Distribution of Tylothyris (Brachiopoda) and its occurrence in the Traverse Group (Middle Devonian) of Michigan

Abstract

The septate, lamellose, spiriferacean brachiopod genus Tylothyris North, 1920, is considered to be a delthyridid of the subfamily Tylothyridinae Carter, 1972. Herein, the diagnosis of the Tylothyridinae is emended to include forms with micro-ornament, and with a myophragm or median septum on the floor of the pedicle valve.For the first time, Tylothyris is formally reported from the Traverse Group rocks of Michigan, Givetian through early Frasnian in age. Two species of the genus occur in the Traverse Group. The first, T. subvaricosa (Hall and Whitfield, 1872), previously described by Stainbrook (1943) from the Cedar Valley Limestone of Iowa, is also known from the Milwaukee Formation of Wisconsin and the Mineola Limestone of Missouri. Tylothyris randalia Stainbrook, 1943, also from the Cedar Valley Limestone, is considered to be synonymous with T. subvaricosa. In the Traverse Group, T. subvaricosa occurs in the Bell Shale, Rockport Quarry Limestone, Ferron Point Formation, Genshaw Formation, Alpena Limestone, Gravel Point Formation, Norway Point Formation, and Potter Farm Formation. The second species, T. rockportensis n. sp., occurs only in the Rockport Quarry Limestone, and appears to be endemic to the Michigan Basin.Tylothyris is considered to have originated in the Illinois area during the late Eifelian, then to have migrated to the Michigan area by the early Givetian. With the onset of Taghanic Onlap during the late Givetian, the genus dispersed from its Illinois–Michigan area of origin, becoming distributed across much of North America during the Frasnian and Famennian, and becoming essentially cosmopolitan in distribution during the Tournaisian. Tylothyris apparently became extinct by the end of the Visean.

A reappraisal of feeding current systems inferred for spire-bearing brachiopods

“…Uncertainty about orientation may arise when a stable life position of the host shell is maintained after death, while still exposed to colonisation on the seabed (unless the hinge or the commissure itself became overgrown, thus revealing post mortem growth) (Chang, 1959;Sparks et al 1980); (iii) arrangement or clustering of skeletobiont guest: in many instances, it may be possible to establish the order of clustering among individuals belonging to several generations of a single species, or ecological succession among representatives of different taxa. This can be achieved by detailed examination and sequencing of local overgrowths, their distance from the umbo at the commencement of growth and their overall pattern of average size increase (Ager 1961;Schumann 1967;Sparks et al 1980;Alvarez & Taylor 1987;Taylor & Wilson 2003); (iv) modification of skeletobiont growth pattern closely matching the growth of the host: such as simultaneous growth halts affecting guest and host, or abrupt termination of an epizoobiont at a prominent growth-line of the underlying shell; colonial organisms may even branch preferentially, or extend sideways, along the commissure, thus implying synchronous growth with the host (Ager 1961;Hoare & Steller 1967;Pitrat & Rogers 1978;Alvarez & Taylor 1987;Taylor & Wilson 2003); (v) perturbation of normal growth due to various interferences between skeletobionts and host: common examples include: the host's commissure may become disrupted, undergoing an abnormal configuration caused by crowded settlement of epizoobionts, the epibionts may develop xenomorphic sculpture as they copy in their own shells incremental growth features of the host (specially rhythmic growth banding), or the secretory regime of the host may be affected, thus building various kinds of bioreactions (tubular, blister-like, etc.) as a response to shell boring or etching (Ager 1961;Biernat 1961;Hoare & Steller 1967;Schumann 1967;Cameron 1969;MacKinnon & Biernat 1970;Rudwick 1970;Chatterton 1975;Cooper 1975, [pl.…”

“…4A-B); on the other hand, settlement of inarticulate brachiopods may be more useful for estimating slope orientation rather than current disposition (Hoare & Steller 1967;Vogel 1975). Likewise, settlement of spirorbids (which lack the ability to change their attachment area, or to increase it appreciably), if preferential, seems to be biased towards vacant spaces on upper surfaces, rather than conditioned by the host's feeding currents (Hurst 1974;Pitrat & Rogers 1978;Alvarez & Taylor 1987;Brice & Mistiaen 1992vs. Ager 1963Gekker 1966).…”