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897 tocA fossil beaked whale from Hokkaido, Japan

Kaylene Butler

 

Beaked whales (Family: Ziphiidae) are part of an easily recognised family of toothed whales known for their elongated beaks. Fossil evidence for beaked whales dates back as far as the Miocene (15 million years ago). Despite a singficant increase in our knowledge of beaked whale evolution, the evolution of their ear bone (periotic) shape in particular is poorly understood. Among the fossil beaked whale, preserved ear bones are rare compared to skulls. However, recently, Yoshihiro Tanaka and his colleagues described a beaked whale fossil! The fossil, described from the Chepotsunai Formation of Hokkaido, Japan was designated TTM-1 includes preserved ear bones, isolated teeth and vertebrate.

 

TTM-1 – A fossil beaked whale

TTM-1, the recently described fossil whale occurred in the latest Miocene (5 – 6 million years ago). The fossil differs from other fossils whales. Dr Tanaka explains:

“TTM-1 is a fossil ziphiid from a Miocene sediment in Hokkaido, Japan (about 6.5 to 5.6 million years ago), but it has more a derived shape compared to other fossil ziphiids. A process on an ear bone (it is called as the anterior process of the periotic) of TTM-1 is more robust than the ones of fossil ziphiids, and is more slender than the ones of modern ziphiids.”

figure2Fragmentary skull of TTM-1, Ziphiidae gen. et sp. indet. in dorsal view (Figure 2 of Tanaka et al. 2019).

 

An intermediate ear condition

In modern whales, the auditory bulla (or tympano-periotic bulla) houses the middle and inner ear. In whales, these bones are often incredibly dense. These ear bones play a role in protecting inner-ear tissues from damage and the pressure of diving beneath the sea. They also allow for better underwater hearing.

TTM-1 fills in a gap in our knowledge of beaked whale evolution by allowing us to study the transition from a more robust to more slender anterior process of the periotic in the ear. Unlike modern beaked whales such as Tasmacetus (the Shepherd's beaked whale), TTM-1 does not have a swollen anterior process of the periotic (inner ear bone). “TTM-1 shows the condition between ancient and modern ziphiids [beaked whales],” Dr Tanaka says, “only a few fossil ziphiids are known their ear bones.”

 

figure9Morphological changes of the periotics among the Ziphiidae. The cladogram is modified from figure 13 of Bianucci et al. (2016) (Figure 9 of Tanaka et al. 2019).

 

figure31-6, Left periotic of TTM-1, Ziphiidae gen. et sp. indet., lateral (1), ventral (2), anterior (3), posterior (4), dorsal (5), medial (6) (Figure 3 of Tanaka et al. 2019).

 

From tooth to nearly- toothless

Fossil beaked whales also differ from their modern counterparts in other ways. Modern beaked whales have lost function teeth and have a somewhat different skull shape. Dr Tanaka points to more excellent research explaining this:

“Fossil ziphiids had a weaker degree of vertex (top of the skull) elevation. Modern ziphiids have a strongly elevated vertex and have also lost functional teeth. Interestingly, this trend happened independently among the family. This fact was reported in a great work by Dr. Giovanni Bianucci and his colleagues.”

In fact modern beaked whales differ from all other toothed whales in that most species only have just one pair of teeth!  Like other many other early beaked whales, TTM-1 had multiple teeth.

figure71-34, Teeth of TTM-1, Ziphiidae gen. et sp. indet., anterior or posterior view (1-8 and 10-17), other side of view of each teeth (18-25 and 27-34), apical view of tooth number 2 (8), lateral view of tooth number 2 (26) (Figure 7 of Tanaka et al. 2019).

As Dr Tanaka states, “TTM-1 preserves ear bones, and gives better idea of ear bone evolution among the Ziphiidae.” This research is invaluable in order to properly understand whale evolution.

Thank you to Dr Tanaka and his team for talking to the PE blog editors about this exciting fossil! To learn more about these fossils, check out the original PE article here.

 

 

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