Trimerellida is an

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

of craniate

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of 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 e ...

s, containing the sole superfamily Trimerelloidea and the

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

Adensuidae, Trimerellidae, and Ussuniidae. Trimerellidae was a widespread family of warm-water brachiopods ranging from the Middle

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

("Llandeilo" / Darriwilian) to the late Silurian (

Ludlow Ludlow ( ) is a market town and civil parish in Shropshire (district), Shropshire, England. It is located south of Shrewsbury and north of Hereford, on the A49 road (Great Britain), A49 road which bypasses the town. The town is near the conf ...

). Adensuidae and Ussuniidae are monogeneric families restricted to the Ordovician of

Kazakhstan Kazakhstan, officially the Republic of Kazakhstan, is a landlocked country primarily in Central Asia, with a European Kazakhstan, small portion in Eastern Europe. It borders Russia to the Kazakhstan–Russia border, north and west, China to th ...

. Most individuals were free-living, though some (namely Australian populations of the genus '' Eodinobolus'') clustered into large congregations similar to modern oyster reefs.

Evolution

Trimerellides probably originated from tropical island arcs in the region of

Kazakhstania Kazakhstania (), the Kazakh terranes, or the Kazakhstan Block, is a geological region in Central Asia which consists of the area roughly centered on Lake Balkhash, north and east of the Aral Sea, south of the Siberian craton and west of the Alta ...

(present-day Kazakhstan) during the "Llandeilo" (late Darriwilian stage). By the late Sandbian and early Katian stages, many dispersed eastward to nearby regions equivalent to

South China South China ( zh, s=, p=Huá'nán, j=jyut6 naam4) is a geographical and cultural region that covers the southernmost part of China. Its precise meaning varies with context. A notable feature of South China in comparison to the rest of China is ...

and Australia. A few managed to populate the vicinity of

Laurentia Laurentia or the North American craton is a large continental craton that forms the Geology of North America, ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of ...

(North America), possibly through its diminishing proximity to the Australian portion of

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

. By the late Katian, trimerellides had also dispersed westward, populating the seas around

Baltica Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains. The thick core of Baltica, the East European Craton, i ...

(eastern Europe), Scotland, and

Siberia Siberia ( ; , ) is an extensive geographical region comprising all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has formed a part of the sovereign territory of Russia and its predecessor states ...

. Trimerellides were an exclusively tropical group, with most genera

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

to a specific region. There are some exceptions: ''Eodinobolus'' and '' Monomerella'' were particularly widespread, found at low-latitude ecosystems worldwide. At their acme in the late Katian, trimerellides reached the highest diversity ever seen among craniiform brachiopods, forming a significant component of brachiopod assemblages worldwide. Trimerellide diversity collapsed during global cooling in the first pulse of the

Late Ordovician mass extinction The Late Ordovician mass extinction (LOME), sometimes known as the end-Ordovician mass extinction or the Ordovician–Silurian extinction, is the first of the "big five" major mass extinction events in Earth's history, occurring roughly 445 mill ...

. This fate was shared by several other orders of " inarticulate" brachiopods. Trimerellides are an example of a

Lazarus taxon In paleontology, a Lazarus taxon (plural ''taxa'') is a taxon that disappears for one or more periods from the fossil record, only to appear again either in later fossil records, or as actual living organisms, and often in isolated, obscure, ...

: their fossils are absent from the Hirnantian (last stage of the Ordovician) and the Rhuddanian (first stage of the Silurian), with only a few new Silurian genera afterwards. Silurian species were most likely descended from relictual survivors in South China and Australia. Trimerellides managed to recover slightly during the late Wenlock Epoch (mid-Silurian), experiencing a mild rediversification at the species level. New species emphasized deeper muscle attachments relative to most of their Ordovician counterparts. Silurian trimerellide species may have been too specialized to adapt to rapid changes, resulting in their total extinction in the

Ludlow Epoch In the geological timescale, the Ludlow Epoch (from 427.4 ± 0.5 million years ago to 423.0 ± 2.3 million years ago) occurred during the Silurian Period, after the end of the Homerian Age. It is named for the town of Ludlow in Shropshire, Engl ...

Anatomy

Trimerellides are massive by the standards of early brachiopods. They have fairly smooth and unornamented shells, which were probably aragonitic in composition. The shells are unequally biconvex (both

valves A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, ...

convex to different degrees), in some cases nearly spherical in shape. There is no opening for the pedicle. Trimerellides show some similarities to

rhynchonelliform Rhynchonelliformea is a major subphylum and clade of brachiopods. It is roughly equivalent to the former Class (biology), class Articulata, which was used previously in brachiopod taxonomy up until the 1990s. These so-called articulated brachiop ...

("articulate") brachiopods, including mixoperipheral shell growth (where the valves converge towards each other) and the development a fixed hinge at the back of the shell. In trimerellides, this hinge is an articulation between a wide plate on the dorsal valve and a socket-like groove on the ventral valve, opposite to the socket-and-teeth articulation of rhynchonelliforms. Like other craniate brachiopods, the

musculature Skeletal muscle (commonly referred to as muscle) is one of the three types of vertebrate muscle tissue, the others being cardiac muscle and smooth muscle. They are part of the somatic nervous system, voluntary muscular system and typically are a ...

consisted of two pairs of large and vertically-oriented adductor muscles (which close the shell) alongside two pairs of horizontally-oriented oblique muscles (which slide each valve past each other). The inner (internal) pair of oblique muscles extend nearly straight back to the dorsal valve hinge plate. This contrasts with craniids and craniopsids, where the oblique internals splay out and attach besides the posterior adductors. A shelf is usually present near the middle of each valve, in front of the attachments for the anterior adductors. In later trimerellides especially, the anterior adductors sockets are deep vaulted pockets hollowed out from the internal surface of each valve.

Subgroups

From the ''

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

References

Bibliography

* Modzalevskaya, Tatyana Lvovna (2003). "Silurian and Devonian brachiopods from Severnaya Zemlya" (Russian Arctic). ''Geodiversitas'' 25 (1). pp. 73–107. Prehistoric brachiopod orders Craniata {{paleo-protostome-stub