''Entothyreos'' is a genus of extinct

panarthropod Panarthropoda is a clade comprising the greatest diversity of animal groups. It contains the extant phyla Arthropoda (Euarthropoda), Tardigrada (water bears) and Onychophora (velvet worms), although the precise relationships among these remained ...

belonging to the group

Lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...

and known from the middle

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

Burgess Shale The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fos ...

British Columbia, Canada British Columbia is the westernmost province of Canada. Situated in the Pacific Northwest between the Pacific Ocean and the Rocky Mountains, the province has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beach ...

. The genus contains a single species, ''Entothyreos synnaustrus'' (meaning "convergent inner shield"), described in 2024. ''Entothyreos'' is significant for possessing a remarkable degree of

sclerotization Sclerotization is a biochemical process that produces the rigid shell of sclerotin that comprises an insect's chitinous exoskeleton. It is prominent in the thicker, armored parts of insects and arachnid Arachnids are arthropods in the Class (b ...

among lobopodians, comparable to that of

arthropod Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

Description

''Entothyreos'' is a stout collinsovermid

lobopodia Lobopodians are members of the informal group Lobopodia (), or the formally erected phylum Lobopoda Cavalier-Smith (1998). They are panarthropods with stubby legs called lobopods, a term which may also be used as a common name of this group as ...

n, capable of reaching roughly in length. The dorsum was protected by numerous paired

sclerite A sclerite (Greek language, Greek , ', meaning "hardness, hard") is a hardened body part. In various branches of biology the term is applied to various structures, but not as a rule to vertebrate anatomical features such as bones and teeth. Instea ...

spines (two pairs per trunk segment). The spines are longest at the middle of the body. These spines are associated (although still dissociable) with subrectangular sclerotic sheets which lie just below the

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

surface. These sclerite sheets may have increased body rigidity, allowing easier erection of the body to allow for

suspension feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a specia ...

from the

water column The (oceanic) water column is a concept used in oceanography to describe the physical (temperature, salinity, light penetration) and chemical ( pH, dissolved oxygen, nutrient salts) characteristics of seawater at different depths for a defined ...

. They are comparable with the

sclerotic rings The scleral ring or sclerotic ring is a hardened ring of plates, often derived from bone, that is found in the eyes of many animals in several groups of vertebrates. Some species of mammals, amphibians, and crocodilians lack scleral rings. The ring ...

found in other luolishaniids, which probably evolved at first to allow for the otherwise soft body to carry numerous large spines. Additionally, the head segment bears two pairs of small sclerite spines (with no apparent underlying sclerotic sheet), and a pair of soft antenniform sensory structures. ''Entothyreos'' possessed eleven pairs of lobopodous limbs, divided into two sets. The anterior set of six

limb Limb may refer to: Science and technology *Limb (anatomy), an appendage of a human or animal *Limb, a large or main branch of a tree *Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb *Limb, in botany, t ...

pairs are elongate and lined with two rows of large, sclerotized

seta In biology, setae (; seta ; ) are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Depending partly on their form and function, protostome setae may be called macrotrichia, chaetae, ...

e, which allowed the animal to

sieve A sieve (), fine mesh strainer, or sift is a tool used for separating wanted elements from unwanted material or for controlling the particle size distribution of a sample, using a screen such as a woven mesh or net or perforated sheet m ...

food particles from the water. These limbs were also covered in numerous shorter, fine setae on their dorsal side, and tipped with a pair of

sickle A sickle, bagging hook, reaping-hook or grasshook is a single-handed agricultural tool designed with variously curved blades and typically used for harvesting or reaping grain crops, or cutting Succulent plant, succulent forage chiefly for feedi ...

-shaped

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

s. The posterior five pairs are thick and

conical In geometry, a cone is a three-dimensional figure that tapers smoothly from a flat base (typically a circle) to a point not contained in the base, called the ''apex'' or '' vertex''. A cone is formed by a set of line segments, half-lines, ...

, each tipped with a single, large, highly developed claw. The annuli of these posterior limbs are highly sclerotized, the margins of which are lined with short setae of alternating lengths. The posterior claws have an additional branch near their base opposing the tip like a claw, unique to ''Entothyreos'', which aided in anchoring to substrate. The final, posterior-most pair of limbs have annuli differentiated into seven sclerotized sheets, which covered at least the dorsal surface of the limb, probably for further protection. While not truly

ized (they are likely not articulated with an arthrodial membrane), the degree of sclerotization present does represent a remarkable convergence with arthropods.

Ecology

''Entothyreos'' is relatively common compared with other

lobopodians, being initially described from a suite of fifty-one fossil specimens. One slab, preserving a large amount of organisms including ''

Anomalocaris ''Anomalocaris'' (from Ancient Greek , meaning "unlike", and , meaning "shrimp", with the intended meaning "unlike other shrimp") is an extinct genus of radiodont, an order of early-diverging stem-group marine arthropods. It is best known fro ...

'', ''

Peytoia ''Peytoia'' is a genus of hurdiid radiodont, an early diverging order of stem-group arthropods, that lived in the Cambrian period, containing two species, ''Peytoia nathorsti'' from the Miaolingian of Canada and '' Peytoia infercambriensis'' fr ...

'', and some

sponge Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and a ...

s, contains nine different individual specimens of ''Entothyreos''. All fossil specimens derive from the Tulip Beds locality of the Burgess Shale (found at the base of the Campsite Cliff Shale Member, making it one of the oldest Burgess Shale localities), which represents a more distal environment, further from the Cathedral Escarpment than the more famous

Walcott Quarry The Walcott Quarry is the most famous quarry of the Middle Cambrian Burgess Shale, located in the Canadian Rockies The Canadian Rockies () or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, i ...

. The Tulip Beds' paleoenvironment is dominated by

sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...

animals, particularly suspension feeders. ''Entothyreos'' likely used its posterior limb pairs to anchor itself to a raised substrate (e.g. sponges), and then used its frontal limb pairs to sieve food particles from the

. The posterior portion of the body is often poorly preserved, which the authors speculate could be caused by kinetic force of burial (a

mudflow A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/se ...

) tearing apart the body, as the anchoring claws are still firmly embedded in substrate. Comparable signs of tearing are seen in other sessile Burgess Shale taxa. Of the fifty-one individual specimens studied, four of them are of a slightly different

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

. This much rarer subset has a more compact body and longer dorsal spines. However, they are roughly the same overall length as the more common morph, and occur on the same bedding planes. This indicates that ''Entothyreos'' may have been

sexually dimorphic Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

References

{{Lobopodia Fossil taxa described in 2024 Lobopodia Prehistoric protostomes Cambrian animals of North America Burgess Shale fossils Cambrian genus extinctions