Lower Pennsylvanian Ammonoids from the Bloyd Formation of Arkansas and Oklahoma

McCaleb, J.A.

doi:10.1130/spe96-p1

Cited by 20 publications

(10 citation statements)

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“…As has been described elsewhere (Stephen, 1997;Manger et al, 1999b), there is little size (maximum diameter) variation among the individuals representing a particular species from Mid dle Carboniferous ammonoid assemblages in northern Arkansas. There is, however, considerable variation in other morphological parameters, as has been noted previously also (McCaleb et al, 1964;McCaleb, 1968;Baker, 1978;Manger and Saunders, 1980). Compressed and depressed conch forms can be identified visually with assurance (Figs.…”

Section: Morphologic Datasupporting

confidence: 67%

Ontogeny and heterochrony in the Middle Carboniferous ammonoidArkanites relictus(Quinn, McCaleb, and Webb) from northern Arkansas

2002

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The reticuloceratid ammonoidArkanites relictus(Quinn, McCaleb, and Webb, 1962) is represented by hundreds to thousands of individuals from horizons isolated both stratigraphically and geographically in northern Arkansas. These assemblages appear to represent mass mortality events resulting from a semelparous reproductive strategy.Arkanites relictusoccurs as a dimorphic pair (depressed, widely umbilicate, cadiconic conchs and compressed, narrowly umbilicate, pachyconic conchs) thought to reflect sexual dimorphism. Late stage ontogenetic modifications, such as septal crowding and change in aperture profile, are widely cited evidence of sexual maturity in ammonoids. Septal crowding begins at a predictable ontogenetic stage in the compressed forms ofA. relictus,but specimens with cadiconic conchs do not have crowded septa even at the largest diameters available.Depending on the trait examined and the proxy for age of individuals, the dimorphism inArkanites relictus(using the depressed form as the reference morph) is the result of acceleration, neoteny, or hypermorphosis plus neoteny. If size (diameter) is considered a proxy for age, the dimorphs were the same age at death, and the septa in the compressed variants developed via acceleration relative to the depressed variants. Regarding conch shape (width vs. diameter), the compressed morphs developed via neoteny relative to the depressed morphs. If septal count is considered a proxy for age, the dimorphs were not the same age at death, and the compressed forms were produced by a combination of hypermorphosis plus neoteny, i.e., they grew longer yet slower than the depressed forms. InA. relictus,the heterochronic processes of hypermorphosis and neoteny may have been operating simultaneously, which is an interesting possibility because it is an example of a combination of both peramorphic and paedomorphic processes.

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Section: Morphologic Datasupporting

confidence: 67%

Ontogeny and heterochrony in the Middle Carboniferous ammonoidArkanites relictus(Quinn, McCaleb, and Webb) from northern Arkansas

2002

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“…However, a few exceptions exist. McCaleb (1968) stated that in the Late Carboniferous Syngastrioceras oblatum, the morphological differences between macro-and microconch are larger in juvenile/preadult growth stages than in the mature stage/the last whorl. Similarly, Rein (2001Rein ( , 2003 suggested that species of the Middle Triassic genus Ceratites show a similar morphological separation of the antidimorphs.…”

Section: Criteria For Dimorphismmentioning

confidence: 99%

Mature Modifications and Sexual Dimorphism

Klug

Zatoń

Parent

et al. 2015

Topics in Geobiology

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Allometric growth between different parts of the shell often hampers the identification of mollusk shells, particularly in such cases where preadult shell growth varies strongly. Especially in gastropods, the terminal aperture is often less variable and yields morphological information essential for species determination (e.g. Vermeij 1993; Urdy et al. 2010a, b). In fossil mollusk shells, the adult aperture (peristome)is often missing, partially due to an early death, and partially due to destructive processes, which occurred post mortem (taphonomy). Therefore, the entire shell ontogeny is known only from a small fraction of all ammonoid taxa (e.g., Landman et al. 2012). Nevertheless, knowledge of the adult shell of ammonoids is very important since it can yield morphological information essential for systematics and for the reconstruction of various aspects of their paleobiology.

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“…In fact, the widespread distribution is additional evidence that the Morrow chlorites are authigenic. If they were detrital, a sizeable terrigenous source area would be required, where type-I chlorites are dominant instead of the common lib of igneous and metamorphic terrains, and where other common Henbest, 1968;McCaleb, 1968).…”

Section: The Morrow Series Arkansas and Oklahomamentioning

confidence: 99%

Polytypism of Chlorite in Sedimentary Rocks

Hayes

1970

Clays and clay miner.

155

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Abstract-Four chlorite polytypes of Bailey and Brown (1962) have been identified by X-ray diffraction in clay-size chlorites of soils, sediments, and sedimentary rocks: (1) lib, the polytype of common metamorphic and igneous chlorites; (2) lb(/3 = 90~ (3) lbffi = 97~ (4) la. An additional stacking arrangement, iba, is defined herein as disordered chlorite which lacks an hOl diffraction band in the 2-4-2.5 A region.Most type-I chlorites are authigenic, as demonstrated by thin-section petrography. Type-1 chlorites form during diagenesis, or less commonly during halmyrolysis, at temperatures and pressures less than those of low-grade metamorphism. A type-I crystallization sequence is proposed, from least to most stable: lbd ---> lb(/3 = 97 ~ --~ Ib(/3 = 90~ Conditions of low-grade metamorphism usually are necessary to cause conversion of lb(/3 = 90 ~ to lib, the most stable and common polytype. Chemical composition has little or no influence upon polytype relative stabilities; temperature is much more important.Sediment source areas with high relief, abundant rainfall, cold climate, and which contain libchlorite-bearing metamorphic rocks, may yield essentially unweathered 1 l b chlorite to sites of deposition. Thus, clay-size lib chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because 11 b may be able to form authigenically at submetamorphic temperatures, because it is the most stable polytype. Petrographic evidence is useful in such cases.Chlorite polytypism as a geothermometer can be applied to several geologic problems: (1) the authigenic versus detrital origin of clay minerals in sedimentary rocks, particularly in graywacke matrix; (2) the recognition of diagenetic facies or gradients, areally and stratigraphically, within given geologic provinces; (3) the detection of hydrothermal and incipient metamorphic effects. Chlorite polytypism merits general application as an interpretive tool. INTRODUCTION CLAY minerals, like most other minerals, record the physical and chemical conditions under which they formed. Clay minerals, once formed, are sensitive to subsequent changes in the physical and chemical environments of their host rocks. Fine grain size, large surface area, structural and compositional complexities make this so. Clay minerals, therefore, can provide a decipherable record of rock history, embracing the processes of weathering, transportation, deposition, diagenesis, and low-grade metamorphism. Clay-size chlorite is a common layer silicate in sedimentary rocks, sediments, and some soils. The main objective of this paper is to show how chlorite polytypism is related to chlorite occurrence, stability, composition, and parent material. From this, chlorite potytypism can be used to interpret origin of clay minerals in sediments and sedimentary rocks.

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Lower Pennsylvanian Ammonoids from the Bloyd Formation of Arkansas and Oklahoma

Cited by 20 publications

References 0 publications

Ontogeny and heterochrony in the Middle Carboniferous ammonoidArkanites relictus(Quinn, McCaleb, and Webb) from northern Arkansas

Ontogeny and heterochrony in the Middle Carboniferous ammonoidArkanites relictus(Quinn, McCaleb, and Webb) from northern Arkansas

Mature Modifications and Sexual Dimorphism

Polytypism of Chlorite in Sedimentary Rocks

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