The Origin and Evolution of Mammals - Moodle

82 THE ORIGIN AND EVOLUTION OF MAMMALS
(a)
Yet further changes occurred later in the Early
Permian, to judge from the North American
sequence (Fig. 2.2a), as seasonal aridity continued
to increase. Ophiacodontids last occur in the Clyde
Formation, and the edaphosaurids only shortly
afterwards in the Arroyo Formation. It is at this
time that the caseids appeared, and they persisted
Lower Permian
(b) Upper Permian
Figure 3.27 The disposition of the continents and occurrences of synapsid fossils in (a) Lower Permian and (b) Upper Permian (Parrish et al. 1986).
S-occurrence of pelycosaurs, O-occurrence of tetrapads but synapsids absent. Other letters-therapsid taxa present.
right through into the highly arid San Angelo and
Flower Pot Formations that mark the start of the
Late Permian. Olson (1975) proposed that during
the Early Permian there existed what he termed a
caseid chronofauna, a fauna adapted to drier, seasonal
conditions. The only evidence for it was the
occasional, erratic occurrence in the fossil record of