Beaked Whales, Ziphiidae, are
small-to-medium sized Toothed Whales, Odontoceti, which hunt Fish, Cephalopod
Molluscs and Crustaceans using echo-location in a similar way to Dolphins.
However unlike Dolphins they are specialized for deep-diving, generally feeding
at depths ranging from a few hundred to a few thousand meters. This is
apparently a very successful strategy, as the group comprise about 25% of all
living Whale species. However while all crown group Beaked Whales (the crown
group comprises all living members of a group, their last common ancestor, and
everything descended from it) are either living species known to practice
deep-water diving or show adaptations to deep-water diving, such as fused
vertebrae, reduced flipper sizes, enlarged sinuses and reduced dentition, the
feeding habits of stem-group Beaked Whales (the stem group comprises members of
a group descended from the last ancestral species not thought to have given
rise to any living species outside the crown group, i.e. species more closely
related to the crown group than to any other group, but still outside the crown
group) is less clear, with the transition between shallow-water feeding and
deep-diving thought to have occurred somewhere in this group.
In a paper published in the
Proceedings of the Royal Society Series B: Biological Sciences on 9 September 2015, a
team of scientists led by Olivier Lambert of Terre et Histoire de la Vie at the
Institut royal des Sciences naturelles de Belgique, describe the discovery of a
remarkable death-assemblage in the Late Miocene Pisco Formation at Cerro
Colorado in southern Peru, comprising a stem-group Beaked Whale and a large
number of Clupeiform Fish (Herring), thought to be closely related to the modern
Pacific Sardine, Sardinops sagax.
The Pisco Formation comprises two
fining-uppward sequences (sequences in which the particle sizes grow smaller
upwards) ranging from course conglomerates (sediments containing mixed
pebble-sized rocks with an infilling of sand or finer particles, typically
associated with beaches, river beds or similar environments) to diatomaceous
mudstones (sediments comprising fine clay particles and the shells of
planktonic Diatoms, typically formed far from shore in the sea or a wide lake),
and separated by an unconformity (missing section in a stratigraphic sequence,
typically caused by exposure of the rocks and erosion associated with a drop in
sealevel), dated to between 9.9 and 8.9 million years old by the presence of
the Late Miocene marine Diatom Lithodesmium
reynoldsii. This formation has produced a rich assemblage of vertebrate
fossils, including Sperm Whales, Beaked Whales, Dolphins, Baleen Whales,
Turtles, Crocodiles, Seabirds, Sharks and Bony Fish.
The specimen described by Lambert
et al. comprises a Beaked Whale
assigned to the species Messapicetus
gregarius, which is well documented from the Pisco Formation with at least
ten known specimens, thought likely to have been female and estimated to have
been between 4.1 and 4.5 m in length. This specimen was buried in an inverted
position (upside down) with between 40 and 60 well-articulated Bony Fish
skeletons surrounding the head and apparently within the chest cavity. These
Fish average 38.8 cm in length, with very little variation, and are placed
within the Pacific Sardine genus, Sardinops,
and are thought to have been very similar to the modern Pacific Sardine, Sardinops sagax.
Lambert et al. believe that these fish represent the last meal of the
Whale, which has been partially regurgitated following the Whale’s death. They
cite a number of lines of evidence to support this theory. Firstly the Whale
and Fish skeletons are located within exactly the same stratigraphic layer, and
are all well-articulated, suggesting they died within a short time-span of
one-another. Secondly the Fish are found only around the head and body cavity
of the Whale, positions consistent with a regurgitated meal. Thirdly, despite
the rich fossil fauna of the formation, this is the only locality where these
Sardines are found, other than a few isolated scales and bones, and the only
Bony Fish death assemblage, suggesting that the locality does represent a
unique localized event, rather than the fortuitous deposition of a Whale
skeleton within a wider Fish bone-bed layer. Fourthly modern Pacific Sardines
consume only planktonic Crustaceans, not large vertebrate carrion, making it
unlikely that these very similar Miocene Fish were feeding on a dead Whale when
they died. Fifthly the size of the Fish closely matches the size of Fish
targeted by living toothed Whales of a similar size to the Pisco Formation
specimen. Finally the size and number of Fish suggests a total mass of about 25
kg, roughly the same as the masses of Fish produced by stomach content analysis
of extant Toothed Whales of similar size.
Fossil remains of the extinct beaked whale Messapicetus gregarious and associated
Clupeid Fish Sardinops sp. cf. S. sagax
found in Cerro Colorado. (a) Photograph and line drawing of the articulated caudal
portion of a skeleton of Sardinops sp.
in left lateral view (note the typically Clupeid urostyle supporting the caudal
fin complex), with a complete skeleton of the modern sardine S. sagax for comparison. (b) Imbricated
large cycloid scales of Sardinops sp.
in right lateral view showing tubercular protuberances in their central region
and curved radii-like lines in their lateral fields, with a body of S. sagax for comparison reporting the
putative collocation of the scale set. (c) Dolomite concretion with the skull and
mandibles of M. gregarious in ventral
view; occipital region, hamular processes of the pterygoids, posteroventral and
apical regions of the mandibles emerge from the concretion. Abbreviations: bv,
articulated bivalve shells; mda, apex of mandibles; mdp, posteroventral part of
mandibles; ph, hamular processes; wd, fragment of fossilized wood. (d) Line
drawing of the skull of M. gregarious
inside the concretion with a reconstructed outline of its body. Multiple
individuals of Sardinops sp. found
around the head and in the chest region are schematically represented. Stippled
line marks the outline of the concretion. Lambert et al. (2015).
The cause of death of the Whale
is not immediately obvious. The specimen appears to have been relatively
healthy at the time of death, not showing any signs of illness or malnutrition,
and apparently feeding normally (typically sick Whales have difficulty
feeding). It shows no sign of physical injury, which would be expected if it
had been the subject of an attack by a predator such as a large Shark or Sperm
Whale. Lambert et al. suggest that it
may have died as a result of poisoning, due to toxins produced by a bloom of
single-celled algae. Such blooms are a known cause of death in modern Whales,
who can either consume the toxic algae along with desired prey, or be poisoned
by consuming prey that had itself ingested the harmful algae. The Pisco
Formation is known to show evidence by testate (shelled) single-celled algae,
and while there is no evidence of such a bloom around the Whale find, lethal
toxins are more usually associated with algae that lack shells. Such algal
blooms do not (and cannot) occur in deep water, as the algae that produce them
are reliant on sunlight to grow.
Modern Pacific Sardines form
shoals of the same age, and therefore size, remaining with these individuals
throughout their lives. All of the Sardines found at the Pisco Formation Whale
death site are of approximately the same size, suggesting that these Fish
exhibited similar behaviour.
Life reconstruction of three individuals of
the extinct Beaked Whale Messapicetus
gregarious preying upon a school of
aged Sardines Sardinops sp. (average body
length 38.8 cm) in the upper part of the water column along the coast of
present-day Peru. The front individual is an adult male, whereas the last in
the background is a female. Alberto Gennari in Lambert et al. 2015).
Modern crown group Beaked Whales
are all deep-water feeders, but the ancient stem group Beaked Whales from which
they descend lack the specialist adaptations that enable their descendants to
do this, suggesting that they were shallow-water feeders. This theory is
supported by the discovery of a Whale from the Miocene of Peru which apparently
died shortly after feeding in a shallow-water environment. The last of these
stem-group Beaked Whales appear to have lived around the end of the Miocene,
after which they disappeared, leaving only the crown group Beaked Whales, which
appeared in the Middle Miocene and which feed in deeper waters. This
disappearance of the shallow-water Beaked Whales coincides with the appearance
of modern Dolphins towards the end of the Miocene, a group which radiated
rapidly and spread quickly around the globe. Lambert et al. suggest that these early Dolphins may have competitively
excluded Beaked Whales from shallow water environments, leading to the demise
of the stem group Beaked Whales.
Phylogenetic tree illustrating the relationships between extant and
part of the extinct Ziphiids. The outgroup is the Eurhinodelphinid Xiphiacetus. Red lines indicate
stratigraphic ranges. Dotted lines indicate uncertainty for the age of some
members of a genus. Separation between epipelagic and deep-diving taxa is based
on morphology, platform versus deep-sea deposits for fossil taxa, stomach
content analysis for Messapicetus,
and optimization of the deep-diving ecology of most extant genera on the
phylogenetic tree.
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