a long temporal range, surviving from the middle Ordovician to late Devonian. In 2021, extremely rare body fossils of ''Panderodus'' from the Wauke ...

''. Bizarrely, animals like

brachiopods Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, whi ...

cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...

clams Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two shel ...

snails A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastrop ...

and

crinoids Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

, which are common in most Silurian environments, are rarely found in this biota, but conulariids are still somewhat common. Interestingly it seems there could be another possible synziphosuran from the same deposits as ''Venustulus'', but the fossils are too fragmentary to consider it a unique genus.

References

{{Reflist, 30em Synziphosurina Prosomapoda Silurian first appearances Silurian arthropods Fossils of the United States Fossil taxa described in 2005