''Erratus'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...

from the

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

. Its type and only species is ''Erratus sperare''. ''Erratus'' is likely one of the most basal known arthropods, and its discovery has helped scientists understand the early evolution of arthropod trunk appendages. Some of the

stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushro ...

-arthropods like radiodonts did not have

legs A leg is a weight-bearing and locomotive anatomical structure, usually having a columnar shape. During locomotion, legs function as "extensible struts". The combination of movements at all joints can be modeled as a single, linear element c ...

, instead they had flap like appendages that helped them swim. ''Erratus'' on the other hand had not only flaps but also a set of primitive

. It also supported the theory that the gills of aquatic arthropods probably evolved into the wings and

lungs The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either si ...

of terrestrial arthropods later in the

Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...

. Fossils of ''Erratus'' have been found in the Chengjiang

Lagerstätte A Lagerstätte (, from '' Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that exhibits extraordinary fossils with exceptional preservation—sometimes including preserved soft tissues. These f ...

of China, dating to around 520 million years ago. ''Erratus'' was a small, nektonic organism with a bivalved

carapace A carapace is a 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 tortoises, the unde ...

that probably ate by deposit feeding or filter feeding.

History of study

The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

consists of specimens XDBZ101 and XDBZ102. The paratype specimen, NWUS92-310, had previously been referred to '' Isoxys auritus''. Both specimens were found in the Helinpu Formation in

Yunnan Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...

. ''Erratus sperare'' was named by Fu and colleagues in 2022. The genus name is Latin for "roaming", in reference to the inferred nektonic habit of the organism, and the species name is

Latin Latin (, or , ) is a classical language belonging to the Italic languages, 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 ...

for "hope". The holotype specimens consist of a part and counterpart of an individual, and an isolated

. The paratype consists of a carapace with an

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

spine, as well as seven anterior body fragments.

Significance for understanding arthropod evolution

The relationship between the components of the biramous limb of many euarthropod groups, which consists of an

exopod The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments (called podomeres) are of Latin origin, and may be confused with terms for bones: ''coxa'' (meaning hip, pl ...

and an endopod (appendages), and the lopopods (stubby legs) and flaps of lobopodians and radiodonts has long been controversial. ''Erratus'' appears to show one of the earliest steps in the evolution of the endopod, with an unsclerotized endopod fused to a flap that is attached to the body wall, rather than being a branch of the appendage like an exopod. Fu et al. 2022 describes the species as having "unique trunk appendages formed of lateral

anomalocaridid Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two origina ...

-type flaps and ventral subconical endopods". The fossils indicate that the species represent "an intermediate stage of biramous limb evolution".

Description

Known specimens of ''Erratus'' measure in length. The arthropod had a bivalved

that covered the top, which possesses an anterior spine but rounded posteriorly. A pair of lateral eyes located near the anterior spine of the carapace. The eyes appear to have no lenses, and the stalks appear to be as long as the eyes. On the bottom of the arthropod were 11 pairs of wide body flaps with gill-like wrinklings. The flaps decreased in size towards the bottom of the arthropod. As in gilled lobopodians and radiodonts, the flaps were connected to the body wall. The anterior 7 pairs of body flaps each possesses a medially-arranged endopod (inner branch). Each endopod was a simple, 7-segmented leg, lacking any evidences of endites (internal projections) and claws. The posterior 4 pairs of body flaps lacking endopods, but the wrinklings are more prominent. Ventral head structures between the eyes and body flaps are yet to be discovered, it is also uncertain if the trunk-end possesses a

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on acco ...

Classification

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

analysis conducted by Fu et al. 2022 found that ''Erratus'' was a basal arthropod, branched before isoxyids ('' Isoxys'' and ''

Surusicaris ''Surusicaris'' is an extinct genus of bivalved arthropod, known from the Cambrian Burgess Shale of British Columbia, Canada. It is considered to be closely related to ''Isoxys,'' and like it has spined grasping frontal appendages. Description ...

'') and other euarthropods ( hymenocarines, fuxianhuiids, megacheirans,

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 A ...

and so on), but more derived than other stem-arthropods like gilled lobopodians (''

Pambdelurion ''Pambdelurion'' is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar ''Kerygmachela'' from the same locality, ''Pambdelurion'' is thought to be closely related to ...

'' and ''

Kerygmachela ''Kerygmachela kierkegaardi'' is a gilled lobopodian from the Cambrian Stage 3 aged Sirius Passet Lagerstätte in northern Greenland. Its anatomy strongly suggests that it, along with its relative ''Pambdelurion whittingtoni'', was a close rela ...

''), '' Opabinia'' and radiodonts (''

Anomalocaris ''Anomalocaris'' ("unlike other shrimp", or "abnormal shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group arthropods. The first fossils of ''Anomalocaris'' were discovered in the ''Ogygopsis'' Shale of the Stephen F ...

'', '' Hurdia'' and so on).

Paleoecology

''Erratus'' was a member of the

Chengjiang Biota The Maotianshan Shales are a series of Early Cambrian deposits in the Chiungchussu Formation, famous for their '' Konservat Lagerstätten'', deposits known for the exceptional preservation of fossilized organisms or traces. The Maotianshan Shales ...

, which dates to the

Cambrian period The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

, 520 million years ago. During this time the area was a tropical region with sea level changes and tectonic activity. Most of the

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...

were primarily

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...

, and were probably buried via

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

s. Brachiopods,

ctenophores Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), ...

, phoronids, lobopodians,

worms Worms may refer to: *Worm, an invertebrate animal with a tube-like body and no limbs Places *Worms, Germany Worms () is a city in Rhineland-Palatinate, Germany, situated on the Upper Rhine about south-southwest of Frankfurt am Main. It had ...

, primitive chordates, and other arthropods are represented in the biota. The site is known for its incredible preservation of fossils similar to that of the younger Burgess Shale.

Notes

References

{{Taxonbar, from=Q110952212 Fossil taxa described in 2022 Prehistoric arthropod genera Cambrian arthropods Maotianshan shales fossils