A SKULL OF THE MINKE WHALE DUG OUT FROM OSAKAHIDEO OMURAWhales Research Institute, TokyoABSTRACT

Among collections of bones of whales dug out from Osaka and now
being kept at the Osaka Museum of Natural History is included ahalf-broken skull of the minke whale, Balaenoptera acutorastrata. Aftercomparing with known specimens of this species this whale wasestimated to be a full grown animal of about 9 m in length.

INTRODUCTION

During several years in 1960's a number of bones of whales have been dugout from various places in Osaka city while large-scale construction works suchas subways, expressways, and buildings were going on (Shindo, 1975). Thesebones are now being kept at the Osaka Museum of Natural History and I hada chance to observe these bones in December 1975 and found a half-broken
skull of the minke whale is included among the collection.
In addition to these, bones of sperm whale were also dug out recently fromunderground of Higashiosaka city. The species of this whale was identified
by Okazaki (1975) by the presence of the characteristic atlas and a tooth among
the collection.
At the period of Jomon Transgression (about 7,000-3,000 B. P. [years beforepresent]) a vast area from Uemachi Upland, where Osaka castle is situated,
eastwards and including Higashiosaka city to Ikoma mountain had been
invaded by sea water, forming a bay called Kawachi-wan. This area is now
the Osaka Plain. The development of the Osaka Plain was studied by
Kajiyama and Itihara (1972) with reference to the radio-carbon dates. They
recognized nine successive stages of the development. In the second or thestage of the Paleo-Kawachi Plain (ea. 9,000 B. P., earliest Jomon age) the sea
level was lower than in the present by 20 m or over. Osaka Bay was alreadyin existence, but the Kawachi area was not covered by sea water. In the thirdor the stage of the Kawachi Bay I (ea. 7,000-6,000 B. P., the first half of early
Jomon age) the sea level reached to that of the present and the Paleo-Kawachi
Plain was widely covered by sea water. In the fourth or the stage of theKawachi Bay II (ea. 5,000-4,000 B. P., latest early Jomon to middle Jomonages) the sea level remained nearly unchanged, but in the next stage of theKawachi Lagoon (ea. 3,000-2,000 B. P., latest Jomon to the first half of Yayoiages) due to the development of the sand bar north of the Uemachi Upland,Kawachi Lagoon was formed and the inner part of the bay was filled with
Sci. Rep. Whales Res. Inst.,
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OMURA

fresh water. The growth of the sand bar continued and it developed
successively into the stages of the Kawachi Lake I and II, Osaka Plain I, andfinally to the stage of the Osaka Plain II or the present.
In the 3rd and 4th, and also in the 5th stages too, it is possible that whales
came into the Kawachi Bay and some of them stranded ashore or dead afloat
and finally sunk. This is the possible explanation why bones of whales dug
out from the alluvial deposits in Osaka. It is also possible, therefore, thatthese bones are of the existing whale species.

OBSERVATION AND CONCLUSION(Plates I and II)

This skull (0. M. N. H., F2639) was dug out from 14 m deep while constructmg
underground railroad at 2-Chome, Oimazato-hommachi, Higashinari-ku, on 11August 1966. The skull was broken transversely towards middle part of thesupraoccipital bone by mishandling during preparation for transportation after
discovering. Its posterior portion is remaining in a very good condition,suggesting the skull was a splendid specimen before it was broken. Exo- andbasioccipitals, and temporals are nearly perfect, enabling me to measure thebreadth at squamosal or the greatest breadth of the skull, as well as othermeasurements concerning posterior region of the skull precisely. Posterior
marginal portion of the vomer is remaining attached, but pterygoids arebroken and tympanic bullae are lacking.
Vertex and facial region of the skull are completely broken, but orbital
process of the frontal of both sides, part of vomer, 170 cm in length, and somefragments of maxillary and premaxillary are remaining separately. Frontals
are broken at their proximal ends and their distal margins or orbits arecomplete and the orbital length was measurered accordingly.
At a glance this skull can be identified to be a species of balaenopterid
whales (Balaenopteridae) and from its comparatively smaller size to be theminke whale. Measurements of the skull are shown in Table 1, comparing
with corresponding measurements of other specimens from the North Pacificand Antarctic, cited from Omura (1975). Measurements numbers in the table
are the same as used in the previous paper (1975). The breadth of the skull
at squamosal of this specimen (F2639) is far greater than in the two specimensfrom the North Pacific (A Y69B, A Y69A), and lying between figures for thetwo specimens from the Antarctic, suggesting the body length of this whale isalso between these two specimens, i. e. 8.5-9.8 m and possibly around 9 m.
These two whales from the Antarctic are physically matured (Omura, 1975)
and since the body length at sexual maturity of minke whales is somewhat
smaller in those from the North Pacific than in those from the Antarctic(Ohsumi et al., 1970) this specimen from Osaka was possibly a full grown whale.
This is also supported by rigid structure of the skull.
Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

MINKE SKULL FROM OSAKATABLE 1.

71

MEASUREMENTS OF THE SKULL (F2639) COMPARED WITH THE
KNOWN SPECIMENS OF THE MINKE WHALE (in mm)

Measurements

F2639
Osaka

AY69B*
N. Pacific

AY69A*
N. Pacific71]2793*
Antarctic71]2883*
Antarctic

16.

Breadth of skull at squamosal

1,180

708

1,256

Length of orbit, frontal, right
Length of orbit, frontal, left

207
206

156

613
146

1,075

19.
20.

157

146

188
190

207
207
886
228

21.

Breadth of occipital bone

822

563

473

760

22.
23.

Breadth across occipital condyle
Height of occipital condyle, right229

157

124

98

154
120

203
131

24.
25.

Height of occipital condyle, left
Breadth of foramen magnum
Height of foramen magnum12479

96
83

100

124

80

70

79

60

90

85

97

127

M. 6.6m
juv.

M. 5.4m
juv.

M. 8.5mad.

F. 9.8m
ad.

26.

Sex, body length, and age

145
142

• Cited from Omura (1975).

In other measurements figures for this specimen are within the ranges ofthe values of the two specimens from the Antarctic or very close to them,except the measurement no. 26 (height of foramen magnum), which is thought
to be of less importance. From this table it can safely be concluded that thisspecimen is a skull of the minke whale, Balaenoptera acutorostrata. Thereremains, however, still a taxonomic problem among minke whale populationsof the world, i. e. a possibility that the Antarctic minke whale, B. bonaerensis,
is in fact a different species or subspecies of the minkes in the northernhemisphere, B. acutorostrata. But the distinctions so far noted in the skull
are in the vertex and facial regions and not concern the posterior portion of
the skull (Omura, 1975). Accordingly this problem does not affect the above
conclusion.
ACKNOWLEDG MENTS
My sincere thanks are due to Mr. Hiroyuki Taruno and other members ofthe Osaka Museum of Natural History. They extended me all possible
arrangement and help for the works done at the museum. Mr. H. Taruno
also read and commented on draft of the manuscript.
REFERENCES
KAJIYAMA, H. and M. ITIHARA, 1972. The developmental history of the Osaka Plain withreferences to the radio-carbon dates. The Memoirs of the GeologiJal Society of Japan. 7: 101-112.
(In Japanese with English abstract).
0HSUMI, S., Y. MASAKI and A. KAWA MURA, 1970. Stock of the Antarctic minke whale. Sci.
Rep. Whales Res. Inst., 22: 75-125.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

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OMURA

OKAZAKI, Y. 1974. Higashiosaka-shi de Makko-kujira no Hakken (Bones of sperm whale
discovered in Higashiosaka city). Nature Study (ed. Osaka Museum of Natural History). 21(12): 143-144. (in Japanese).
OMUR.-\, H. 1975. Osteological study of the minke whale from the Antarctic. Sci. Rep. Whales
Res. Inst., 27: 1-36.
SHl'.'iDO, N. 1975. History of whales in the Inland Sea. Hyogo. 89pp.

EXPLANATION OF PLATESPLATE I
Half-broken skull of the minke whale dug out from Osaka and now beingkept at the Osaka Museum of Natural History. F2639.
Fig. I. Posterior view.
Fig. 2. Anterior view.
PLATE II
The same specimen shown in Plate I.
Fig. I. Superior view.
Fig. 2. Inferior view.

Sci. Rep. Whales Res. Inst.,
No. 28, 1976.

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PLATE IIOルfURA

Sci.Rep.Whales Res.Inst.
,No.28,1976.