Uza

BSpiriferida

1 Spiriferinida

Figure 12.2 Classification and stratigraphic distribution of the Brachiopoda. (Courtesy of Sandra Carlson.)

at the time of deposition. The ratio of isotopes within the crystal lattice of the brachiopod shell was often controlled by the provenance of the chemical elements (marine or terrestrial) and temperature and salinity of the sea-water. Carbon, oxygen and strontium isotopes are particularly useful. Devonian brachiopod shells from North America, Spain, Morocco, Siberia, China and Germany analyzed for stable isotopes (513C, 518O and 87Sr/86Sr) have provided many new data on the termination of the Caledonian Orogeny (decrease in the 87Sr/86Sr ratio due to limited influx of freshwater), uplift during the Variscan Orogeny (increase in 87Sr/86Sr ratio due to increased influx of freshwater) and Devonian climate warming (negative 518O excursions) together with increased rates of carbon burial signaled by positive 513C excursions (van Geldern et al. 2006).

Box 12.2 The brachiopod fold hypothesis and the search for stem-group brachiopods

Already by the Early Cambrian a range of diverse brachiopods populated nearshore environments. But where can we find their ancestors and what sort of animals are we looking for? Many have assumed that a prototype brachiopod probably arose in the Late Precambrian with a phosphatic shell substance and an apparently simple Lingula-like morphology. But did it evolve from a burrow-dwelling sessile organism or from a mobile, slug-like ancestor? A careful study of the early development of the non-articulated brachiopod Neocrania by Claus Nielsen (University of Copenhagen) has yielded a few, exciting clues. During ontogeny the embryo actually curls over at both ends (Fig. 12.3). The resulting embryo has the posterior end of the animal forming the dorsal surface (or valve) and the anterior end, the ventral surface. This process, subsequently called the brachiopod fold hypothesis (Cohen et al. 2003), provides an elegant model for how a brachiopod could have evolved from a flat, possibly worm-like, animal with shells at its anterior and posterior ends. Care must be taken in locating such possible ancestors. Halkieria, for example, has shells at its anterior and posterior end but is a mollusk (see p. 331); however shells such as Micrina and Mickwitzia may have belonged to a slug-like stem-group brachiopod. The mystery may be solved only when some exceptionally well-preserved fossil is found.

Posterior Anterior <-►

Dorsal (brachial) valve

Ventral (pedicle) valve

Anterior dorsal (brachial) valve Plane of brachiopod fold Posterior dorsal (brachial) valve

Figure 12.3 (a) The traditional body plan with an upper dorsal and a lower ventral shell. (b) The brachiopod fold hypothesis plan implies that the brachial valve is the anterior one and the pedicle posterior - both were previously on the dorsal surface of the animal. (From Cohen et al. 2003.)

Brachiopod Larve

Figure 12.4 Brachiopod larvae. (a) Ventral and (b) dorsal valves of the brachiopod Onniella. Black arrows indicate the anterior extent of the larval shell. Scale bars, 200 |im. (From Freeman & Lundelius 2005.)

Figure 12.4 Brachiopod larvae. (a) Ventral and (b) dorsal valves of the brachiopod Onniella. Black arrows indicate the anterior extent of the larval shell. Scale bars, 200 |im. (From Freeman & Lundelius 2005.)

Posterior Anterior <-►

"Spirifer" - type

"Atrypa" - type "Terebratula" - type

"Spirifer" - type

"Atrypa" - type "Terebratula" - type

Fossil Atrypa

increasing hydroenergy

Figure 12.5 Morphological variation in Terebratalia from the San Juan islands related to changing hydrodynamic conditions. (From Schumann 1991.)

increasing hydroenergy

Figure 12.5 Morphological variation in Terebratalia from the San Juan islands related to changing hydrodynamic conditions. (From Schumann 1991.)

External periostracal layer -Inner bounding membrane Mucoprotein Protein cement

Vacuole

Vacuole

-Periostracum

— Primary shell layer Secondary shell layer

- Cellular epithelium

Axis of rotation Generative zone

Outer bounding membrane Secretion droplet Mucopolysaccharide

Figure 12.6 Shell secretion at the margins of Notosaria. (Based on Williams, A. 1968. Lethaia 1.)

-Periostracum

— Primary shell layer Secondary shell layer

- Cellular epithelium

Axis of rotation Generative zone

Outer bounding membrane Secretion droplet Mucopolysaccharide

Figure 12.6 Shell secretion at the margins of Notosaria. (Based on Williams, A. 1968. Lethaia 1.)

Distribution in time: extinctions and radiations

The make up of the Cambrian, Paleozoic and Modern brachiopod faunas are fundamentally different, represented by a dominance of different orders; some key representatives are illustrated in Fig. 12.7. Cambrian faunas were dominated by a range of non-articulated groups together with groups of disparate articulated taxa such as the chileides, nauka-tides, obolellides, kutorginides, billingsellides, protorthides, orthides and pentamerides.

These brachiopods were members of a variety of loosely-structured, nearshore paleocommunities.

During the Ordovician radiation, the delti-diodont orthides and strophomenides dominated faunas. These first evolved around Early Ordovician island complexes and came to dominate the shelf benthos, where they began to move offshore and diversify around carbonate mounds. These communities formed the basis of the Paleozoic brachiopod fauna.

Figure 12.7 Representatives of the main orders of non-articulates and articulates. Non-articulates: (a) Pseudolingula (Ordovician lingulide), (b) Nushibella (Ordovician siphonotretide), (c) Numericoma (Ordovician acrotretide), (d) Dinobolus (Silurian trimerellide) and (e) Crania (Paleogene craniide). Articulates: (f) Sulevorthis (Ordovician orthide), (g) Rafinesquina (Ordovician strophomenide), (h) Grandaurispina (Permian productide), (i) Marginifera (Permian productide), (j) Cyclacantharia (Permian richthofeniid), (k) Neospirifera (Permian spiriferide), (l, m) Rostricelulla (Ordovician rhynchonellide) and (n, o) Tichosina (Pleistocene terebratulide). Magnification approximately x2 (a, e-g, l, m), x8 (b), x60 (c), x1 (d, h-k, n, o). (Courtesy of Lars Holmer (a), Michael Bassett (g), Robin Cocks (j) and Richard Grant (h, i, k, l).)

Figure 12.7 Representatives of the main orders of non-articulates and articulates. Non-articulates: (a) Pseudolingula (Ordovician lingulide), (b) Nushibella (Ordovician siphonotretide), (c) Numericoma (Ordovician acrotretide), (d) Dinobolus (Silurian trimerellide) and (e) Crania (Paleogene craniide). Articulates: (f) Sulevorthis (Ordovician orthide), (g) Rafinesquina (Ordovician strophomenide), (h) Grandaurispina (Permian productide), (i) Marginifera (Permian productide), (j) Cyclacantharia (Permian richthofeniid), (k) Neospirifera (Permian spiriferide), (l, m) Rostricelulla (Ordovician rhynchonellide) and (n, o) Tichosina (Pleistocene terebratulide). Magnification approximately x2 (a, e-g, l, m), x8 (b), x60 (c), x1 (d, h-k, n, o). (Courtesy of Lars Holmer (a), Michael Bassett (g), Robin Cocks (j) and Richard Grant (h, i, k, l).)

Figure 12.8 Teeth of articulated brachiopods: (a) deltidiodont and (b) cyrtomatodont dentition.

Figure 12.8 Teeth of articulated brachiopods: (a) deltidiodont and (b) cyrtomatodont dentition.

The brachiopods experienced five main extinction events followed by recoveries and radiations of varying magnitudes. The end-Ordovician event occurred in two phases against a background of glaciation and accounted for the loss of almost 80% of bra-chiopod families. The recovery and subsequent radiation is marked by the decline of deltidiodont groups such as the orthides and strophomenides, whereas the spire-bearing atrypides, athyridides and the spiriferides with cyrtomatodont dentition (Fig. 12.8), together with the pentamerides, achieved greater dominance, particularly in carbonate environments. Late Devonian events, at the Frasnian-Famennian Stage boundary, were also associated with climate change and removed the atrypides and pentamerides and severely affected the orthides and strophomenides, whereas the spiriferides and rhynchonellides survived in deeper-water environments and staged an impressive recovery. A particular feature of the post-Frasnian fauna was the diversity of recumbent brachio-pod megaguilds (see p. 91), dominated by the productides. The Carboniferous and particularly the Permian were intervals of spectacular experimentation: some brachiopods mimicked corals or developed extravagant clusters of spines while a number of groups reduced their shells, thus presenting soft tissues to the outside environment.

Not unexpectedly, the end-Permian mass extinction saw the demise of over 90% of brachiopod species, including some of the most ecologically and taxonomically diverse groups. The post-extinction fauna was first dominated by a variety of disaster taxa (see p. 179), including lingulids; nevertheless the brachiopod fauna later diversified within a relatively few clades dominated by the rhyn-chonellides and terebratulides. The end-Trias-sic event removed the majority of the remaining spiriferides and the last strophomenides. The agenda set by the end-Permian event, involving the subsequent dominance of rhynchonel-lide and terebratulide groups, was continued after the end-Triassic event. The end-Cretaceous event may have been responsible for the loss of about 70% of chalk brachiopod faunas in northwest Europe; nevertheless, many genera survived to diversify again in the Danian limestones. Despite the post-Permian decline of the phylum, Modern brachiopods exhibit a remarkable range of adaptations based on a simple body plan and a well-defined role in the fixed, low-level benthos.

Ecology: life on the seabed_

Living and fossil brachiopods have developed a wide range of lifestyles (Fig. 12.9). Most were attached by a pedicle cemented to a hard substrate or rooted into soft sediment. A

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