Mesozoic Arms Race

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Figure 13.26 Stratigraphie range of the main mollusk groups.

benthos of the Paleozoic, although many more localized, often nearshore, assemblages were dominated by mollusks.

During the Paleozoic, bivalves were common in nearshore environments, often associated with lingulide brachiopods, although the class also inhabited a range of deeper-water clastic environments; and by the Late Paleozoic bivalves had invaded a variety of carbonate environments. However, at the end of the Paleozoic, the appearance of more typical bivalves in shallow-water belts may have displaced the Paleozoic associations seaward. During the Late Mesozoic and Ceno-zoic the significant radiation of infaunal taxa may have been a response to increased predation.

The majority of Paleozoic gastropods were Eogastropoda that commonly dominated shallow-water marine environments and some carbonate reef settings. The Mesozoic, however, was dominated by the Mesogastropoda, which grazed on algal-coated hard substrates. The Cenozoic, marks the acme of the group with the radiation of the siphonal carnivorous neogastropods, and with a further diversification of mesogastropods (Fig. 13.26).

The cephalopods evolved through the development of a chambered shell with a sip-huncle, which gave them considerable control over attitude and buoyancy; this system was refined in the nautiloid groups. The evolution of complex folded sutures in the ammonoids, the exploitation of a pelagic larval stage and a marginal position for the siphuncle apparently set the agenda for the further radiation of the group during the Mesozoic.

Throughout the Phanerozoic, the fleshy mollusks provided a source of nutrition for many groups of predators. The evolution of the phylum was probably in part influenced by the development of predator-prey relationships and minimization of predator success. Thick armored shells were developed in some groups while the evolution of deep-infaunal life modes was also part of a defensive strategy. Predation and the development of avoidance strategies, together with the so-called arms race, had an important influence on molluskan evolution. Predators develop a particular search image when seeking their favored prey. Living terrestrial snails show a wide range of color patterns and the purpose of this variability may be to confuse predators like the song thrush by presenting a wide range of images. If a predator targets as prey one particular variant in the population, then other variants would be free

Box 13.9 Mesozoic marine revolution

The post-Paleozoic seas and oceans were probably different in many ways from those before. One key difference is the more intense predator-prey relationships, signaled by the Mesozoic marine revolution (MMR). During this interval, shell predation by, for example, crushing and drilling, became commoner. A Mesozoic arms race, with predators evolving more highly developed weapons of attack, was balanced by prey evolving better defensive mechanisms and structures. Thus whereas crustaceans developed the efficiency of their claws, jaws and pincers, mollusks grew thicker, more highly-ornamented shells and perhaps burrowed deeper and faster into the sediment. This form of escalation is somewhat different from the mechanism of coevolution; organisms adapt to each other rather than merely change together. In this system, predators will always be one step ahead of their prey. Liz Harper (2006) has reviewed the evidence for post-Paleozoic escalation, plotting the ranges of durophagous body and trace fossils that may have been predatory together with evidence for crushing and drilling of shells (Fig. 13.27). The MMR may have been a complex series of events: (i) a Triassic radiation of decapods, sharks and bony fishes; (ii) Jurassic-Cretaceous radiations of malacostracans and marine reptiles; (iii) a Paleogene explosion of neogastropods, teleosts and sharks; and (iv) the Neogene appearance of mammals and birds.

Body fossil record

Trace fossil record drill holes repaired crushes

Tertiary

Cretaceous

Jurassic

Triassic

My 20 40 60 80 100 120 140 160 180 200 220 240

Taphonomic void? ** St Cassian

* Sporadic evidence of crushing

* marine reptiles?

Crushers

Drillers

Figure 13.27 Stratigraphie relationships between predators and prey during the Mesozoic marine revolution. The St. Cassian Formation, Italy has excellent preservation of aragonitic gastropods. Double asterisks show the level of the St. Cassian Formation, while single asterisks indicate sporadic evidence of crushing. (From Harper 2006.)

to recover until a switch in images was produced. Although such relationships are documented for some Mesozoic (Box 13.9) and Cenozoic faunas, data are sparse for the

Paleozoic. On the other hand, close relatives of the mollusks, the annelids, may have been important predators equipped with an efficient jaw apparatus (Box 13.10).

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