Dorsal valve of Acrotreta, from the University of Tartu.

Belongs within: Linguliformea.

The acrotretids: micro-brachiopods from the dawn of… brachiopods
Published 15 July 2015
Ventral valve of Acrotreta sp., copyright Ivo Paalits / TÜ geoloogiamuuseum.

When brachiopods have been featured on this site before, they have generally been representatives of the group known as the articulates. Today’s subjects, the Acrotretidae, are instead members of the inarticulate brachiopods. Whereas the shells of articulate brachiopods have a hinge connecting the two valves, the shells of inarticulates do not. Instead, the valves of inarticulates are held together purely by the muscle and tissue around them. Fewer of the living brachiopods are inarticulates than articulates, and the inarticulates have been less diverse over most of brachiopod history.

The Acrotretidae are one of the earliest known families of brachiopods in the fossil record, first appearing in the early Cambrian. They were most diverse in the later Cambrian and early Ordovician, becoming less so in the later Ordovician. Only a single genus is known to have survived into the Silurian (Holmer & Popov 2000). This may be something of a pseudo-extinction: the ‘Acrotretidae’ as currently defined is probably ancestral to other families of the order Acrotretida that post-dated it. Nevertheless, the acrotretid lineage as a whole became extinct during the Devonian. At one time it was thought that some living brachiopod families (the craniids and discinids) might be descendants of the acrotretids; they are now believed to not be closely related.

Reconstruction of the anatomy of the acrotretid Linnarssonia constans (with a boring parasite at lower left) from Bassett et al. (2004).

The first feature that springs to attention about the acrotretids is that they were tiny. In general, their shells were only one or two millimetres across. The two valves of the shell were generally quite distinct for each other. The dorsal valve was generally low and convex, whereas the ventral valve was more or less a deep lop-sided cone. A rounded or oval opening was present in the ventral valve, usually just behind the point of the cone. In life, this would have been the opening through which extended the pedicel, the fleshy stalk that would have attached the stalk to its substrate. In brachiopods as small as acrotretids, the lophophore would have been fairly simple. Living forms with such simple lophophores open the shell wide when feeding and hold the lophophore filaments in a bell-shape; water containing food particles is drawn into the centre of the ‘bell’ and pushed out laterally through the filaments (Rudwick 1965).

An alternate model of the acrotretid anatomy was proposed by Chuang in the early 1970s. He compared acrotretids to the living inarticulate brachiopod Lingula, in which the pedicel does not pass through an opening in the ventral valve but instead is positioned in the centre rear of the animal, passing between the two valves. Chuang suggested that the acrotretid pedicel did likewise, and that the opening in the conical valve (which he interpreted as dorsal rather than ventral) was used to expel water after it was drawn over the lophophore. In support of this model, he conducted an experiment in which he drilled holes in a comparable position in the dorsal valve of living craniid brachiopods (demonstrating once again the concept that one can get away with anything so long as one is experimenting on ‘lower lifeforms’), through which the brachiopods did indeed expel water. However, Chuang’s model was dismissed by Rowell (1977) who identified a number of features confirmed that the perforate valve of acrotretids was indeed ventral. Lingula, despite being the best-known inarticulate in the modern brachiopod fauna, is a poor model for acrotretids due to its adaptations to an infaunal lifestyle buried in mud, including the modification of the pedicel into a supersized structure for digging and anchoring itself. As for Chuang’s experimental observations, Rowell argued that the only thing they demonstrated was that “a system under pressure leaks when perforated”, noting that “This relationship… applies equally to bicycle tires and brachiopods”.

So how did acrotretids make their living? The impression I’ve gotten while researching this post is that they are common in deposits that would have been part of the outer continental shelf. In particular, they are often found in black shales, a rock type that was originally formed from anoxic mud. Obviously, few animals are actually able to make a living in an environment lacking oxygen. Some do, such as the “rat-tailed maggot” larvae of hoverflies that possess a long breathing tube with which to obtain air, but it is difficult to imagine acrotretids functioning in this way. The other animals found fossilised in black shales alongside acrotretids are planktonic and nektonic forms, such as graptolites or cephalopods. It is possible that many acrotretids were pseudoplankton, living attached to other organisms or objects floating in the water, such as floating seaweeds (not floating wood, though, because wood didn’t exist yet). When the acrotretid died, or its host substrate disintegrated, then it would begin the long descent towards eventual fossilisation in the black muds deep below.

Systematics of Acrotretidae
    |  i. s.: Hisingerella Henningsmoen 1948R65
    |           |--*H. nitens (Hisinger 1838) [=Atrypa nitens]R65
    |           `--H. tenuisW97
    |         Keyserlingia Pander 1861 [=Kayserlingia (l. c.)]R65
    |           `--*K. reversa (de Verneuil 1845) [=Orbicula reversa]R65
    |         Opisconidion arcticon Ludvigsen 1974HB93
    |         FascicomaHC04
    |    |--Ceratreta Bell 1941R65
    |    |    `--*C. hebes Bell 1941R65
    |    `--Clistotrema Rowell 1963 [=Orbicella d’Orbigny 1847 non Dana 1846]R65
    |         `--*C. buchii (de Verneuil 1845) [=Orbicula buchii, *Orbicella buchii]R65
    |    |--Artiotreta Ireland 1961R65
    |    |    `--*A. parva Ireland 1961R65
    |    |--Rhysotreta Cooper 1956R65
    |    |    `--*R. corrugata Cooper 1956R65
    |    `--Scaphelasma Cooper 1956R65
    |         |--*S. septatum Cooper 1956R65
    |         `--S. micaW97
    |    |--Neotreta Sobolev 1976RNP03
    |    |    |--*N. tumida Sobolev 1976RNP03
    |    |    `--N. davidi Popov, Berg-Madsen & Holmer 1994RNP03
    |    `--Rhondellina Rowell 1986RNP03
    |         |--*R. dorei Rowell 1986RNP03
    |         |--R. albertensis Robson, Nowlan & Pratt 2003RNP03
    |         `--R. karatauensis Koneva 1992RNP03
    |    |--Opisthotreta Palmer 1954RNP03
    |    |    `--*O. depressa Palmer 1954RNP03
    |    |--Stilpnotreta Henderson & MacKinnon 1981RNP03
    |    |    `--*S. magna Henderson & MacKinnon 1981RNP03
    |    |--Acrothyra Matthew 1901R65
    |    |    `--*A. proavia (Matthew 1899) [=Acrotreta proavia]R65
    |    |--Linnarssonia Walcott 1885 [incl. Pegmatreta Bell 1941]R65
    |    |    |--*L. transversa (Hartt 1868) [=Obolella transversa]R65
    |    |    |--*Pegmatreta’ perplexa Bell 1941R65
    |    |    `--L. sagittalisR65
    |    `--Picnotreta Henderson & MacKinnon 1981RNP03
    |         |--*P. debilis Henderson & MacKinnon 1981RNP03
    |         |--P. karakichiensis Holmer et al. 2001RNP03
    |         |--P. lophocracenta Robson & Pratt 2001RNP03
    |         `--P. robusta Holmer, Popov & Lehnert 1999RNP03
         |--Apsotreta Palmer 1955R65
         |    `--*A. expansa Palmer 1955R65
         |--Spondylotreta Cooper 1956R65
         |    `--*S. concentrica Cooper 1956R65
         |--Undiferina Cooper 1956R65
         |    `--*U. rugosa Cooper 1956R65
         |--Angulotreta Palmer 1954RNP03
         |    |--*A. triangularis Palmer 1954RNP03
         |    `--A. postapicalisW97
         |--Conotreta Walcott 1889 (see below for synonymy)R65
         |    |--*C. rusti Walcott 1889R65
         |    `--C. multisinuataR65
         |--Prototreta Bell 1938 [incl. Homotreta Bell 1941]R65
         |    |--*P. trapeza Bell 1938R65
         |    `--*Homotreta’ interrupta Bell 1941R65
         |--Linnarssonella Walcott 1902RNP03
         |    |--*L. girtyi Walcott 1902RNP03
         |    |--L. elongata Bell 1941RNP03
         |    `--L. tubicula Robson, Nowlan & Pratt 2003RNP03
         `--Acrotreta Kutorga 1848W77
              |--*A. subconica Kutorga 1848R65
              |--A. gemmaW77
              |--A. pyxidicula White 1874W77
              `--A. subsidua White 1874W77

Conotreta Walcott 1889 [incl. Geinitzia Hall 1889 (n. n.) nec Gemmellaro 1892 nec Handlirsch 1906 nec Dietz 1911]R65

*Type species of generic name indicated


Bassett, M. G., L. E. Popov & L. E. Holmer. 2004. The oldest-known metazoan parasite? Journal of Paleontology 78 (6): 1214–1216.

[HB93] Harper, D. A. T., C. H. C. Brunton, L. R. M. Cocks, P. Copper, E. N. Doyle, A. L. Jeffrey, E. F. Owen, M. A. Parkes, L. E. Popov & C. D. Prosser. 1993. Brachiopoda. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 427–462. Chapman & Hall: London.

[HC04] Harper, D. A. T., L. R. M. Cocks, L. E. Popov, P. M. Sheehan, M. G. Bassett, P. Copper, L. E. Holmer, J. Jin & Rong J. 2004. Brachiopods. In: Webby, B. D., F. Paris, M. L. Droser & I. G. Percival (eds) The Great Ordovician Biodiversification Event pp. 157–178. Columbia University Press.

Holmer, L., & L. Popov. 2000. Lingulata. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda, Revised vol. 2. Linguliformea, Craniiformea and Rhynchonelliformea (part) pp. 30–146. Geological Society of America: Boulder, and University of Kansas: Lawrence.

[RNP03] Robson, S. P., G. S. Nowlan & B. R. Pratt. 2003. Middle to Upper Cambrian linguliformean brachiopods from the Deadwood Formation of subsurface Alberta and Saskatchewan, Canada. Journal of Paleontology 77 (2): 201–211.

[R65] Rowell, A. J. 1965. Inarticulata. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 1 pp. H260–H296. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas Press: Lawrence (Kansas).

Rowell, A. J. 1977. Valve orientation and functional morphology of the foramen of some siphonotretacean and acrotretacean brachiopods. Lethaia 10: 43–50.

Rudwick, M. J. S. 1965. Ecology and paleoecology. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda vol. 1 pp. H199–H214. The Geological Society of America, Inc., and The University of Kansas Press.

[W77] White, C. A. 1877. Report upon the invertebrate fossils collected in portions of Nevada, Utah, Colorado, New Mexico, and Arizona, by parties of the expeditions of 1871, 1872, 1873, and 1874. U.S. Geographical Surveys West of the One Hundredth Meridian 4 (1): 1–219, pls 1–21.

[W97] Williams, A. 1997. Shell structure. In: Kaesler, R. L. (ed.) Treatise on Invertebrate Paleontology pt H. Brachiopoda, Revised vol. 1. Introduction pp. 267–320. The Geological Society of America: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

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