Evolution of brachiopods

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.

Origins

Brachiopod fold hypothesis

The long-standing hypothesis of brachiopod origins, which has recently come under fire,[1] suggests that the brachiopods arose by the folding of a Halkieria-like organism, which bore two protective shells at either end of a scaled body.[2] The tannuolinids were thought to represent an intermediate form, although the fact that they do not, as thought, possess a scleritome means that this is now considered unlikely.[3] Under this hypothesis, the Phoronid worms share a similar evolutionary history; molecular data also appear to indicate their membership of Brachiopoda.[4]

Under the Brachiopod Fold Hypothesis, the "dorsal" and "ventral" valves would in fact represent an anterior and posterior shell. This would make the axes of symmetry consistent with that of other bilaterian phyla[4] and appears to be consistent with the embryological development, in which the body axis folds to bring the shells from the dorsal surface to their mature position.[4] Further support has been identified from the gene expression pattern during development,[4] but on balance, developmental evidence speaks against the BFH.[5]

More recent developmental studies have cast doubt on the BFH. Most significantly, the dorsal and ventral valves have significantly different origins; the dorsal (branchial) valve is secreted by dorsal epithelia, whereas the ventral (pedicle) valve corresponds to the cuticle of the pedicle, which becomes mineralized during development.[6] Moreover, the dorsal and ventral valves of Lingula do not display the Hox gene expression patterns that would be expected if they were ancestrally 'anterior' and 'posterior'.[7]

Tommotiids

An alternative to the BFH suggests that brachiopods arose through the shortening of a tube-like organism consisting of many shell plates. It is possible that they arose from within the tommotiid group in this fashion.[8] The more derived tommotiid Paterimitra has a pair of brachiopod-like shells at its rear, in just the arrangement one would expect of a brachiopod.[1] This is supported by the similarities in mineralogy between the Tommotiids and the earliest brachiopods.[9]

Crown group

The earliest unequivocal brachiopod fossils appeared in the early Cambrian Period.[10][11] The oldest known brachiopod is Aldanotreta sunnaginensis from the lowest Tommotian Stage, early Cambrian of the Siberia was confidently identified as a paterinid linguliforms.

Mineralization

The Lingiliformea brachiopods have apatite shells, which contrasts with the calcitic exoskeleta of the other two brachiopod subphyla. This split occurred very early – the earliest brachiopod assemblages, from the Tommotian, already contain both apatite- and calcite-secreting organisms.[9] Since the minerals used to form exoskeleta rarely change,[12] one might expect these two forms to represent two discrete lineages – but in fact, early brachiopods used a wide range of techniques and materials in shell construction, drawing from phosphatic, calcitic and organic building blocks, and sometimes employing all three.[9] Deducing the original method of mineralisation is tricky; however, it appears that the tommotiids – probably the closest stem group to the brachiopoda, assuming that the Brachiopod Fold Hypothesis is false – produced the same shell microstructre as the earliest known brachiopods. Their shells had a relatively high concentration of phosphate and organic material, though this decreased over time.[9]

Palaeozoic dominance

Brachiopods are extremely common fossils throughout the Palaeozoic. During the Ordovician and Silurian periods, brachiopods became adapted to life in most marine environments and became particularly numerous in shallow water habitats, in some cases forming whole banks in much the same way as bivalves (such as mussels) do today. In some places, large sections of limestone strata and reef deposits are composed largely of their shells.

The major shift came with the Permian extinction, as a result of the Mesozoic marine revolution. Before the extinction event, brachiopods were more numerous and diverse than bivalve mollusks. Afterwards, in the Mesozoic, their diversity and numbers were drastically reduced and they were largely replaced by bivalve molluscs. Molluscs continue to dominate today, and the remaining orders of brachiopods survive largely in fringe environments.

Mesozoic decline

Throughout their long geological history, the brachiopods have gone through several major proliferations and diversifications, and have also suffered from major extinctions as well.

It has been suggested that the slow decline of the brachiopods over the last 100 million years or so is a direct result of the rise in diversity of filter-feeding bivalves, which have ousted the brachiopods from their former habitats; however, the bivalves have undergone a steady rise in diversity from the mid-Paleozoic onwards, and their abundance is unrelated to that of the brachiopods; further, many bivalves occupy niches (e.g. burrowing) which brachiopods never inhabited.[13]

Alternative possibilities for their demise include the increasing disturbance of sediments by roving deposit feeders (including many burrowing bivalves); the increased intensity and variety of shell-crushing predation; or even chance demise – they were hard hit in the End-Permian extinction and may simply never have recovered.

See also

• Brachiopods−related topics
  • Paleozoic brachiopods
  • Mesozoic brachiopods

References

1. Skovsted, C. B.; Holmer, E.; Larsson, M.; Hogstrom, E.; Brock, A.; Topper, P.; Balthasar, U.; Stolk, P.; Paterson, R. (May 2009). "The scleritome of Paterimitra: an Early Cambrian stem group brachiopod from South Australia". Proceedings of the Royal Society B: Biological Sciences. 276 (1662): 1651–1656. doi:10.1098/rspb.2008.1655. ISSN 0962-8452. PMC 2660981. PMID 19203919.
2. Sigwart, J. D.; Sutton, M. D. (Oct 2007). "Deep molluscan phylogeny: synthesis of palaeontological and neontological data". Proceedings of the Royal Society B: Biological Sciences. 274 (1624): 2413–2419. doi:10.1098/rspb.2007.0701. PMC 2274978. PMID 17652065. For a summary, see "The Mollusca". University of California Museum of Paleontology. Retrieved 2 October 2008.
3. G. Giribet C. W. Dunn G. D. Edgecombe A. Hejnol M. Q. Martindale G. W. Rouse. "Assembling the spiralian tree of life" (PDF). In M. J. Telford; D. T. J. Littlewood (eds.). Animal Evolution — Genomes, Fossils, and Trees. pp. 52–64.
4. Cohen, B. L.; Holmer, L. E.; Luter, C. (2003). "The brachiopod fold: a neglected body plan hypothesis" (PDF). Palaeontology. 46: 59–65. doi:10.1111/1475-4983.00287.
5. Altenburger, Andreas; Martinez, Pedro; Budd, Graham E.; Holmer, Lars E. (2017). "Gene Expression Patterns in Brachiopod Larvae Refute the "Brachiopod-Fold" Hypothesis". Frontiers in Cell and Developmental Biology. 5: 74. doi:10.3389/fcell.2017.00074. PMC 5572269. PMID 28879180.
6. Altenburger, A.; Wanninger, A.; Holmer, L. E. (2013). "Metamorphosis in Craniiformea revisited: Novocrania anomala shows delayed development of the ventral valve". Zoomorphology. 132 (4): 379. doi:10.1007/s00435-013-0194-3.
7. Luo, Yi-Jyun; Kanda, Miyuki; Koyanagi, Ryo; Hisata, Kanako; Akiyama, Tadashi; Sakamoto, Hirotaka; Sakamoto, Tatsuya; Satoh, Noriyuki (2018). "Nemertean and phoronid genomes reveal lophotrochozoan evolution and the origin of bilaterian heads". Nature Ecology & Evolution. 2: 141–151. doi:10.1038/s41559-017-0389-y. PMID 29203924. (see Supplementary Information for discussion)
8. Skovsted, C. B.; Brock, G. A.; Paterson, J. R.; Holmer, L. E.; Budd, G. E. (2008). "The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny". Geology. 36 (2): 171. Bibcode:2008Geo....36..171S. doi:10.1130/G24385A.1.
9. Balthasar, Uwe (August 2009). "The Evolution of Shell Composition in Brachiopods" (PDF). In Smith, Martin R.; O'Brien, Lorna J.; Caron, Jean-Bernard (eds.). Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009). Toronto, Ontario, Canada: The Burgess Shale Consortium (published 31 July 2009). ISBN 978-0-9812885-1-2.
10. Alwyn Williams; Leonid E. Popov; Lars E. Holmer; Maggie Cusack (1998). "The diversity and phylogeny of the paterinate Brachiopods" (PDF). Palaeontology. 41 (2): 241–262. Archived from the original (PDF) on 2011-07-16. Retrieved 2009-10-10.
11. Valentine, James W. (2004). On the origin of phyla. Chicago: University of Chicago Press. p. 638. ISBN 978-0-226-84548-7.
12. Porter, S. M. (Jun 2007). "Seawater chemistry and early carbonate biomineralization". Science. 316 (5829): 1302. Bibcode:2007Sci...316.1302P. doi:10.1126/science.1137284. ISSN 0036-8075. PMID 17540895.
13. Gould, S. J.; Calloway, C. B. (1 October 1980). "Clams and Brachiopods — Ships that Pass in the Night". Paleobiology. 6 (4): 383–396. doi:10.1017/s0094837300003572. ISSN 0094-8373. JSTOR 2400538.