Ｈｏｒｓｅｓｈｏｅ ｃｒａｂｓ ｉｎ Ａｕｓｔｒａｌｉa
When you watch the title of “Horseshoe crabs in
Australia", you may think that there are horseshoe crabs in Australia now.
However, there are not any horseshoe crabs in the Australia zone including
Australia now. This report is the story of horsesoe crabs which had been used
to live in Australia and were found as fossils.
On the other hand, three species of horseshoe crabs now live in east south
Asia, that is, the Oriental zone. The boundary of the Oriental zone and the
Australia zone is the Wallace's line or the Weber's line. The Wallace's line
situates at the west of the Sulawasi (Celebes) island, and the Weber's line situates at
the east of the same island. There is a report that a species of horseshoe crab,
Tachypleus tridentatus lives in the Sulawasi island (Sekiguchi, 1988). Therefore in future,
we may find some living horseshoe crabs in natural condition at north west Australia.
There are now four species of horseshoe crabs in the world. Limulus polyphemus lives in the east coast of north America continent. Other three species
live in Asia continent. Tachypleus tridentatus lives in Japan, south China, north Vietnam and islands of east south
Asia. T. gigas lives the area of Asia continent from south Vietnam to east India
and the islands of east south Asia. Carcinoscorpius rotundicauda lives the area of Asia continent from south China to east India and
the islands of east south Asia. We can notice those three Asian species live in
the Oriental zone except Japan.
The ancester of all four species is seemed to be the same species called Mesolimulus walchi which inhabited in Europe 200 million years ago.
There are only four species of horseshoe crabs
in the present world, but there used to be many species of horseshoe crabs in
the whole world. In Australia, there were many horseshoe crabs. Almost of
those horseshoe crabs in Australia lived in fresh water. We are surprised in this fact,
because all present horseshoe crabs live in sea water or brackish water.
We visited to Australia in 2001 and could get the informations about
fossils of horseshoe crabs there. We now report you about the rise and fall
of Australian horseshoe crabs.
More than 200 milion years ago, all continents
were gathered into only one land in the earth. Australia was face to the south
pole continent and India and close to Africa continent. At the time, Australia
was situated at the higher latitude than present one. We call the gathered
land Pangaea. As there is a ocean call Tethys Sea, Pangaea was divided into two
parts. We call the north part Laurasia continent or Angara continent, and
the south part Gondwana continent.
Mesolimulus walchi used to inhabit in the Europe which was in Laurasia continent, therefore
we can regard they inhabit on the north coast of Tethys sea. The horseshoe crabs, which moved toward
west, became Limulus polyphemus and another horseshoe crabs, migrated toward east, became Asian horseshoe crabs.
We can suppose to think the horseshoe crabs came into India after when
the Indian subcontinent hit the Laurasia and became one land the Eurasian continent.
At 600 million ago, the beginning of the Paleozoic era, the very strong
living organism appeared and prospered. That is trilobites (【Subphylum】 Trilobitomorpha).
The trilobites became extinct at the end of the Paleozoic era, 230 million ago. On the other hand, its direct descendant 【Subphylum】 Chelicerata including horseshoe crabs
still have survived until the present, somehow. Horseshoe crabs seem
to have been evolved from 【Order】 Redlichiida in trilobites at the
first period of the Paleozoic era. Somebody say horseshoe crabs evolved
from 【Suborder】 Redichia of 【Order】 Redlichiida in trirobites, but we can
see more similarity with Olenellus vermontanus of 【Suborder】 Olenellia in same 【Order】 Redlichiida .
The famous sea scorpions (【Class】 Eurypteria)
are also relative with horseshoe crabs. Both animals belong to 【Subphylum】
Chelicerata. One kind of sea scorpions reached the land and was evolved into
land scorpions and spiders, that is, 【Class】 Arachnida.
Both trilobites and sea scorpions inhabited
widely but they became exinct in the Paleozoic era. Only the horseshoe crabs
have still alive in the present sea among Trilobitomorpha and marine Chelicerata. Consequently,
we should protect horseshoe crabs.
The process of evolution such as the evolvement from 【Subphylum】 Trilobitomorpha
to 【Subphylum】 Chelicerata, the appearance of 【Class】 Xiphosura and the
appearance of 【Order】 Xiphosurida seem to occur in Laurasia continent according
to fossil unearthing. When we call horseshoe crabs in paleontology, it stands for
the species of 【Order】 Xiphosurida. So that 【Order】 Aglaspida in same 【Class】
Xiphosura is not called horseshoe crabs (Table 1).
Most fossils of horseshoe crabs and its
relatives are definitely found in Laurasia continents and very a few of those
fossils are found in Gondwana continent including Australia. Only five kinds
of fossil horseshoe crabs have been reported in Australia. However, it is
certain that horseshoe crabs used to inhabit in Australia.
The oldest fossil horseshoe crabs in Australia are Kasibelinurus amicorum, founded in the stratum of late Devonian period, 400 million to 350 million years old. The word Kasibelinurus means a company of Belinulus. Belinulus is the horseshoe crab founded in Europe and it means tails such as the
arrow. The word amicorum means friendly. The length of Kasibelinurus amicorum fossile is 63 mm and it was founded at near the Parkes in New South Wales (Fig. 1; Pickett 1993). As
it was founded at the sea layers, it is likely to be saltwater horseshoe crabs. To ours interests,
other horseshoe crabs in Australia were found in the fresh water strata.
There are reports of the fossil horseshoe crabs as the same genus as
Kasibelinurus species in Europe and North America continents so that the genus widely
spread in the earth.
Fig.1. Kasibelinurus amicorum. The figure was written by Pickett (1993), and the photograph was taken by us at Australian museum.
Kasibelinurus belongs to 【Suborder】 Limulina. There are some opinions in relation with
the belong 【Family】. We feel it can belong to 【Family】 Paleolimulidae,
but Pickett (1933) made it belongs to the new 【Family】 Kasibelinurus. We can not agree to make many 【Family】. It
is very difficult. We think the horseshoe crabs of the Paleozoic era are classified
into 【Family】 Paleolimulidae except the species belonging to 【Suborder】 Synziphosurina, and ones of the Mesozoic era into 【Family】 Mesolimulidae, and
ones of the Cenozoic era including the present ones into 【Family】 Limulidae.
That is, there are only three families.
There is a same problem about 【Superfamily】. We
think 【Superfamily】 is not needed. Somebody think 【Superfamily】 Belinuracea and
【Superfamily】 Euproopacea. However, we can not feel that there is big
differences between Belinuracea, Euproopacea and Paleolimulidae, so that it is unnecessary for
them to divide many 【Superfamily】. We propose Belinurus and Euproops should
belong to 【Family】 Paleolimulidae in 【Suborder】 Limulina.
We can see the body segments of 【Suborder】 Synziphosurina have been
independent and each segments work individually. On the other hand, the
segments of 【Suborder】 Limulina fuse. The segments of 【Family】 Paleolimulidae
is clear in spite of fuse.
The fossil of Synziphosurina, similar to Kasibelinurus, has founded from the strata of early stage of Devonian period in
Bolivia of south America (Eldredge, 1974). This Bolivian fossil has been
recorded as the oldest horseshoe crab in Gondowana continent. The theory
of that the evolutionary process from 【Suborder】 Synziphosurina to 【Suborder】
Limulina happened in Laurasia continent, is strong possibility but we can
not deny that the process might have happened in Gondowana continent.
An Aglapida fossil has found from the strata of the late Cambrian period in the
early Paleozoic era at north Tasmania, Australia (Jago and Baillie, 1992). Aglaspida is not horseshoe crabs, but it is close
order of Xiphosurida, that is, horseshoe crabs (Table 1).
A fossil, Hemiaspis tunnecliffei has found from the strata of Silurian period and another fossil, Pincombella belomontensis has founded from the strata of late Permian period. Both species were
reported as horseshoe crabs by Chapman (1932). However, as the result of detailed investigation, it elucidated that Hemiaspis tunnecliffei was a species of trilobites and Pincombella belomontensis was an insect (Pickett, 1984).
The fossil, certainly judged as a horseshoe crab, is a Paleolimulidae
which has founded from the strata of the late Permian period in Poatina in Tasmania island
(Fig. 2, Ewington etal., 1989). It located between 75 to 80
degrees north latitude and suppose to be cold climate at the time. And the fossils has found from the strata, made from
fresh water, so that the horseshoe crabs suppose to be fresh water horseshoe
crabs. The length is 3 cm, except for tail telson.
Fig.2. The horseshoe crab fossil of Paleolimulidae from Tasmania. From Ewington & Banks (1989).
Horseshoe crabs, born in Paleozoic era, lived
over the last Paleozoic era which trilobites came exinct and also horseshoe
crabs inhabit during the period of dinosaurs, Mesozoic era. Horseshoe crabs in
Australia also lived over the time of extinct at the end of Paleozoic era.
The fossil of Dubbolimulus peetae has been found from the strata of the middle Triassic period of Mesozoic era
at Dubbo in New South Wales (Fig, 3, Pickett, 1984). As it is a horseshoe crab in Mesozoic era,
it is possible to belong to the 【Family】 Mesolimulidae, but Pickett
(1984) proposed it belongs to new 【Family】 Dubbolimulidae. Surely we can see
the wide prosoma which is peculiar to Australian horseshoe crabs, on the Dubbolimulus. Dubbolimulus lived in flesh water. The total length is 4 cm, width of prosoma
is 27.8 mm, and the length of prosoma is 14.0 mm.
Fig.3. Dubbolimulus peetae. The figure was written by Pickett (1984), and the photograph was taken by us at Australian museum.
An astonishing, symbolic Australian fossil horseshoe crab was found from the 200
million years old strata of Beacon hill at Brookvale of Sydney in New South
Wales. It is Austrolimulus fletcheri (Fig. 4, Rick, 1955, 1968). It may belong to 【Family】 Mesolimulidae, but
Rick (1955, 1968) has proposed new 【Family】 Austrolimulus. You can see
the figure widens remarkably. Two fossils have found and the bigger one's total
length is 14.6 cm, the length excluding telson is 5.7 cm. The width is
17.8 cm. You can recognize the width is longer than the length. There
is no stings on the abdomens and telson is longer than the body. This
symbolic Australian horseshoe crab also lived in fresh water.
Fig.4. Austrolimulus fletcheri. The photograph was gotten at Australian museum.
The last horseshoe crab in Australia has found from the strata of early Cretacean
period at Koonwarra in Victoria. It is Victalimulus mcqueeni (Fig. 5, Rick and Gill, 1971). This horseshoe crab is the latest one
in Australia. Cretaceous is the last period of Mesozoic era and the wild extinction happened and
dinosaurs also became exinct at the time. Although Australian horseshoe
crabs lived over the last Paleozoic era, they seemed not to be able to
live over the last Mesozoic era and they became extint in the same way as dinosaurs. This Victalimulus also found from the strata of fresh water so that they seem to have lived
in fresh water. However, Rick and Gill (1971) mentioned it might have
lived in salt water or brackish water, considering that there are stratas
of salt water near there. If the conjecture was true, it became to think
that they had gone into the river or lake and became a fossil after millions
years. This Victalimulus looks like Mesolimulus walchi which lived in Europe before 200 millions years and the ancestor of present horseshoe crabs.
We think it should be studied more about the origin of Victalimulus.
Fig.5. Victalimulus mcqueeni. From Riek & Gill (1971).
Mesolimulus walchi found from strata of salt water and the four kinds of living horseshoe
crabs are all living in salt water. But, They prefer weak salt water than
strong salt water, nevertheless there is a record of Carcinoscorpius rotundicauda was found in the river which is 150 km far from the river mouth. Limulus polyphemus vitaly live in near 50% sea water.
After all, Australian horseshoe crabs are said
to all became extinct at the end of Mesozoic era, but we expect that living
horseshoe crabs in fresh water might still exist in some rivers or lakes of
Australia, because Australia is the mysterious continent.
Acknowledgement
We deeply thank Dr. Robert Jones and staffs' cooperation in Sydney
Museum, staffs of Seahorse Museum, and Dr. Karen Gowlett-Holms and staffs of
Eagle Howk Diver Center. We would like to express our heartfelt thanks to
Dr. Chip Clark of Smisonian Museum in Washington D.C.
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