Living Nautilus has allowed biologists and paleontologists to model the functions and life modes of the ancient ammonites by using a modern analog. But despite the similarity of their respective shells, coleoids are, in fact, more closely related to ammonites than modern nautiloids, and thus better behavioral analogs may be found within the coleoids (Jacobs & Landman 1993). It is probable that coleoid-type swimming mechanisms probably evolved prior to the loss of the body chamber in the coleoids. Ammonoids thus probably had a coleoid-like mantle and thus may have operated quite differently from living Nautilus. But how far could an empty ammonite shell travel? Ryoji Wani and his colleagues (2005) have demonstrated that the phragmocone of living Nautilus pompilius becomes waterlogged only after the mantle tissue decomposes. Water is then sucked into the shell because of its lower internal gas pressures. This is actually more common for smaller shells, generally with diameters less than 200 mm, and these fill up with water more quickly. Only larger shells had the ability to drift long distances. Since the ratio of volumes of the body chamber to the phragmocone in nautiloids is similar to that of the ammonoids, they probably behaved similarly. The small shells sank and the large shells drifted.
ontogeny of the animal. The phragmocone is chambered, with each chamber marking successive occupation by the animal, and sealed off from previous chambers by a septum, complex in structure at its margins, like a sheet of corrugated iron. Where the septum is welded to the shell, a suture is developed, commonly with a complex pattern of frilled lobes and saddles.
Five main sutural types are recognized among cephalopods (Fig. 13.17). The ortho-ceratitic pattern, with broad undulations or
rounded lobes and saddles, characterizes mainly nautiloids ranging in age from Late Cambrian to Late Triassic. Anarcestid and agoniatitic patterns, however, have a narrow mid-ventral lobe and a broad lateral lobe with additional lobes and saddles, and range in age from the Early to Mid Devonian. Goniatitic sutures are characterized by sharp lobes and rounded saddles, and are found in Late Devonian-Permian ammonoids. Ceratitic sutures show frilled lobes and undivided saddles, and ammonitic patterns have both the lobes and saddles fluted and frilled. Based on these sutural patterns, three groups among the ammonoids can be recognized in a general way: the goniatites are typical of the Devonian-Permian, the ceratites of the Triassic, and the ammonites dominated the Jurassic and Cretaceous. Nevertheless, these sutural patterns may be cross-stratigraphic, with Cretaceous taxa having both goniatitic and ceratitic grades of suture in homeomorphs of more typical Devonian and Triassic forms.
The siphuncle connects the outer body chamber with the phragmocone that includes all the empty, previous chambers. Septal necks act like washers, guiding the passage of the siphuncle through each septum. Excepting the clymeniids, the siphuncle is situated along the outer ventral margin of the shell. Seawater may be pumped in or out of the chambers through the siphuncle in order to alter the buoyancy of the ammonite, similar to mechanisms in the nautiloids and in submarines.
The body chamber contains the soft parts of the ammonite. The aperture may be modified laterally with lappets and ventrally with the rostrum. In many taxa, aptychi sealed the aperture externally, although these plates may also have been part of the jaw apparatus.
The subclass Ammonoidea is currently split into nine orders. The first three, the Anarces-tida, the Clymeniida and the Goniatitida, have goniatitic sutures and are included in the order Goniatitida. The anarcestides characterize Early to Mid Devonian faunas when forms such as Anarcestes and Prolobites displayed tightly coiled shells together with a ventral siphuncle. The clymeniids were the only ammonoids with a dorsal siphuncle; they radiated in Late Devonian faunas in Europe and North Africa, where the group is important for biostratigraphic correlation. The order developed a variety of shell shapes: Pro-gonioclymenia is evolute with simple ribs, Soliclymenia evolved triangular whorls, and Parawocklumeria is a globular involute form with a trilobed appearance. As a whole, the goniatitides ranged in age from the Mid Devonian to Late Permian, with typical goniatitic sutures consisting of eight lobes and ventral siphuncles. Goniatites, for example, was a spherical inflated form with spiral striations,
t dorsal lobe t dorsal lobe
' prong of ventral lobe
' prong of ventral lobe cadicone cadicone sphaerocone sphaerocone platycone platycone
serpenticone serpenticone ellipticone oxycone ellipticone
Figure 13.16 Morphology and shape terminology of the ammonoids: (a) external morphology, (b) suture pattern, and (c) shell shapes.
goniatitic agoniatitic orthoceratitic
Figure 13.17 Evolution of suture patterns: the five main types; arrows point towards the frontal aperture.
whereas Gastrioceras was a depressed, tuber-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 ceratitic. Prolecanites, for example, was evolute with a wide umbilicus. The ceratitides include most of the Triassic ammonoids with ceratitic suture patterns and commonly elaborate ornamented shells. Nevertheless, some taxa developed ammonitic-grade sutures and a number of lineages evolved heteromorphs (Box 13.7).
The ammonites proper (Fig. 13.18) comprise four orders, the Phylloceratida, the Lytoceratida, the Ammonitida and the Ancy-loceratida. The ammonitides appeared first in the Early Triassic with ammonitic sutures, 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 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 Cretaceous with virtually no change, after having generated many of the major post-Triassic lineages. The phylloceratides were smooth, involute (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. Although the group had a near-cosmopolitan distribution, its members were most common in the Tethyan province, but were characteristic of open-water environments.
The lytoceratides originated near the base of the Jurassic, with evolute (all previous 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; however, it too generated many other groups of Jurassic and Cretaceous ammonites.
The ammonitides included the true ammonites and ranged from the Lower Jurassic to the Upper Cretaceous, whereas the ancyloc-eratides included most of the bizarre hetero-morph ammonites, ranging from the Upper Jurassic to the Upper Cretaceous.
Figure 13.18 (opposite) Ammonite taxa: (a) Ludwigia murchisonae (macroconch) from the Jurassic of Skye, (b) cluster of Ludwigia murchisonae (microconchs) from the Jurassic of Skye, (c) Quenstedtoceras henrici from the Jurassic of Wiltshire, (d) Quenstedtoceras henrici (showing a characteristic suture pattern) from the Jurassic of Wiltshire, and (e) Peltomorphites subtense from the Jurassic of Wiltshire, (f) Placenticeras (Cretaceous), (g) Lytoceras (Jurassic), (h) Hildoceras (Jurassic) and (i) Cadoceras (Cretaceous). Magnification x1 (a-e), XÜ.5 (f-i). (a-e, courtesy of Neville Hollingworth.)
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