A SKULL OF BALEEN WHALE DREDGED FROM
THE INLAND SEA OF JAPANHIDEO OMURAWhales Research Institute, TokyoAND
TOSHIO KASUYA
Ocean Research Institute, University ef Tokyo, TokyoABSTRACT

A half-broken skull was dredged by a fishing vessel in the Inland Sea ofJapan in August 1978. This skull was studied from the taxonomic standpoint
and was thought to be a skull of the Bryde's whale, Balaenoptera edeni.
INTRODUCTION

It was reported in a local press in Oita Prefecture dated 27 August 1978 a big boneof unknown animal was dredged by a small fishing vessel while the operation ofbottom drag net for shrimps and other bottom fish. The bone was so heavy thatthe fishermen could not haul it on to the deck they ceased fishing operation andbrought back the bone, hanging it outside the boat, to the fishing port of Kizuki
City (Fig. 1).
The bone was eventually brought to Kizuki-shiritsu Minzoku Shiryokan(Kizuki Municipal Folklore Museum) for preservation and identification of theanimal species. At first they consulted with Oita Marine Palace, a nearby aquarium and was told that the bone may be a skull of a w.hale and examination byspecialist is needed. Then they took photographs of the bone from various angles
and sent them to Dr Teruo Tobayama of the Kamogawa Sea World for identification of the species, who in turn passed them to the Whales Research Institute.
From the photograph we understood that the bone is half-broken skull ofa baleen whale, but identification of the species is very difficult without close
observation of the bone. And this is our great concern in the light of rather poor
scientific knowledge of whales hitherto obtained from the Inland Sea, though
there is a good reason to think that a number of whales came into the Inland Sea
and out annually until about less than one hundred years ago (Shindo, 1975;Omura, 1974). Thus we made a trip to Kizuki City, Oita Prefecture, towards
the beginning of October 1978.
OBSERVATIONThe skull is completely lacking the maxillaries and premaxillaries.
Sci. Rep. Whales Res. Inst.,

No. 31, 1979, 85-92

Possibly thisOMURA AND KASUYA

86

is caused by loose articulation of bones in the facial region of the skull of the baleenwhales, contrary to the toothed whales. Fortunately the vomer is remaining(Pl. I, Figs 1, 2 and 3). Its upper margin and anterior tip are broken to someextent, but presumably only a few centimeters, and thus protruding forward greatlyand giving an impression of a flat rostrum in profile. Nasals are completely
lacking (Pl. I, Fig. 1). On the inferior side of the vomer there still attached thepalatines, to which pterygoids are also attached, but their posterior ends or hamularprocesses are broken (Pl. I, Fig. 2).
130°E

SEA OF JAPANz0PACIFIC OCEANz
Fig. I. Chart showing the position where the Kizuki specimen was dredged bya fishing vessel. Marked with a cross.

The brain case of the skull is rather in a good condition. The supraoccipitalbone has a broken hole just above the occipital condyles, and its outer margins
especially of superior regions are broken to some extent, but still it shows a characteristic feature of the balaenopterid whales, being roughly triangle in posteriorview and the surface being relatively flat and having no deep concavity (Pl. I,Fig. 1 and Pl. II). Exo- and basi- occipitals are nearly complete. Temporals
are in good condition, except extreme end of the right zygomatic process whichis broken slightly. Orbital processes of the frontals are also in good condition,
especially in posterior portion and orbits, but the anterior portions are broken.
Parietals are also remaining on the lateral sides of the brain case and extend forSci. Rep. ~Whales Res. Inst.,
No. 31, 1979SKULL OF BALEEN WHALE
TABLE I.

87

MEASUREMENTS OF SKULL, KIZUKI SPECIMEN
Measurements:

Lengthin mm% ofbreadthGreatest breadth of skull at zygomatic process
Breadth of skull at orbital process of frontal, posterior
centerBreadth of skull at mastoid process of temporal
Length of orbit, right*, leftLength of vomer, from broken tip*
Length of palatine, greatest, right, left, along median line
Length of skull from broken tip of vomer to occipital condyle*
Breadth of occipital condyle
Height of occipital condyle, right, left
Breadth of foramen magnum
HeightHeight of supraoccipital bone from foramen magnum
Breadth of supraoccipital bone
Vertical height of skull from vertex to vomer
Horizontal length from vertex to occipital condyle

1,287
1,264
1, 181986
195
210
2, 125
582
583
559
2,560
259
160
166
8596
623
871
431
668

100.0

98.2
91.8
76.6
15.2
16.3
165.1
45.2
45.3
43.4
198.9
20.1
12.4
12.9
6.6
7.5
48.4
67.7
33.5
51.9

* Least value due to breakage.
TABLE 2.

COMPARISON OF THE KIZUKI SPECIMEN TO BRYDE'S,
SEI AND MINKE WHALES
Kizuki specimen

l\1easurements

Lengthmm%

1l

Cited from Omura, 1959.

2)

Lengthmm%~45 ft(13.5m)
3,480 215.5
43927.2

Sex and body length
Condylo-premaxillary length
Tip of premax. to anterior end ofvomer
Greatest breadth of skull
Breadth of skull at orbit (center)
Breadth of occipital condyle
Height of occipital condyle
Breadth of foramen magnum
Height of foramen magnum
Height of supraoccipital bone fromforamen magnum
Length of orbit

Bryde's 1'

Sei 1lLengthmm

Minke 2l%

Lengthmm

%

~9.8m

~43ft

(12.9m)
3,060 229.4

2,350
290

187.1
23.1
100
97. I
18.2
11.4
6.3
10.1
51.8
16.5

1,287
1,181
259
163
85
96
623

100
91.8
20. l
12.7
6.6
7.5
48.4

1,615

100

1,335

100

2661745345835

16.5
10.8
3.3
2.8
51. 7

251
167
84
97
768

18.8
12.5
6.3
7.3
57.5

1,256
1,219
228
143
79
127
650

210

16.3

234

14.5

233

17.5

207

Cited from Omura, 1975.

ward beyond the line of the vertex.
We made some measurements of the skull. The greatest breadth of theskull at zygomatic processes of the squamosals, and other measurements concerning
breadth of the skull are fairly accurate. Results of the measurements are shown
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No. 31, 1979

88

OMURA AND KASUYAin Table 1 and these measurements are compared with corresponding measurements of the Bryde's, sei and minke whales in Table 2.

DISCUSSION

In profile the skull of the Kizuki specimen is very fiat (Pl. I, Fig. 3). Although it
lacks maxillaries and premaxillaries the vomer is remaining in almost all its length,
and we can safely assume that the ventral border of the vomer represents that ofthe rostrum itself. From this fact and combined with other evidence we candeny the possibility of the specimen of being Balaenidae as well as Eschrichtiidae.
In Balaenidae the rostrum is curved downwards greatly and roughly Tee shapedin dorsal aspect of the skull, but the Kizuki specimen shows no resemblance of
these character. In Eschrichtiidae the rostrum is curved downwards in less extent
than Balaenidae but more curved than in Balaenopteridae. Further in Eschrichtiidae the supraoccipital bone presents three deep concavities, and on the superior
portion two prominent and peculiar rugosities, and also present similar rugosities
upon the basisphenoid and basioccipital bones (Andrews, 1914), but the Kizuki
specimen has neither such clear concavity nor rugosity. Thus it is clear that theKizuki specimen belongs to a species of the family Balaenopteridae.
Among the family Balaenopteridae there are two genera of Megaptera (onespecies) and Balaenoptera (five species). We can erase the posibility of the humpback whale, Megaptera novaeangliae, because of its slender shape of the Kizuki
specimen. The skull of the humpback whale is robust and it has maximum relative zygomatic width in the family Balaenopteridae, or 57-66.9% of the condylopremaxillary length (Tomilin, 1967). In the Kizuki specimen the ratio is lessthan l,287/2,560x 100=50.3 (see Table 1). The skull length of 2.56 m is thelength from the broken tip of the vomer to the occipital condyles. Probably wecan add to this figure some 30-40 cm as the length from the tip of premaxillary
to the broken end of the vomer (see Table 2), which makes condylo-premaxillary
length to be 2.90-3.00 m, resulting the ratio as small as 43-44%. From this fact
alone the possibility of the humpback whale will be erased.
We can safely conclude, therefore, that the Kizuki specimen belongs to aspecies of the genus Balaenoptera, and there remains five species to be considered.
First of all we can exclude the possibility of the minke whale, B. acutorostrata, because of the size of the skull. The condylo-premaxillary length of the minke whale
usually does not exceed 2 m (Tomilin, 1967) and the largest skull ever recorded,
2.35 m in length, was obtained from a 9.8 m female in the Antarctic (Omura,1975). The minimum estimate of the skull length of the Kizuki specimen is 2.56 m(Table 1), but the actual condylo-premaxillary length would have been 2.903.00 m as already stated.
The inferior aspect of the skull of the Kizuki specimen denotes two specialfeatures important for the identification of the species (Pl. I, Fig. 2). One is theshape of the palatines. In the Kizuki specimen the mesial border of anterior partSci. R.ep. Whales R.es. Inst.,
No. 31, 1979SKULL OF BALEEN WHALE

89of the palatines situate near the inferior edge of the vomer and in parallel with theskull axis, but in the posterior portion they diverge outwards and the inner edges
do not articulate to the inferior edge of the vomer. In the Kizuki specimen theright palatine was broken at its posterior portion along the edge, but in the leftpalatine the posterior edge is nearly complete, except a semi-round broken holejust anterior to the curved posterior edge. Such a shape of the palatine is usual
in all the Balaenoptera species except the blue whale, B. musculus. The skull of theblue whale differs from those of other Balaenoptera in the shape of palatines which
terminate abruptly, without forming left and right processes. Such form of thepalatines is observed also in the pygmy blue whale, B. musculus brevicauda (Omuraet al., 1970). In this feature this species is similar to the humpback whale (Tomilin,1967). From the above it is clear that the Kizuki specimen is not a blue whale.
Another feature of importance is the shape of the vomer at its posterior end.
Tomilin (1967) shows that the posterior end of the vomer is greatly expanded in
the adult fin whale, B. physalus, differing from the sei whale (Fig. 26 of his book).
Since no such expansion is observed in the Kizuki specimen it is also possible that
this specimen should not be assigned to the fin whale.
Among five species of the genus Balaenoptera we have already excluded threespecies, i.e. the minke, the blue and the fin whales. The remaining two species
or the sei whale, B. borealis, and the Bryde's whale, B. edeni, are very close relatives
and it is very difficult to separate the two only from the skull. One feature of theskull of the Bryde's whale is the flat rostrum, compared with somewhat downwardbending rostrum of the sei whale (Junge, 1950; Omura, 1959). The shape of
rostrum in the Kizuki specimen, presumed from the vomer left on the skull, resembles the Bryde's whale rather than the sei whale (Pl. I, Fig. 3). Another character separating the two species is the state of the front margin of the nasals. The
anterior margin is concave in the Bryde's whale, but straight in the sei whale.
In the Kizuki specimen, however, nasals are completely lacking and this character
is of no use in this case (Pl. I, Fig. 1).
Miller (1924) considers the unusually deep and narrow sulcus formed at theregion of juncture between the squamous and articular portion of the squamosalin his Pablo Beach specimen of the pollack whale (No. 236680, US. NationalMuseum) as a specific character of this species or B. borealis. According to Junge(1950) this sulcus is narrower and deeper in most specimens of B. borealis than it
is in B. physalus and B. edeni. In the Bryde's whale on the coast of Japan this sulcus
is less developed than in the sei whale (Omura, 1959). In this respect the Kizuki
specinen denotes sulcus of B. edeni type, being wide and shallow. In addition, inB. edeni the basicranial part of the skull exposed behind the palatine is much longer
than broad (Junge, 1950). This also observed in the Kizuki specimen (Pl. I, Fig.
2). It seems that, from the above, the Kizuki specimen shows some charactersof the Bryde's whale rather than the sei whale.
There are, however, some doubts in reliability of such skeletal characters.
Mead (1977) found, in several specimens, some of the characters generally used
to separate B. borealis from B. edeni either intermediate or contradictory, particularly
Sci. Rep. Whales Res. Inst.,
No. 31, 1979

90OMURA AND KASUYA

the shape of the nasal bones. He examined records of sei whales from the Atlantic
coast of the United States and identified the above-mentioned Miller's specimenof the pollack whale (No. 236680, U.S. National Museum) as B. edeni. He describes that this was confirmed by an examination of the baleen, which is clearly
that of B. edeni. The most remarkable difference, however, which separates B.
edeni from B. borealis in the form of vertebrae is the strong backward inclination of
the spinous processes in the former (Anderson, 1879; Lonnberg, 1931; Junge,1950; Omura, 1959). This inclination is observed in vertebrae from about 7thdorsal and increases till its maximum at about 7th lumbar. In the Miller's specimen of Pablo Beach, however, no such backward inclination is observed (Pl. 12of Miller, 1924). Thus the problem is much complicated and we are not able toconclude whether the Kizuki specimen is really a Bryde's whale or a sei whale.
The sei whale on the coast of Japan, however, does not usually migrate to
the south coast of western Japan, but it moves from the Bonin Islands directly toSanriku and further north. The Bryde's whales, on the other hand, were takenin these regions in the days of old whaling as well as in the days of modern whaling(Omura, 1977). These facts suggest that the Kizuki specimen possibly representsa Bryde's whale, B. edeni. Size of this whale is estimated to be 12 m, assumingthat the skull is about 25% of the whale body.
Dating of the Kizuki specimen was not made, but there is a record kept at
the Japan Whaling Association that two Iwashikujira were taken at Saganoseki(see Fig. 1) in 1926. At present the name Iwashi-kujira only means the sei whale,
B. borealis, but in these days no distinction was made between the sei and Bryde'swhales. In any case this will suggest that the Bryde's whale (or sei whale) usuallycame into the Inland Sea until about 50 years ago. As far as we know a whaling. landstation was established at Saganoseki in 1926, but it was closed soon due to
unexpected scarcity of whales coming there.
ACKNOWLEDGMENTS

We are much indebted to Mr Morosuke Ono and Mr Shoichi Abe of the Kizukishiritsu Minzoku Shiryokan, who gave us all the help we needed while measuring
and photographing the specimen. Our sincere thanks are due to Mr Shiro Takamatsu of the Oita Marine Palace and Dr Teruo Tobayama of the Kamogawa Sea
World who gave us the opportunity to study this specimen.
REFERENCES.
ANDERSON, J., 1879. ,Zoological results of the two expeditions to western Yunnan in 1868 and 1875, I: 551-564.
London: B. Quaritch.
ANDREWS, R. C., 1914. Monographs of the Pacific cetacea. I. The California gray whale (Rhachianectesglaucus Cope). Mem. Amer. Mus. Nat. Hist. (New Ser.) 1: 227-287.
JUNGE, G.C.A., 1950. On a specimen of the rare fin whale, Balaenoptera edeni Anderson, stranded on Pulu
Sugi near Singapore. ,Zoologische Verhandelingen, 9: 1-26.
Li:iNNBERG, E., 1931. The skeleton of Balaenoptera brydei 6. Olsen. Arkiv. ,Zool., 23: 1-23.
Sci. Rep. Whales Res. Inst.,
No. 31, 1979SKULL OF BALEEN WHALE

91

MEAD, J. G., 1977. Records of sei and Bryqe's whales from the Atlantic coast of the United States, theGulf of Mexico, and the Caribean. Rep. int. Whal. Commn (special issue 1): 113-116.
MILLER, G. S., 1924. A pollock whale from Florida presented to the National Museum by the Miami
Aquarium Association. Proc. U.S. .Nat. Mus., Vol. 66, art. 9: 1-15.
OMURA, H., 1959. Bryde's whale from the coast of Japan. Sci. Rep. Whales Res. Inst., 14: 1-33.
OMURA, H., 1974. Possible migration route of the gray whale on the coast of Japan. Sci. Rep. Whales Res.
Inst., 26: 1-14.
OMURA, H., 1975. Osteological study of the minke whale from the Antarctic. Sci. Rep. Whales Res. Inst.,
27: 1-36.
OMURA, H., 1977. Review of the occurrence of Bryde's whale in the northwest Pacific. Rep. int. Whal.
Commn (special issue 1): 88-9 l.
OMURA, H., T. lcHIHARA, and T. KASUYA, 1970. Osteology of pygmy blue whale with additional information
on external and other characteristics. Sci. Rep. Whales Res. Inst., 22: 1-27.
SHINDO, N., 1975. History of whales in the Inland Sea. Hyogo Dietists' Institute. Hyogo. 89 pp.
ToMILIN, A. G., 1967 Mammals of the U.S.S.R. and adjacent countries, IX, Cetacea. Israel Program for ScientificTranslations. Jerusalem. 717 pp.

Sci. Rep. Whales Res. Inst.,.No. 31, 1979

92

OMURA AND KASUYAEXPLANATION OF PLATESPLATE I
Fig. 1.
Fig. 2.
Fig. 3.

Dorsal view of the Kizuki specimen.
Inferior view of the Kizuki specimen.
Lateral view of the Kizuki specimen.

Fig. I.

Posterior view of the Kizuki specimen.

PLATE II

Sci. Rep. Whales Res. Inst.,
No. 31, 1979

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