''Emeraldella'' is a genus 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 ...

known from the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

of North America. The type species ''E. brocki'' was described in 1912 from the

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 ...

. 21 specimens of ''Emeraldella'' are known from the Greater

Phyllopod bed The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess Shale fossil ''Lagerstätte''. It was quarried by Charles Walcott from 1911–1917 (and later named Walcott Quarry), and was ...

, where they comprise < 0.1% of the community. A re-study on the species was done in 2012. A second species ''E. brutoni'' is known from the

Wheeler Shale The Wheeler Shale (named by Charles Walcott) is a Cambrian ( 507 Ma) fossil locality world-famous for prolific agnostid and ''Elrathia kingii'' trilobite remains (even though many areas are barren of fossils) and represents a Konzent ...

, which was described in 2011. An additional specimen of ''E. brutoni'' was described in 2019, which revealed more of the anatomy. It has been placed as a basal member of the clade

Vicissicaudata Vicissicaudata is an unranked group of artiopodan arthropods, containing Cheloniellida, Aglaspidida and several other genera outside these groups like ''Sidneyia'' and ''Emeraldella.'' Description Vicissicaudatans are similar to most other ...

within

Artiopoda Artiopoda is a clade of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomor ...

, a group of arthropods containing

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s and their relatives.

Description

''Emeraldella'' species reached around in length, excluding the long

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

/tailspine at the end of the body, which is longer than the trunk region of the body. The semicircular head bore an elongate pair of antennae, sometimes exceeding over 80 segments in ''E. brocki''. Eyes were present in ''E. brutoni'', but absent in ''E. brocki''. The head has 3 pairs of post-antennae limbs, the first of which was uniramous (only composed of one branch), composed of an

endopod 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, : ...

(leg-like part) with nine segments/podomeres, which were covered in spines, while the other two pairs were biramous (two branched). The trunk is made up of 10 (in ''E. brutoni'') or 11 in (''E. brocki'') segments/

tergites A ''tergum'' (Latin for "the back"; : ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'margin'. ...

, which have well developed pleurae, which are followed by a cylindrical segment to which "caudal flaps" attached, followed by the tailspine. Pairs of

biramous 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, ...

(two branched) limbs run down the body. The shape of the segments ( podomeres) of the endopods (inner, leg-like branch) of the biramous limbs strongly varied in proportion along the body length, the endopods generally curved outward towards where it attached to the body and then downwards near their tips at the fifth segment/podomere. The

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 (a ...

(outer branch of the biramous limb) is paddle-shaped and divided into three segments, the one closest to the body was covered in lamellae, while the two further away were covered in

setae 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 ...

(hair-like structures). The caudal flaps were modified exopods, and were also fringed with setae.

Taxonomy

In 1923, ''Emeraldella'' was placed, along with ''

Sidneyia ''Sidneyia'' is an extinct marine arthropod known from fossils found from the Early to the Mid Cambrian of China and the Mid Cambrian Burgess Shale of British Columbia, Canada. Description Sidneyia size comparison.svg, Size comparison of ''Si ...

'', as part of the group "Xenopoda". Today, both ''Sidneyia'' and ''Emeraldella'' are placed as part of the clade Vicissicaudata within

, which includes trilobites and other arthropods with similar bodyforms. However, ''Sidneyia'' and ''Emeraldella'' are usually not recovered as each others closest relatives within Vicissicaudata, rendering "Xenopoda" invalid. Cladogram after McCoy et al. 2025:

References

External links

* * {{Taxonbar, from=Q3052157 Burgess Shale fossils Artiopoda Prehistoric arthropod genera Cambrian genus extinctions Wheeler Shale