TABLE 1. Fission-track dating result. Dating was carried out by the external detector method and internal crystal surface was used. Ages were calculated using dosimeter glass NIST-SRM612 and samples were irradiated at pneumatic tube of graphite facility (Tc-pn) of Kyoto University Reactor (KUR). Zeta value 367. 1±4. 1 (Ohira, 2004) was used. ρ=track density; N=total number of tracks counted; s for spontaneous, i for induced and d for dosimeter; P(χ2)=probability of obtaining χ2 value for v degrees of freedom (where v=number of crystals-1) (Galbraith 1981); r=correlation coefficient between ρs and ρi; U=average uranium content.
| Sample Name |
No. Crys. |
ρs (Ns) (×106/cm2) |
ρi (Ni) (×106/cm2) |
P(χ2) % |
ρd (Nd) (×105/cm2) |
r | U ppm |
Age (Ma) (±1σ) |
| KG2018 | 34 | 1. 1791(836) | 5. 1396(3644) | 23. 7 | 2. 1417(7173) | 0. 85 | 328 | 9. 0±0. 4 |
TABLE 2. Measurements in mm of Megabalaena sapporoensis ( SMAC 2731) skull and mandible following Buono et al. (2017), and Tanaka and Taruno (2019). For skull and mandible, distances are either horizontal or vertical, unless identified as point to point. + shows an incomplete measurement, because of erosion.
| Skull | |
| Bizygomatic width | 2232 |
| Width of the skull at the level of the exoccipitals | 1350 |
| Supraoccipital width anterior to foramen magnum | 900 |
| Transverse diameter of left occipital condyle | 130 |
| Dorsoventral diameter of left occipital condyle | 150+ |
| Transverse diameter of foramen magnum | 100 |
| Distance between lateral border of left occipital condyle and lateral border of left exoccipital | 550 |
| Width of the occipital condyles and foramen magnum | 310 |
| Anteroposterior diameter of the supraorbital process of the frontal of the distal edge | 290 |
| Transverse diameter of the supraorbital process of the frontal | 870+ |
| Width of the optic canal in its medial portion | 20 |
| Width of the optic canal in its lateral portion | 220 |
| Length of the optic canal | 680+ |
| Orbit length (between preorbital and postorbital process) | 200 |
| Mandible | |
| Height of mandible, from coronoid process to ventral margin | 350 |
| Maximum preserved width of mandible | 290+ |
| Height of body just anterior to coronoid process | 320 |
TABLE 3. Measurements in mm of Megabalaena sapporoensis ( SMAC 2731) left periotic and tympanic bulla. Dimensions follow Tsai and Fordyce (2015). Distances are either horizontal or vertical, unless identified as point to point. + shows an incomplete measurement, because of erosion. Measurements are rounded to the nearest 0. 5 mm.
| Periotic | |
| Length of anterior process of periotic (right side) | 45.5 |
| Maximum transverse width of the body of periotic | 86.5+ |
| Length of pars cochlearis | 29.0+ |
| Width of pars cochlearis | 48.0+ |
| Dorsoventral diameter of fenestra rotunda | 7.5 |
| Width of fenestra rotunda | 11.5 |
| Length of the posterior process of the periotic | 343.5 |
| Bulla | |
| Greatest length, in lateral view | 108.5 |
| Greatest length of tympanic cavity | 71.5 |
| Greatest width, in lateral view | 54.0 |
| Length of anterior lobe, from lateral furrow to anterior tip of tympanic bulla | 35.5 |
TABLE 4. Measurements in mm of Megabalaena sapporoensis ( SMAC 2731) vertebrae. Distances are either horizontal or vertical, unless identified as point to point. + shows an incomplete measurement, because of erosion.
| length of body | width of body | height of body | maximum preserved height |
maximum preserved width |
neural spine height |
neural spine length |
|
| Atlas | 140 | - | - | 260 | - | - | - |
| Axis and 3rd cervical vertebra | 100 | - | - | - | 490 | - | - |
| Axis | - | 390 | 90 | - | - | - | - |
| 3rd cervical vertebra | 260 | 150 | - | - | - | - | - |
| 4th cervical vertebra | 50 | 250 | 190 | - | 370 | - | - |
| 5th cervical vertebra | 45 | 255 | 220 | - | 290 | - | - |
| 6th cervical vertebra | 50 | 240 | 190 | - | 230 | - | - |
| 7th cervical vertebra | 50 | 245 | 195 | - | 320 | - | - |
| 1st thoracic vertebra | 70 | 230 | 200 | - | 300 | - | - |
| 2nd thoracic vertebra | 80 | 240 | 200 | - | 310 | - | - |
| 3rd thoracic vertebra | 90 | 237 | 190 | - | 301 | - | - |
| 4th thoracic vertebra | 100 | 240 | 190 | - | 435 | - | - |
| 5th thoracic vertebra | 110 | 235 | 190 | - | 410 | - | - |
| 6th thoracic vertebra | 110 | 240 | 190 | - | 440 | - | - |
| 7th thoracic vertebra | 126 | 219 | 194+ | - | - | - | - |
| 8th thoracic vertebra | 135 | 275 | 181+ | - | 475 | - | - |
| 9th thoracic vertebra | 135 | 246 | 190 | - | 445 | - | - |
| more posterior vertebra 1 | 155 | 252 | 209 | - | 345 | - | - |
| more posterior vertebra 2 | 178 | 241 | 196 | - | 360 | - | - |
| more posterior vertebra 3 | 185 | 280 | 191 | - | 406 | - | - |
| more posterior vertebra 4 | 187 | 248+ | 185 | - | 349 | - | - |
| more posterior vertebra 5 | 202 | 233 | 185 | - | 313 | - | - |
| more posterior vertebra 6 | 197 | 250 | 228 | - | 429 | - | - |
| more posterior vertebra 7 | 203 | 244 | 220 | - | 439 | - | - |
| more posterior vertebra 8 | 214 | 254 | 221 | - | 405 | 440 | 192 |
| more posterior vertebra 9 | 210 | 270 | 231 | - | 304 | - | - |
| more posterior vertebra 10 | 220 | 243 | 230 | - | 278 | - | - |
| more posterior vertebra 11 | 238 | - | - | - | - | - | - |
| more posterior vertebra 12 | 245 | - | 235 | - | - | - | - |
| more posterior vertebra 13 | 250 | - | - | - | - | - | - |
| more posterior vertebra 14 | 285 | - | - | - | - | - | - |
TABLE 5. Measurements in mm of Megabalaena sapporoensis ( SMAC 2731) forelimb. Distances are either horizontal or vertical, unless identified as point to point. + shows an incomplete measurement, because of erosion.
| Scapula | ||
| length of glenoid fossa | 280 | |
| width of glenoid fossa | 200 | |
| Humerus | ||
| proximodistal length | 560 | |
| maximum anteroposterior length at proximal end | 255 | |
| maximum width of proximal end | 199 | |
| minimum length at shaft | 135 | |
| maximum length of distal end | 180 | |
| maximum width of distal end | 145 | |
| Ulna | ||
| proximodistal length | 490+ | |
| minimam length at shaft | 95 | |
| maximum length of distal end | 170 | |
| maximum width of distal end | 65 | |
| Radius | ||
| proximodistal length | 545 | |
| minimum length at shaft | 140 | |
| maximum length of distal end | 195 | |
| maximum width of distal end | 85 |
TABLE 6. Estimated total length and size classifications of balaenids. Estimated total length of extinct balaenids except Megabalaena sapporoensis were taken from Bisconti et al. (2021). Modern adult total length taken from Jefferson et al. (2008). Age of Balaena ricei, Balaenella brachyrhynus, Balaenula astensis and Balaena montalionis were taken from supplementary file of Marx and Fordyce (2015).
| Species | Specimen | total length in meters |
size | age | epoch | locality | reference | |
| Morenocetus parvus | MPL 5-11 | 4.2 | small | Early Miocene | Burdigalian | 19.8-18.2 | Trelew, Argentina | Bisconti et al., 2021 |
| Peripolocetus vexillifer | CAS 905 | 5.9 | middle | early Middle Miocene | Langhian | 16.0-15.2 | Sharktooth Hill, California | Bisconti et al., 2021 |
| Peripolocetus vexillifer | SDSNH 99766 | 4.5 | small | early Middle Miocene | Langhian | 16.0-15 2 | - | Bisconti et al., 2021 |
| Megabalaena sapporoensis | SMAC 2731 | 12.7 | large | early Late Miocene | Tortonian | 9.4-8. 6 | Hokkaido, Japan | This study |
| Eubalaena shinshuensis | SFMCV-0024 | At least 12.0 to 10. 8 | large | late Late Miocene | Messinian | 6.1-5. 9 | Shinshishinmachi, Nagano, Japan | Bisconti et al., 2021; this study |
| Archaeobalaena dosanko | FCCP 1049 | 5.8 | middle | Early Pliocene | Zanclean | 5.2-3. 5 | Hokkaido, Japan | Bisconti et al., 2021 |
| Balaenella brachyrhynus | NMB 42001 | 5.2 | small | Early Pliocene | Zanclean | 5.0-4.4 | Kallo, Belgium | Bisconti et al., 2021 |
| Balaena ricei | USNM 22553 | 7.1 | middle | Early Pliocene | Zanclean | 4.9-4.4 | Rice’s Pit, Virginia | Bisconti et al., 2021 |
| Balaena montalionis | MSNTUP I-12357 | 7.3 | middle | Early Pliocene | Zanclean | 4.8-4.4 | Casina, Italy | Bisconti et al., 2021 |
| Charadrobalaena valentinae | MCRE 232834 | - | large? | Early Pliocene | Zanclean | 3.8-3.6 | Emilia Romagna, Italy | This study |
| Balaenula astensis | MSNTUP I-12555 | 6.5 | middle | Early Pliocene | Zanclean | 4.0-3.0 | Portacomaro, Italy | Bisconti et al., 2021 |
| Balaenidae sp. | MCRE 232834 | 10.8 | large | Early Pliocene | Zanclean | 3.81-3.6 | - | Bisconti et al., 2021 |
| Eubalaena sp. | MCMV M_PAL_002-005 | 14.3 | gigantic | Late Pliocene | Piacenzian | 3.6-2.6 | Tuscany, Italy | Bisconti et al., 2021 |
| Antwerpibalaena liberatlas | IRSNB M 2325 | 10.7 to 9.5 | large | Late Pliocene | Piacenzian | 3.21-2.76 | Belgium | Bisconti et al., 2021 |
| Eubalaena ianitrix | IRSNB M. 879 | 8.2 | middle to large | latest Early to Late Pliocene | - | - | Antwerp, Belgium | Bisconti et al., 2021 |
| Balaena mysticetus | - | 20 female, 18 male | gigantic | modern | Holocene | 0 | - | Jefferson et al., 2008 |
| Eubalaena glacialis | - | 17 | gigantic | modern | Holocene | 0 | - | Jefferson et al., 2008 |
| Eubalaena australis | - | 17 | gigantic | modern | Holocene | 0 | - | Jefferson et al., 2008 |
| Eubalaena japonica | - | Over 17 | gigantic | modern | Holocene | 0 | - | Jefferson et al., 2008 |
TABLE 7. Ratio of the humerus and radius in the Balaenidae. Larger number indicates more slender humerus.
| Humerus; Proximodistal length/minimum anteroposterior length | Humerus; Proximodistal length/maximum anteroposterior length | Radius; Proximodistal length/minimum anteroposterior length | Radius; Proximodistal length/maximum anteroposterior length | specimen numbers | Reference | |
| Yamatocetus canaliculatus | 3.9 | 2 4 | 4.6 | 3.6 | KMNH VP 000,017 | Okazaki, 2012 |
| Megabalaena sapporoensis | 4.1 | 2.2 | 4.2 | 2.9 | SMAC 2731 | This study |
| Antwerpibalaena liberatlas | 2.6 | 1.9 | 3.5 | 1.6 | IRSNB M2325 | Duboys de Lavigerie et al., 2020 |
| Charadrobalaena valentinae | - | - | 4.0 | 2.7 | MCRE 232834 | Bisconti |
| Balaena ricei | 2.6 | 1.7 | 2.8 | 2.3 | USNM 22553 | Duboys de Lavigerie et al., 2020 |
| Balaena mysticetus | 2.7 | 1.6 | 3.5 | 2.5 | ZMUC 1596 | Duboys de Lavigerie et al., 2020 |
| Eubalaena australis | 2.8 | 1.7 | 2.5 | 1.7 | MNHNP A2929 | Benke 1993 |
| Eubalaena australis | 2.1 | 1.4 | 2.7 | 1 7 | NMNZ MM002239 | Duboys de Lavigerie et al., 2020 |
TABLE 8. Comparison of the conditions of cervical vertebra fusion among the family Balaenidae. “Sep” means separated. “Fu” means fused.
| Species | age | Specimen number | Ontogenetic state | Atlas | Axis | C3 | C4 | C5 | C6 | C7 | reference | |
| Morenocetus parvus | Early Miocene | 19.8-18 2 | MLP 5-11 | subadult | ? | ? | ? | ? | ? | ? | ? | Cabrera 1926, Buono et al., 2017 |
| Megabalaena sapporoensis | Early Late Miocene | 9.4-8.6 | SMAC 2731 | adult | Sep | Fu | Fu | Sep | Sep | Sep | Sep | This study |
| Balaena ricei | Early Pliocene | 4.9-4.4 | USNM 22553 | physically immature | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Westgate and Whitmore 2002 |
| Charadrobalaena valentinae | Early Pliocene | 3.8-3.6 | MCRE 232834 | older than juvenile | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Bisconti et al., 2023 |
| Antwerpibalaena liberatlas | Late Pliocene | 3.21-2.76 | IRSNB M2325 | adult | Sep | Fu | Fu | Fu | Fu | Fu | Fu | Duboys de Lavigerie et al., 2020 |
| Balaenula balaenopsis | Zanclean/Piacenzian | ? | - | young | Sep | Fu | Fu | Fu | Fu | Fu | Sep | Van Beneden 1872 |
| Balaenotus insignis | Zanclean/Piacenzian | ? | - | - | Sep | Fu | Fu | Fu | Fu | Fu | Sep | Van Beneden 1872, Abel 1941 |
| Balaena mysticetus | modern | - | adult | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Jefferson et al., 2008 | |
| Balaena mysticetus | modern | - | young | Fu | Fu | Fu | Fu | Fu | Fu | Sep | Nishiwaki and Kasuya 1970 | |
| Eubalaena glacialis | modern | - | adult | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Jefferson et al., 2008 | |
| Eubalaena australis | modern | - | adult | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Jefferson et al., 2008 | |
| Eubalaena japonica | modern | - | adult | Fu | Fu | Fu | Fu | Fu | Fu | Fu | Jefferson et al., 2008 |
