Invertebrate Paleontology
Spring 2004
Lab 7: Brachiopoda
Brachiopods on the web:
Goals of this lab: Learn to distinguish brachiopod features used in
taxonomy; become familiar with common Brachiopod groups and when they
lived.
There are 4 assemblages of Brachiopods (1-4). Assume for each that
the fossils lived at the same time and at the same place.
TABLE : Some characteristic morphologic features of articulate brachiopods
Note: H=hinge, L=length, PV = pedicle valve
Order/
Suborder Convexity Pedicle
Opening Hinge Other Orthida Unequally
biconvex triangular strophic Diverging
costae Strophomenida/
Strophomenidina Plano-concavo-convex absent strophic Thin-shelled Strophomenida/Chonetidina Plano-concavo-convex absent strophic Spines on
hinge Strophomenida/
Productidina Plano-concavo-convex absent Strophic Spines on
PV Pentamerida biconvex absent astrophic Medial
septum Rhynchonellida biconvex absent astrophic Strongly
plicate Spirifirida/
Atrypidina biconvex small astrophic Often
costat Spirifirida/
Spiriferidina biconvex triangular strophic Wing-shaped Spirifirida/
Athyridina biconvex Small,
circular astrophic Often
smooth Terebratulida biconvex circular astrophic Smooth
Assemblage 1:
'Inarticulate'
groups Articulate,
Orthida Articulate,
Pentamerida Lingula obtusa: Lingulida,
Lingulata Palmorthis elegantula Sieberella roemeri Praebolus bretonensis: uncertain
position (Obolellida) Hebertella occidentalis Lingulepsis paradoxides: Lingulida,
Lingulata Hesperorthis tricenaria Lingulella starri: Lingulida,
Lingulata Rhipidomella sp. Hesperorthis sp. Dinorthis subquadrata
In this assemblage there are 4 inarticulate groups, 6 Orthida, and one Pentamerid. Note straight hingeline, fine ribbing of the Orthida, usually rather flat, biconvex, no sulcus. The absence of articulate brachiopods other than Orthida (dominant) and Pentamerida and the abundant occurrence of inarticulate groups places this assemblage in the Cambrium (p. 238/243 text book, lingulid-orthid assemblage). Ecology: this must have been a mixed environment, with a probably muddy substrate because of the common lingulids, but rocky outcrops where the orthids could become attached.
Assemblage 2:
Terebratulida Rhynchonellida Terebratula sp. Rhynchonella vanuxemi Terebratula insignis Rhynchonella platybola Terebratula globosa Columna teres
This assemblage consists of various, species
that mainly belong to the genera Terebratula and Rhynchonella (T and
R). They can be grouped into two types: one with ribs, rather
triangular in outline (R), and one smooth, with an oval outline (R).
Both have curved, not well defined hingelines. Note the large pedicle
foramen in theTerebratulids, and the rather sharp top ('beak') and
coarse ribs in theRhynhconellids. The Terebratulids lived attached by
large pedicle, as did the Rhynchonellids by their thinner ones. The
latter probably had (for a brachiopod) rather high rates of flow of
water (p. 237/242). The absence of other brachiopoda suggests that
this assemblage post-dated the Permian extinction and probably even
the Triassic (the age is, in fact, Early Cretaceous). Both groups
live attached by their pedicle, so we are probably looking at an
assemblage formed on a rocky substrate. Note the absence of bivalves:
this was probably a place unfavorable for bivalves, maybe because of
an absence of mud for burrowing, or presence of strong currents.
Assemblage 3:
Strophomenida Spiriferida Rhynchonellida Terebratulida Dictyoclostus inflatus
(productid) Phriadothyris perplexa Rhynchonella wortheri Terebratula bovidens Chonetes sp. Neospirifer triplicatus Echinoconchus punctatus
(productid) Strophomena sp. Strophonelloides
reversus Productus sp.
(productid) Productus costatus
(productid) Juresania nebraskensis
(productid)
This assemblage is dominated by diverse Strophomenids, including some typical Strophomena-like forms (very flat, D-shaped), and the highly concavo-convex group of the Productinida. Note the straight hingeline, usually no foramen. Note the dominance of the productid-type strophomenids, and the presence of some spirifers (biconvex; straight hinge line; rather shallow sulcus), one Rhynchonellid (biconvex, note coarse ribs, curved hingeline), and one small Terebratulid (high, narrow shell, strongly curved hinge line). In productid forms: note rather rugged shells of some forms (J. nebraskensis); probably had spines during life. Also use the openings in E. punctatus (really pseudo-punctate, actually; see p. 231/236; rods have fallen out).
The dominance of productid strophomenids suggest that this fauna lived in the Carboniferous; all other groups could also have lived at that time (Figure 13 .9A).
As to ecology: forms without pedicle dominate heavily, suggesting a fairly shallow marine, muddy area, where the flat strophomenid could lie on/slide over the mud (Chonetes is thought to have slided); the productids were problably living partially in the mud. The Rhynchonellid and Terebratulid may have lived attached to other brachiopods (lacking a rocky substrate). Alternatively, there may have been places with thin mud cover over a rocky substrate where the Spiriferida, Rhynchonellid and Terebratulid lived.
Asssemblage 4.
Spirifirida Orthida Pentamerida Rhynchonellida Strophomenida Acrospirifer murchisoni Orthostrophia
strophemenoides Pentamerus sp. Rhynchotrema capax Strophomena rugosa Atrypa rockfordiensis Atrypa sp. Composita sp. Athyris nettelrothi Eospirifer macropleura Spirifer mucronatus Spirifer acuminatus Spirifer sp.
This assemblage is heavily dominated by various types of Spiriferida, but has components of 4 other groups. Note the typical Spirifer-related forms with the small height/width ratio and the long, straight hingeline, deep sulcus. Also note the more oval, biconvex or concavo-conxex groups of Atrypa and Athyris (p. 234, 235), with sulcus, short hinge line, failry smooth shells. Note typical strongly biconvex shell with high ratio of height to width in Pentamerus, the flat concavo-convex Strophomenid. Strong dominance of Spiriferida, and high diversity of Brachiopod groups suggests that this assemblage was Devonian (Fig. 13.9A).
Ecology is rather difficult because of the diversity of brachiopods. The water was probably fairly shallow (shelf; few hundreds of meters). The substrate was probably not covered with deep mud, since it must have given enough opportunity for attachment. Many Spirifer-shaped brachiopods are thought to have been stabilized in sitting on soft sediment by the long 'wings' to the side. There are few attached-living forms (which may have become attached to other brachiopods), so there were probably not many outcrops of rock in this shallow marine area.