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Of all the mysterious,
secretive creatures in the ocean, the seahorse is
probably the most legendary. It looks like a creature
that could only exist in someone's imagination.
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Hippocampus,
photo by Seahorse.org |
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Seahorses and pipefish belong to the
Syngnathidae,
a teleost family whose oldest fossils date back to the Eocene (Lutetian:
52 mya; cf. Teske et al. 2004)*. The family also includes the pygmy
pipehorses (grouped with seahorses in the subfamily Hippocampinae),
pipehorses and seadragons (Solegnathinae), flag-tail pipefishes
(Doryrhamphinae), and pipefishes (Syngnathinae).
Till recently
little was known about their origin. When, how and where did
they originated? For several decades fossil seahorses have been
known only from the 5 million years old, Lower Pliocene (and
perhaps also Upper Miocene)* Merecchia River Formation, Poggio
Berni, Rimini Province in Italy. However, in the year 2005 much
older fossils of these amazing and exotic animals have been
found in the Tunjice hills (Slovenia). They are of the Middle
Miocene age (Lower Sarmatian, 13 million years), therefore
representing the oldest known seahorse fossils.
Seahorses
appear quite often in the beds of the Coproltic Horizon - a
Konservat - Lagerstätte, where also several fossil insects and
medusae fossils have been found. Most specimens are juvenile and
therefore very small, however adult specimens can be found as
well. Most
often only the head and the spine are preserved. Some specimen show that juvenile seahorses
might have been almost transparent.
In last year we have spent considerable time
comparing these fossil seahorses to other related fish, such as
pipefish and pipehorses. Luckily, one specimen has been
recovered, which still has bony plates preserved, covering the
body. All important macroscopic features are well visible, which
allow to reconstruct this seahorse species with relatively high
accuracy. Taking this specimen into consideration, it is clear
that the seahorses, we are finding in the Tunjice Hills,
represent a relatively well developed seahorse species.
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This fossil
seahorse specimen is the most important fossil found in
the beds of Coprolitic Horizon and perhaps represents
the most recognizable Slovenian fossil. It is excellently preserved. Even the
smallest features are visible, allowing to reconstruct
all macroscopic characteristics of this ancient
seahorse species. |
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When did the seahorses originated in the Earth history?
As Teske et al. (2004)
write, the world's tropical marine faunas can be divided into those
associated with an Atlantic Ocean biome (including the Caribbean and
Mediterranean), and those associated with an Indo-Pacific biome. It has
been suggested that this pattern arose after the closure of the Tethyan
seaway, a tectonic event that resulted from the convergence of the
African and Eurasian plates during the late Oligocene and Miocene.
Seahorses are found throughout the tropical and temperate regions of
both the Atlantic and Indo-Pacific biomes, but their origin and
evolutionary history are not well understood.
In a study on
cytochrome b sequences, Casey (1999) (cf. Teske
et al. 2004) concluded that the genus Hippocampus probably evolved in the Atlantic
biome. An Atlantic origin is also supported by the fact that most
species of the closely related pipefish genus Syngnathus are associated with the Atlantic
biome, as well as the fact that to date the only known seahorse fossils
have been found in Italy. On the other hand, it is interesting to note
that the majority of seahorse species are found in the Indo-West Pacific
region (>27
species).
Several small
seahorses Hippocampus sp. and their oldest known
ancestor. (photo on the right by Robert Pearce)
Based on the study of
RP1 and 16S rRNA sequences
Taske et al. (2004) have proposed a different hypothesis.
The large genetic distance of the
pygmy seahorse, H. bargibanti,
to all other seahorses suggests an ancient divergence of this group from
the main clade of seahorses. H. bargibanti
is widely distributed
throughout the western Pacific, but the fact that this species is highly
adapted to parasitise a certain species of Muricella gorgonian
coral suggests that it is unlikely to disperse readily beyond
the region where this species occurs. Among the species associated with the main clade of seahorses,
the most basal positions are occupied by H. breviceps and H. abdominalis.
Both species are associated with the Australian continent, suggesting
that this may be the region from which seahorses originated. An
Australian or south-west Pacific origin of seahorses is also supported
by the distributions of the three possible sister genera of the genus
Hippocampus. Pygmy pipehorses
of the genus
Amphelikturus are
restricted to the Atlantic biome, the genus Acentronura is widely distributed throughout the
Indo-Pacific, and all known specimens of Idiotropiscis have been found in Australian
waters. Among these three genera, the species of the genus Idiotropiscis,
and particularly a recently discovered species from southern New South
Wales, are most seahorse-like in appearance.
Where did the seahorses
originated?
Based on the
present-day evidence it is diffucult to decide, where seahorses
originated (Peter Taske, personal communication). Fossil seahorses found
in the Tunjice hills (Slovenia) and also the Italian fossil seahorses confirm the
presence of these seahorses in the Paratethys and Mediterranean close to the time when the Tethyan seaway
closed. The Paratethys and Mediterranean were once probably
species-rich, and it is possible that the ancestors of all of the extant
forms of pygmy pipehorses, pygmy seahorses and proper seahorses were
once present there, and then spread towards the Indopacific. Likewise,
it is possible that the form we are finding in the Tunjice Hills, was
once present in the Indopacific and then colonised the Paratethys and
perhaps also the Meditrranean (Peter Taske, personal comunication). This
could happen already before the Early Sarmatian, because the migration
paths connecting Paratehys and Mediterranean with the Indopacific
existed till the Late Badenian (Roegel 1999). Later in the
Early Sarmatian, the seaway into the Indopacific closed, and another
opened, connecting Paratethys and Mediterranean.
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Paleogeographic map, seacurrents and possible migration paths for the seahorses in the Late
Badenian. (After Roegel 1999). |
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Paleogeographic map, seacurrents and possible migration paths for the seahorses in the
Early Sarmatian. (After Roegel 1999). |
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