Page 361 - Introduction to Paleobiology and The Fossil Record
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348 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
commonly ornamented shells and ventral sip-
huncles. The first members of the order Phyl-
loceratida, such as Leiophyllites, appear in
Lower Triassic faunas and, according to some,
this stem group probably gave rise to the
ammonitic entire ammonite fauna of the Jurassic and
Cretaceous (Fig. 13.19). The morphologically
conservative Phylloceras survived from the
Early Jurassic to near the end of the Creta-
ceous with virtually no change, after having
generated many of the major post-Triassic lin-
ceratitic
eages. The phylloceratides were smooth, invo-
lute (with the last whorl covering all the
previous ones), compressed forms; the suture
had a marked leaf-like or phylloid saddle and
a crook-shaped or lituid internal lobe.
goniatitic Although the group had a near-cosmopolitan
distribution, its members were most common
in the Tethyan province, but were character-
istic of open-water environments.
The lytoceratides originated near the base
of the Jurassic, with evolute (all previous
agoniatitic
whorls visible), loosely coiled shells, as
seen in Lytoceras itself, which had a near-
cosmopolitan distribution particularly during
high stands of sea level. Like the phyllocera-
tides, the order remained conservative;
orthoceratitic however, it too generated many other groups
of Jurassic and Cretaceous ammonites.
The ammonitides included the true ammo-
Figure 13.17 Evolution of suture patterns: the nites and ranged from the Lower Jurassic to
five main types; arrows point towards the frontal the Upper Cretaceous, whereas the ancyloc-
aperture.
eratides included most of the bizarre hetero-
morph ammonites, ranging from the Upper
whereas Gastrioceras was a depressed, tuber- Jurassic to the Upper Cretaceous.
culate form.
The order Ceratitida includes the suborders
Prolecantida and Ceratitida. The prolecanti-
dines (Early Carboniferous to Late Permian)
had large, smooth shells with wide umbilici,
and sutures grading from goniatitic to cera- Figure 13.18 (opposite) Ammonite taxa: (a)
titic. Prolecanites, for example, was evolute Ludwigia murchisonae (macroconch) from the
with a wide umbilicus. The ceratitides include Jurassic of Skye, (b) cluster of Ludwigia
most of the Triassic ammonoids with ceratitic murchisonae (microconchs) from the Jurassic of
suture patterns and commonly elaborate Skye, (c) Quenstedtoceras henrici from the
ornamented shells. Nevertheless, some taxa Jurassic of Wiltshire, (d) Quenstedtoceras henrici
developed ammonitic-grade sutures and a (showing a characteristic suture pattern) from
number of lineages evolved heteromorphs the Jurassic of Wiltshire, and (e) Peltomorphites
(Box 13.7). subtense from the Jurassic of Wiltshire, (f)
The ammonites proper (Fig. 13.18) com- Placenticeras (Cretaceous), (g) Lytoceras
prise four orders, the Phylloceratida, the (Jurassic), (h) Hildoceras (Jurassic) and (i)
Lytoceratida, the Ammonitida and the Ancy- Cadoceras (Cretaceous). Magnifi cation ×1 (a–e),
loceratida. The ammonitides appeared fi rst in ×0.5 (f–i). (a–e, courtesy of Neville
the Early Triassic with ammonitic sutures, Hollingworth.)
