Geology o(n)f Whale-falls

"They say the sea is cold, but the sea contains the hottest blood of all,
and the wildest, the most urgent.
Whales Weep Not! D.H. Lawrence (1885-1930)

The deep-sea is sometimes compared with a desert where only occasionally oasis exist - hydrothermal vents are locally oasis where the heat and chemicals coming from earth allow life in a cold and vast subaqueous desert.
But how got this habitat colonized by animals on the first time, and how the first organism reached this places and evolved to survive under these extreme conditions?
Biota of such vents and biota living on and off decomposing whale carcasses share peculiar chemosynthetic adaptations to exploit these resources, and this shared ability has suggested that they somehow are related each to another.

Surely since the Mesozoic, when large sea dwelling animals evolved, carcasses reached the bottom of the sea, for example some chemosynthetic molluscs appeared long before the modern great whales, and others share the last common ancestor with species living on shelf habitats 30 million years ago, when some of the modern whale lineages first evolved. Also molecular data suggest that specialization found in deep sea may have first appeared at shelf depths, and bone-eating worms related to vent polychaetes may represent intermediate stages of evolution and adaption.

Taphonomic processes in deep-water environments differ markedly from those in shallow waters. The preservation of modern whale skeletons is possibly influenced by the depth of the water column, increased hydrostatic pressure at greater depths is presumed to prevent the whale carcass from floating.

Once arrived on the bottom of the sea, the decomposition of whale carcasses follows a general succession of carrion feeders (like observed on terrestrial carcasses). The flesh is removed first by larger animals, like sharks, then the body and surroundings are colonized by animals that feed both on the carcass and on the bacterial mats developing on it. When only the bones remain, after some months or with larger animals after years, very specialized species community can found feeding on the bones itself.
In modern deep-sea carcasses abundant lipids are exploited by bacteria living within bones. Bone-dwelling mussels and polychaetes, in symbiotic relationship with bacteria, are abundant.
Modern natural whale falls are rare on shelves, but in Neogen shelf sediments often fully articulated skeletons were discovered. Research on this kind of discoveries is still rare.

One of this extraordinary discoveries, that may shade light on the evolution of this communities, was found in Pliocene (3,19-2,82Ma) sediments of the paleo-Tuscan archipelago, near the modern city of Orciano Pisano. During the excavation of the 10m long mysticete the species richness and position of the macrofauna with respect to the skeleton was recorded.
The results were compared with modern and fossil species assemblages, especially molluscs of different habitats, ranging from coastal to bathyal sample localities (0-1.600m depth). The studied fossil taphonomic assemblages comprise examples of Pliocene whales from various Italian localities, like Ponte a Elsa, Castelfiorentino, Castellarano and Castell' Arquato.

Fig.1. Taphonomy and paleoecology of Orciano Pisano "whale fall community". A) Glossus humanus in life position below neurocranium (dashed line outlines scapula). B) Megaxinus incrassatus in vertical position and C) in horizontal position. D) Tip of skull, with Amusium cristatum (long arrows) and M. incrassatus (short arrow). E) Articulated M. incrassatus below heavenly damaged large bones. F) Large tooth of Charcarodon carcharias. G) Tympanic bulla with deeply cut marks. H) view showing articulated, but damaged caudal vertebrae and lacking dorsal vertebrae. H1) Sketch of the skeleton with position of M. incrassatus (circles), G. humanus (stars) and teeth of Carcharodon carcharia (open triangles) and Prionace glauca (black triangle) (from DOMINICI et al. 2009).

The fossil whale fall communities showed some similarities to modern colonization of whale carcasses, but also important differences. Even if the specific taphonomic pathways those carcasses undergo in different water depths differ, some general observations can be made.

1) The high numbers of found shark teeth’s suggests that sharks are a frequent cause of initial flesh and bone consumption. Also they may provide access to the interior of the whale for other animals.

2) Organic enrichment in the sediments around the carcass provides an advantage for heterotrophic organisms (like molluscs and sea urchins).

3) Observed alteration of fossil bones are compatible with the occurrence of bone-eating worms, from a Spanish fossil whale in a recent study a new icnogenus was described, that represents the borings of animals maybe comparable to the modern bone-eating polychaetes Osedax.

4) In contrast to modern whale fall colonization, in the fossil record a "reef stage" is documented, in form of bone-cemented epibionts.

The importance of bone eating animals and chemosynthetic specialists increase from shallow to deep waters. The analyse of molluscs suggest that shelf whale-fall communities are structured similarly to shelf vent and seep communities, where local food webs exploit photosynthesis-derived carbon and animals gain enough energy from heterotrophy, and chemosynthetic symbiosis never dominate. This provides a possible insight in the evolution of these communities, whale-fall in different depths provided "oasis" for colonization of animals coming down from the shelf area or shelf area limits, step by step these refuges enabled organism to adapt to more severe conditions in the deep sea and finally colonize sea vents. Modern whale carcasses maybe still play an important role to connect these rare habitats among each other.

ALLISON et al. (1991): Deep-water taphonomy of vertebrate carcasses: a whale skeleton in the bathyal Santa Catalina Basin. Paleobiology 17(1): 78-89
DOMINICI et al. (2009): Mediterranean fossil whale falls and the adaption of molluscs to extreme habitats. Geology V.37(9):815-818; doi: 10.1130/G30073A.1
KAIM, A. et al. (2008): Chemosynthesis−based associations on Cretaceous plesiosaurid carcasses. Acta Palaeontol. Pol. 53(1): 97–104

Introduction imagine: Pliocene whale skeleton from Tuscany, Italy from the front cover of Geology 37(9) 2009

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