Belongs within: Linguliformea.
Contains: Acrotreta.
The Acrotretidae are a family of inarticulate brachiopods with a generally conical ventral valve known from the Lower Cambrian to the Silurian (Holmer & Popov 2000).
The acrotretids: micro-brachiopods from the dawn of… brachiopods
Published 15 July 2015
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.
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
Characters (Holmer & Popov 2000): Ventral valve conical, more rarely convex; ventral pseudointerarea commonly bisected by intertrough or interridge; pedicle foramen circular or elongate oval, apical or immediately posterior to beak; foramen completely or partly within larval shell; apical process variable, usually with muscle platform directly anterior to pedicle tube; dorsal interior usually with median septum or ridge; median buttress commonly present; cardinal muscle fields usually thickened, becoming wider anterolaterally.
<==Acrotretidae [Anabolotretidae, Kotujotretidae]HP00
|--NeotretinaeRNP03
| |--Neotreta Sobolev 1976RNP03
| | |--*N. tumida Sobolev 1976RNP03
| | |--N. davidi Popov, Berg-Madsen & Holmer 1994RNP03
| | `--N. kargailensisHP00
| `--Rhondellina Rowell 1986RNP03
| |--*R. dorei Rowell 1986RNP03
| |--R. albertensis Robson, Nowlan & Pratt 2003RNP03
| `--R. karatauensis Koneva 1992RNP03
|--AcrotretinaeRNP03
| | i. s.: Apsotreta Palmer 1954R65, HP00
| | `--*A. expansa Palmer 1954HP00
| | Angulotreta Palmer 1954RNP03
| | |--*A. triangularis Palmer 1954RNP03
| | `--A. postapicalisW97
| | Prototreta Bell 1938 [incl. Homotreta Bell 1941]R65
| | |--*P. trapeza Bell 1938R65
| | |--P. attenuataHP00
| | `--‘*Homotreta’ interrupta Bell 1941R65
| | Linnarssonella Walcott 1902RNP03
| | |--*L. girtyi Walcott 1902RNP03
| | |--L. elongata Bell 1941RNP03
| | |--L. minuta (Hall & Whitfield 1877)W08
| | |--L. nitens Walcott 1908W08
| | |--L. tennesseensis Walcott 1902W08
| | |--L. transversa Walcott 1908W08
| | |--L. tubicula Robson, Nowlan & Pratt 2003RNP03
| | `--L. urania Walcott 1908W08
| |--AcrotretaHP00
| `--+--Spondylotreta Cooper 1956HP00, R65
| | `--*S. concentrica Cooper 1956R65
| `--Conotreta Walcott 1889 (see below for synonymy)HP00
| |--*C. rusti Walcott 1889R65
| |--C. multisinuataR65
| `--C. siljanensisHP00
`--LinnarssoniinaeRNP03
| i. s.: Opisthotreta Palmer 1954RNP03
| `--*O. depressa Palmer 1954RNP03
| Acrothyra Matthew 1901R65
| |--*A. proavia (Matthew 1899) [=Acrotreta proavia]R65
| `--A. uraniaHP00
|--Stilpnotreta Henderson & MacKinnon 1981HP00, RNP03
| `--*S. magna Henderson & MacKinnon 1981RNP03
|--Picnotreta Henderson & MacKinnon 1981HP00, RNP03
| |--*P. debilis Henderson & MacKinnon 1981RNP03
| |--P. karakichiensis Holmer et al. 2001RNP03
| |--P. lophocracenta Robson & Pratt 2001RNP03
| `--P. robusta Holmer, Popov & Lehnert 1999RNP03
`--+--Physotreta Rowell 1966HP00
| `--*P. spinosa (Walcott 1905) [=Acrotreta spinosa]HP00
`--+--Quadrisonia Rowell & Henderson 1978HP00
| |--*Q. minor Rowell & Henderson 1978HP00
| `--Q. lavadamensisLM05
`--+--Linnarssonia Walcott 1885HP00, R65 [incl. Pegmatreta Bell 1941R65]
| |--*L. transversa (Hartt in Dawson 1868) [=Obolella transversa]HP00
| |--L. ophirensisHP00
| |--‘*Pegmatreta’ perplexa Bell 1941R65
| `--L. sagittalisR65
`--+--Hadrotreta Rowell 1966HP00
| `--*H. primaea (Walcott 1902) [=Acrotreta primaea]HP00
`--Eurytreta Rowell 1966 [=Euritreta Popov in Nazarov & Popov 1980]HP00
|--*E. curvata (Walcott 1902) [=Acrotreta curvata]HP00
`--E. sublataLM05
Acrotretidae incertae sedis:
Hisingerella Henningsmoen in Waern, Thorslund & Henningsmoen 1948HP00
|--*H. nitens (von Hisinger 1837) [=Atrypa nitens]HP00
`--H. tenuisW97
Fascicoma Popov in Nazarov & Popov 1980HP00
`--*F. obscura Popov in Nazarov & Popov 1980HP00
‘Aktassia’ Popov in Nazarov & Popov 1976 non Pritykina 1968HP00
`--*A. triangularis Popov in Nazarov & Popov 1976HP00
Amictocracens Henderson & MacKinnon 1981HP00
`--*A. teres Henderson & MacKinnon 1981HP00
Anabolotreta Rowell & Henderson 1978HP00
`--*A. tegula Rowell & Henderson 1978HP00
Anelotreta Pelman in Ermak & Pelman 1986HP00
`--*A. magna Pelman in Ermak & Pelman 1986HP00
Aphelotreta Rowell 1980HP00
`--*A. minuta Rowell 1980HP00
Araktina Koneva 1992HP00
`--*A. intermedia Koneva 1992HP00
Canthylotreta Rowell 1966HP00
`--*C. marjumensis (Walcott 1908) [=Acrotreta marjumensis]HP00
Cyrtonotreta Holmer 1989HP00
`--*C. depressa (Cooper 1956) [=Conotreta depressa]HP00
Dactylotreta Rowell & Henderson 1978HP00
`--*D. redunca Rowell & Henderson 1978HP00
Dicondylotreta Mei 1993HP00
`--*D. parallela Mei 1993HP00
Ditreta Biernat 1973HP00
`--*D. dividua Biernat 1973HP00
Eschatelasma Popov 1981HP00
`--*E. rugosum Popov 1981HP00
Galinella Popov & Holmer 1994HP00
`--*G. retrorsa (Lermontova 1951) [=Acrotreta retrorsa]HP00
Hansotreta Krause & Rowell 1975HP00
`--*H. acrobela Krause & Rowell 1975HP00
Kotylotreta Koneva 1990HP00
`--*K. undata Koneva 1990HP00
Longipegma Popov & Holmer 1994 [incl. Kotujotreta Ushatinskaya 1994]HP00
|--*L. gorjanskii Popov & Holmer 1994HP00
`--‘*Kotujotreta’ memorabilis Ushatinskaya 1994HP00
Olentotreta Koneva, Popov & Ushatinskaya in Koneva et al. 1990HP00
`--*O. olentensis Koneva, Popov & Ushatinskaya in Koneva et al. 1990HP00
Ottenbyella Popov & Holmer 1994HP00
`--*O. carinata (Moberg & Segerberg 1906) [=Acrotreta carinata]HP00
Satpakella Koneva, Popov & Ushatinskaya in Koneva et al. 1990HP00
`--*S. inconstans Koneva, Popov & Ushatinskaya in Koneva et al. 1990HP00
Semitreta Biernat 1973HP00
`--*S. maior Biernat 1973HP00
Treptotreta Henderson & MacKinnon 1981HP00
`--*T. jucunda Henderson & MacKinnon 1981HP00
Vandalotreta Mergl 1988 [incl. Luhotreta Mergl & Šlehoferová 1990]HP00
|--*V. vafra Mergl 1988HP00
`--‘*Luhotreta’ pompeckji Mergl & Šlehoferová 1990HP00
Conotreta Walcott 1889 [incl. Geinitzia Clarke 1889 (n. n.) nec Gemmellaro 1892 nec Handlirsch 1906 nec Dietz 1911]HP00
*Type species of generic name indicated
References
Bassett, M. G., L. E. Popov & L. E. Holmer. 2004. The oldest-known metazoan parasite? Journal of Paleontology 78 (6): 1214–1216.
[HP00] Holmer, L. E., & L. E. 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. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).
[LM05] Lehnert, O., J. F. Miller, S. A. Leslie, J. E. Repetski & R. L. Ethington. 2005. Cambro-Ordovician sea-level fluctuations and sequence boundaries: the missing record and the evolution of new taxa. Special Papers in Palaeontology 73: 117–134.
[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.
[W08] Walcott, C. D. 1908. Cambrian geology and paleontology. No. 3.—Cambrian Brachiopoda: descriptions of new genera and species. Smithsonian Miscellaneous Collections 53: 53–137.
[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).