Llanocetus and the Earliest "Toothed" Baleen Whales.



As the human-made atom bomb fell, the explosive radiation allowed an instantaneous x-ray-like view of all creatures within the epicentre. Joking aside, the evolutionary split between baleen whales (Mysticeti) and echolocating toothed whales (Odontoceti) occurred ~ 34 million years ago. Sourced from http://jibrael.blogspot.com

Travel back 34 Million years. We'll be straddling two major epochs in earth’s history, whereby the Eocene is giving way into the freezing cold climate of the earliest Oligocene. A hasty descent in global temperatures and the development of the emergent Antarctic Circumpolar Current (the most important of the modern era, responsible for three quarters of global marine export) is creating a new glut of food within its nutrient rich waters. The archaic forms of whale (the gigantic yet slender Basilosaurids, with diminutive back-legs) are being outcompeted by new, much stranger forms of giant whales. Some of these early whales are the ancestors of the true giants that still exist today: the baleen whales (otherwise known as the "Mysticetes"). 
After finally creating a machine that would turn him into a giant, Salvador Dali would wreck havoc upon the world. After his death, his moustache was preserved for future generations to gaze upon the horror that was his reign. Picture via Keri Rogers of the baleen of a Bowhead whale.

 Baleen is a strange structure. 

 The largest baleen currently known belongs to the modern Bowhead whale (Balaena mysticetus) and can be a whopping 2.7 metres in length! The Bowhead whale gulps in vast quantities of water and subsequently filters out the tiny morsels of food (such as krill) via these keratinous plates hanging from the upper palate of the mouth. But it wasn’t always like this...  
It makes logical sense that baleen whales would be derived from tooth bearing ancestors. However, to make things really weird and convoluted, there were forms of"toothed" baleen whales (with raptorial appetites) until the end of Oligocene ~23 million years ago. It's hugely confusing; they culminated in the northern "Aetiocetids" and southern "Mammalodontids". They had a huge period of co-existence with their baleen bearing sister group. Image via A. Gennari
The above picture depicts the toothed whale Mystacodon, the earliest known baleen whale in the fossil record.  Mystacodon did not have baleen but instead was a suction feeder, slurping up prey such as squid and shrimp and occasionally munching on larger prey items. It's a critical specimen, but in this blog I'll be focusing on the only other toothed-baleen whale known from the latest Eocene, the 34 million year old Llanocetus (known in the literature for decades, but only formally described in 2018). 
The implications of Llanocetus and its biology are far reaching...  
After letting off a smelly fart in the classroom, it was clear that Llanocetus was waiting for his classmates to smell his disgrace. The face of Llanocetus. Image via Carl Buell. 

A 2018 paper (Fordyce and Marx, Gigantism Precedes Filter Feeding in Baleen Whale Evolution, Current Biology (2018), https:// doi.org/10.1016/j.cub.2018.04.027) has recently been published illuminating the answer to how baleen whales obtained baleen. Pictured above is Llanocetus, the 34 million year old Mysticete from Antarctica. The description of this ancient whale is a landmark moment in whale palaeontology, allowing us to understand the sequential order of how baleen came about in the largest creatures to ever grace the earth... 

Breaking news: Llanocetus did not eat bananas. Image via Ewan Fordyce, of the cheek tooth of Llanocetus
But first, lets get into the biology of Llanocetus before understanding the implications of its feeding strategies. 


Llanocetus is gigantic for the period, at a body length of 8 metres (for a presumed juvenile). Its jaw still retains teeth, but these are widely spaced; the cheek teeth are relatively robust/highly ornate and it has a dental formula like its Basilosaurid forebears. Its skull is considered archaic for Mysticetes, but it is strikingly broad at the rostrum, with a wide palate and clear facial telescoping ("stretching" of the face).  


The preparation of this cetacean was excruciatingly difficult; like "preparing eggshells out of concrete". Image via Ewan Fordyce, showing the immense size of Llanocetus’ skull. 


But what about the osteological correlates that provide the baleen racks? Are they present in Llanocetus?

 Their palate displays something known as sulci (a number of grooves), which are usually interpreted as blood supply for baleen. But these "sulci" do something different; they converge to the gums (around the tooth sockets). The authors interpret this animal not a filter feeder, but rather as a possible raptorial (using its teeth to tear flesh) and suction feeder, giving way to a new sequential order of how baleen arose in Mysticetes: rather than have baleen with teeth, Mysticetes must have lost their teeth completely via the use of suction feeding, before developing their characteristic baleen. It has been suggested that the baleen may have been used as a mechanism to better keep the swallowed prey in the mouth in latter iterations of the baleen whale form. 


 Llanocetus was a dead end experiment in baleen whale evolution, with no living descendants today. The skull of this creature provides a unique morphological snapshot of the baleen whale through time and for this reason, Llanocetus is critical to our understanding of Mysticete lineage. 


These whales may represent a dead end for the evolution of their immediate kin, but other varieties of toothed mysticetes would soon flourish past the Eocene; the Australian and New Zealand Mammalodontids represent a bizarre yet fascinating phase to the baleen whale story, still being unraveled to this day...  

References

 Fordyce et al 2018. Gigantism Precedes Filter Feeding in Baleen Whale Evolution, Current Biology (2018), https:// doi.org/10.1016/j.cub.2018.04.027)
F.G. Marx, O. Lambert, M.D. Uhen Cetacean Paleobiology John Wiley & Sons, Chichester (2016)

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