| Machaeridia Temporal range: | |
|---|---|
 | |
| Turrilepas specimens. | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Annelida |
| Class: | † Machaeridia |
| Families | |
Machaeridia is an extinct group of armoured, segmented annelid worms, known from the Early Ordovician (Late Tremadoc) to Carboniferous. It consists of three distinct families: the plumulitids, turrilepadids and lepidocoleids. [2]
Only the calcitic sclerites ("armour plates") of these worms tend to be preserved in the fossil record. These are tiny, and usually found disarticulated: articulated specimens reach about a centimeter in length, and are incredibly rare – hence the limited degree of study since their description in 1857. [3]
The machaeridians are characterized by having serialized rows of calcitic shell plates. The dorsal sclerites were convex and almost isometric; lateral sclerites were flatter and longer. [4] The plates comprised two calcite layers: the outer layer is thin and formed by lamellar deposition, whereas new elements were added to the thicker inner layer as it grew. [5] Scales are ridged with growth lines, implying that they grew episodically. [4] A few taxa experimented with different approaches to scale formation; some were only very weakly calcified and may have mainly been organic in nature. [4] They were never moulted, and each scale could be moved with an attached muscle. [4]
The front two segments of the machaeridians were commonly different from the rest, bearing fewer spiny projections. [4]
The plumulitids are flattened from above and looks much like the coat of mail armour of chitons. The two other families are laterally compressed and some lepidocoleids formed a dorsal hinge, which make these machaeridians look like a string of bivalves.
Machaeridians are often found in association with stylophorans - the cornutes and mitrates. This suggests that they possessed a similar ecology. They probably fed on organic detritus, perhaps even the faeces of the accompanying stylophorans. [6]
Their scales almost certainly performed a defensive role. [4]
The organisms would have had limited ability to flex to the right and left (in the sagittal plane), but would have been able to roll up. [4] While most possessed bilateral symmetry, the scales on the right and left side of Turrilepas wrightiana are different in shape and form. [4] The Plumulitid machaeridians would have moved across the surface of the sea floor using parapodia, whereas the fully armoured Turrelepids and Lepidocoelids burrowed in a peristaltic fashion reminiscent of their evolutionary cousins, the earthworms. [7] This burrowing role has subjected them to the same evolutionary pressures which affect burrowing bivalves; convergent evolution as a result of their shared function probably contributed to early suggestions that the machaeridians should be classified with the molluscs. [7]
Historically the group has been assigned to the echinoderms, barnacles, annelids and mollusks. Relationships to other Cambrian forms (such as the Halkieriids) have been proposed and discounted. [8] In 2008, the discovery of a fossil preserving soft tissue (including chaetae and parapodia) established an annelid affinity. Machaeridians represent the only instance of this group developing calcitic armour (notwithstanding certain polychaetes that integrate calcite into their chaetae). The exact position with annelids remains unresolved, though some characters indicate a relationship to Aphroditacean annelids (Vinther et al. 2008). In an accompanying commentary, Jean-Bernard Caron suggested that machaeridians must be a stem group based on number of specialised features. However, one cannot assess crown group/stem group affinities based on autapomorphies, but on shared morphological traits or the lack thereof. He also suggested that machaeridians might be polyphyletic, but machaerdians are a well defined group with a number of shared characters and morphological gradations among all three families.[ citation needed ]
Study in 2019 recognized machaeridian as Phyllodociids based on their jaws. [9]
Articulated machaeridians are known from:
| Taxon | Location | Date | Reference |
|---|---|---|---|
| Lepidocoleus sarlei (Lepidocoleidae) | Rochester Shale Lagerstätten, New York State | Wenlock, Silurian | [10] |
| Lepidocoleus hohensteini (Lepidocoleidae) (with soft tissue) | Hunsrück Slate, Germany | Lower Emsian, Devonian | [11] |
| Plumulites bengtsoni (Plumulitidae) | Fezouata Formation, Morocco | Lower Ordovician | [12] |
| Turrilepas wrightiana (Turrilepidae) | Gotland, Sweden | Hemse, Silurian | [13] |
| Lepidocoleus sp. | Bois d’Arc Formation, Cravat Member. Coal County, Oklahoma | Helderbergian, Lower Devonian | [13] |
| Plumulites richorum (Plumulitidae) | Humevale Formation, Victoria, Australia | Lower Devonian | [14] |
| Deltacoleus crassus (Turrelipidae) | Balclatchie Formation, Scotland | Upper Ordovician | [15] |
| Turrilepas wrightiana (Turrelipidae) | Welsh borderlands, UK | Wenlock, Middle Silurian | [15] |
| Turrilepas modzalevskae (Turrelipidae) | Podolia, Ukraine | Lower Ludlow, Silurian | [15] |
... and possibly elsewhere
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 well. While the definition of lobopodians may differ between literatures, it usually refers to a group of soft-bodied, marine worm-like fossil panarthropods such as Aysheaia and Hallucigenia.
Polychaeta is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. More than 10,000 species are described in this class. Common representatives include the lugworm and the sandworm or clam worm Alitta.
Pikaia gracilens is an extinct, primitive chordate animal known from the Middle Cambrian Burgess Shale of British Columbia. Described in 1911 by Charles Doolittle Walcott as an annelid, and in 1979 by Harry B. Whittington and Simon Conway Morris as a chordate, it became the "one of the most famous early chordate fossils," or "famously known as the earliest described Cambrian chordate". It is estimated to have lived during the latter period of the Cambrian explosion. Since its initial discovery, more than a hundred specimens have been recovered.
Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe. The living animal would have measured up to 5 cm (2 inch) when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres (0.079 in) long.
Aplacophora is a presumably paraphyletic taxon. This is a class of small, deep-water, exclusively benthic, marine molluscs found in all oceans of the world.
Dinocaridida is a proposed fossil taxon of basal arthropods that flourished in the Cambrian period with occasional Ordovician and Devonian records. Characterized by a pair of frontal appendages and series of body flaps, the name of Dinocaridids comes from Greek, "deinos" and "caris", referring to the suggested role of some of these members as the largest marine predators of their time. Dinocaridids are occasionally referred to as the 'AOPK group' by some literatures, as the group compose of Radiodonta, Opabiniidae, and the "gilled lobopodians" Pambdelurion and Kerygmachela. It is most likely paraphyletic, with Kerygmachela and Pambdelurion more basal than the clade compose of Opabiniidae, Radiodonta and other arthropods.
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte, in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
Odontogriphus is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot and a "shell" on its back that was moderately rigid but of a material unsuited to fossilization.
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 arthropods, combining characteristics of "lobopodians" with those of primitive arthropods.
Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980 and Thambetolepidea Jell 1981, loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
Hurdia is an extinct genus of hurdiid radiodont that lived 505 million years ago during the Cambrian Period. Fossils have been found in North America, China and the Czech Republic.
Tommotiids are an extinct group of Cambrian invertebrates thought to be early lophophorates.
The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.
Plumulites is an extinct genus of machaeridians, extinct annelid group.
The Fezouata Formation or Fezouata Shale is a geological formation in Morocco which dates to the Early Ordovician. It was deposited in a marine environment, and is known for its exceptionally preserved fossils, filling an important preservational window beyond the earlier and more common Cambrian Burgess shale-type deposits.
Multiplacophora is a stem-group of chitons with a number of plates arranged in 7 rows along the body. They date to at least the Upper Cambrian, but two lower Cambrian fossils- Ocruranus and Trachyplax - may extend the range downwards.
The camenellans, consisting of the genera Camenalla, Dailyatia, Kennardia, Kelanella, Wufengella and Lapworthella, are a group of Tommotiid invertebrates from the Cambrian period, reconstructed as sister to all others. They are primarily known from isolated sclerites, but are believed to have a scleritomous, Halkieria-like construction. This was confirmed by the discovery of Wufengella, known from articulated remains, which showed camenellans to be mobile, worm-like animals.
Lepidocoleus is a genus of extinct armored annelid worm in the class Machaeridia. Two notable species are L. caliburnus or the "Excalibur worm", and L. shurikenus, or the "shuriken worm". The creature had a "suit" of armor running down its body in the form of overlapping calcite crystals. This worm probably lived in shallow water reefs feeding on organic waste. It lived from the Hirnantian of the upper Ordovician to the Famennian of the Devonian.
Collinsovermis is a genus of extinct panarthropod belonging to the group Lobopodia and known from the middle Cambrian Burgess Shale in British Columbia, Canada. It is monotypic having only one species, Collinsovermis monstruosus. After its initial discovery in 1983, Desmond H. Collins popularised it as a unique animal and was subsequently dubbed "Collins' monster" for its unusual super armoured body. The formal scientific description and name were given in 2020.
Wufengella is a genus of extinct camenellan "tommotiid" that lived during the Early Cambrian. Described in 2022, the only species Wufengella bengtsonii was discovered from the Maotianshan Shales of Chiungchussu (Qiongzhusi) Formation in Yunnan, China. The fossil indicates that the animal was an armoured worm that close to the common ancestry of the phyla Phonorida, Brachiozoa and Bryozoa, which are collectively grouped into a clade called Lophophorata.