The Origins of Deep-Sea Fauna

If you would understand anything, observe its beginning and its development. –Aristotle

To understand the biogeography of the modern deep sea, we must examine the history of the ocean floor and the establishment of deep-sea fauna. The paleoceanography of the deep-sea is an account of intense fluctuations in temperature, oxygen, and circulation. In the past ~55mya, since the Eocene/Paleocene boundary, the deep sea cooled nearly 15˚C. A major cooling event is also evident in the Mid-Cenozoic at the Oligocene/Eocene boundary ~34 mya. Little is known of deep seafloor temperatures prior to 100 mya given the paucity of deep-sea sediments older than the Cretaceous. Apparent is there have been major shifts in deep ocean circulation driven by two different processes leading to density differences between water masses. In an excellent review on the implications of ocean circulation on the age of the deep-sea fauna, Horne (1999) contrasts two ocean types of the past, those dominated by thermohaline circulation (THC) driven by temperature-induced density differences at the poles resulting in cold deep water and halothermal circulation (HTC) driven by salinity-induced density differences in equatorial regions resulting in warm saline deep water. During HTC, circulation is reduced, as density differences tend not to be as great as during THC.

In addition to differences between deep-water temperature during THC and HTC, the two states differ greatly in oxygen concentrations. Due to poorer circulation and poorer ventilation, along with reduced carrying capacity for dissolved oxygen and increased oxygen demand of organisms resulting from higher temperatures, HTC states experience much lower oxygen concentrations than THC states often resulting in wide-spread deep-water anoxic events. Since the Oligocene/Eocene transition the deep sea has been dominated by THC conditions of high oxygen concentrations, a “two layered” ocean, and thermohaline circulation. Indeed, large-scale anoxic events have been absent since the Paleocene. However, prior to the Oligocene/Eocene in the HTC phase dating back to the Triassic, deep-water anoxia was frequent and often widespread.

I mention all of this because most of the recent dialogue on deep-sea faunal origins focuses on timing and effects of anoxic events. However, earlier workers in the late 19th century viewed the deep-sea as buffered against ongoing climate change and extinctions. This combined with early finds of “living fossils”, led many to believe the deep sea a fossil refuge comprised mainly of Pre-Cambrian and Cambrian relics. This “tenacious idea” is largely dispelled with findings that Paleozoic relics are rare, the presence of more primitive forms in shallow water, and evidence of recent and impressive radiations of some deep-sea fauna.

The remaining hypotheses about the origins include much of the rest of the Phanerozoic, with the exceptions of the Jurassic and Triassic. Most of these hypotheses center on “extinction and replacement” of the deep-sea fauna and thought to be triggered by severe anoxic events. Indeed, the idea of anoxic-triggered replacement has led to a new “tenacious idea”, the Mid-Cenozoic Replacement hypothesis. In this scenario, the deep-sea fauna is relatively contemporary dating back to the last major anoxic event at the Oligocene/Eocene boundary. However, several authors have questioned the ability of anoxic events to “reset” the deep-sea fauna.

Modern deep-sea faunas appear to actually represent a composite of clades with multiple originations throughout the Phanerozoic. For example, benthic deep-sea foraminifera genera represent 5% originating in the early Paleozoic, 45% from the pre-Cretaceous, and the remaining 50% from the Eocene. Whereas Asellota, dominating the deep-sea isopods, originated as late as the Jurassic with a subsequent and impressive deep-sea radiation, flabelliferan isopods conform to a relatively recent colonization in the Cenozoic. Bivalves and  gastropods are thought to originated in the late Ordovician, with known fossil assemblages dating to the Lower Cretaceous. In contrast deep-sea octopods, represent relatively recent origins dating back to 30 mya with a subsequent radiation at 15 mya. Similarly all modern deep-sea ostracods arose in the Oligocene and later. Holasteroid echinoids participated in at least four deep-sea invasions, three in the Late Cretaceous and one as early as the Miocene. The Late Cretaceous also gave rise to the stylaserid corals.

One final question remains. What is the site of invasion of shallow taxa into the deep sea? In general, the locations fall into the 1) Antarctic, 2) Mediterranean, and 3) multiple source locations. In the first two scenarios, locations of isothermal conduit is though to be key to migration.  In later, scenario shallow-water migration is thought to occur unimpeded and occurred globally. Evidence of the amazing pressure and temperature tolerances of embryos and larvae of marine invertebrates supports this idea supports the later. However, evidence exists that several clades did originate form Antarctic fauna , although there is also evidence that Antarctic fauna in parts derives from the deep sea.

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