Morphological Abnormalities and Dwarfism in Maastrichtian Foraminifera from the Cárdenas Formation, Valles–San Luis Potosí Platform, Mexico: evidence of paleoenvironmental stress

Resumen

During the early
Maastrichtian an assemblage rich in large foraminifera, mostly
composed of orbitoidal foraminifera, developed in the shallow-water
deposits of the Cárdenas Formation (Valles–San Luis Potosí Platform,
Mexico). Among them, the most abundant is Lepidorbitoides minima,
some of which display a morphologically abnormal test consisting of
polyvalent forms, conical tests and budding, which could be induced
by abrupt changes in environmental factors. The occurrence of the
abnormalities preceded the demise of this species and of all the
larger foraminiferal association in the upper part of the Gansserina
gansseri Zone (early Maastrichtian). The abnormalities could be
induced by environmental stress, probably related to interplay of
different events such as increased terrigenous input and the
transition to deeper facies due to Laramide tectonic instability
before the K/P boundary.

A late Maastrichtian community of dwarfed planktic foraminifera
composed of Trinitella scotti Brönnimann, Plummerita reicheli
Brönnimann, Rugoglobigerina cf. macrocephala Brönnimann,
Rugoglobigerina hexacamerata Brönnimann, Heterohelix punctulata
(Cushman), Pseudoguembelina costulata (Cushman), Pseudotextularia
intermedia de Klasz, Globigerinelloides asperus Bolli, Guembelitria
cretacea Cushman, Rugoglobigerina rugosa (Plummer), Rugotruncana
sp., Globotruncanita stuarti (de Lapparent), and Globotruncana arca
(Cushman) developed in the upper part of the sequence. This uncommon
morphology could also be related to the highly stressed
paleoenvironmental conditions related to global climatic change and
local tectonic activity. Associated with this planktic assemblage is
a deeper-water dwarfed benthic foraminiferal community composed of
Gyroidinoides nitidus (Reuss), Coryphostoma incrassata (Reuss),
Pseudouvigerina plummerae Cushman, P. cretacea Cushman, Bolivina
cretosa Cushman, Planulina texana Cushman and Cibicides harperi
(Sandidge).