The Cetaceous Kings of the Lizards (and the Other King too)

Over two weeks without posting anything, what a shame. I by all means haven't stopped blogging, but for some reason I just decided to write four quasi-complete posts. Well, here's post number two:

Having already talked about the most attenuated living cetaceans, I figured I might as well bring up the basilosaurids. These were some of the last archaeocetes with some Basilosaurus specimens apparently just crossing into the Oligocene (Manning, 2003). In some classifications the family (Basilosauridae) is paraphyletic and includes the sub-family Dorudontinae as well as their close relatives, modern whales (Autoceta - see here). Names used more recently (e. g. Naish 2004) and occasionally in the past seem to indicate that Dorudon and relatives got bumped up to family status (Dorudontidae) which modern whales are either the sister group of or part of. Uhen 1999 does have a distinct Pontogeneus/Basilosaurus/Dorudon clade and a Zyghorhiza/Autoceta clade, but let's not get into this. The point is that while basilosaurids are fairly close to the mysticete/odontocete split and are often used as an example of whale evolution (mostly for the hind legs) it seems that they were up to something a bit...different. Superficially the dorudontids resembled modern whales proportion-wise, but basilosaurids were far more elongated. The generally unsatisfying reconstructions of Basilosaurus often are too fat or resemble rorquals down to the pleats, and mostly just look like stretched out modern cetaceans. I wonder what a reconstruction of Lissodelphis would look like if we only had the skeleton (incidentally, I've never seen a good image of this). Could we predict the tiny flukes, lack of a dorsal fin/ridge and remarkable swimming speed. Well, eventually we would, but probably not from a superficial glance. When basilosaurids are given a closer look, it seems that they are very strange indeed...

While everybody knows that Basilosaurus is the "king of the lizards" despite being cetaceous, the "other king" Basiloterus hussaini is seldom mentioned outside of one paper (Gingerich et al., 1997). Well, okay, it is only known from two lumbar vertebrae from Pakistan and perhaps another from England, but Basiloterus is still fairly interesting. The ~40 million year old vertebrae are pretty large (~20 cm/4 in, if complete) and elongated, but not as extreme as Basilosaurus. It appears that either this is the most basal basilosaurid or their next nearest relative. The dorudontid Pontogeneus was fairly large (4/3 the size of Dorudon) and also comparable in morphology to Basilosaurus too, and Uhen 1998 and Uhen 1999 place them in a clade together. Basiloterus was excluded, but would presumably be even more Basilosaurus-like. Unfortunately I can't find illustrations of the vertebrae from that species, but Gingerich et al. 1997 presents both Basiloterus and Basilosaurus drazindai vertebrae:

Basiloterus is on the left and B. drazindai is on the right. I unwittingly drew these to scale (Basiloterus is ~20 cm and B. drazindai is ~30 cm). The metapophyses are at quite different angles, in B. drazindai they are nearly perpendicular to the neural spine, but in Basiloterus they are much more upwards angled. Also noteworthy is the more rounded neural spine of B. drazindai and the much more elongated centrum.

Basilosaurus drazindai is another basilosaurid seldom seen outside of Gingerich et al. 1997, and was from the same time and location as Basiloterus. And yes, it too has another vertebrae from England that may be assigned to it. The single described vertebrae was probably either a lumbar or an anterior caudal and is of similar size to B. cetoides and B. isis, although it is notable for having a larger neural arch and metapophysis than the latter species.

Gingerich et al. use the discussion of vertebrae to segue into a discussion of one of the more problematic aspects of Basilosaurus: how it moved. They implied that with the relatively short gap in between B. drazindai and B. isis and the subsequent reduction in the neural arch and metapophyses had something to do with a fundamental change in locomotion. But, what could it have been changing to? The authors (well, beforehand) discuss previous theories; such as Howell's in 1930 that since the vertebrae did not interlock and the processes were short it could not power a fluke, but instead a "continuous symmetrically-placed bilateral finfold running the length of the tail". However, Buchholz 2001 points out that the shrinking vertebrae at the end of the tail point to the presence of flukes which would only take up around 3% of the total length. Next up was Kellogg (1936) who noted that since the vertebrae were connected only by the ends of their centra, it would have been quite maneuverable. Buchholtz also regarded this as a highly maneuverable species, and regarded the thick ribs as a feature which would have allowed it to level off. Slijper in 1946 interpreted the large metapophyses and low neural spine in some of the posterior lumbars and caudals as a sign of it moving by horizontal undulations. Gingerich et al note that there appears to be more going on than simple enlargement, and raise the possibility that increasing centrum height and length would mean a great deal of the vertebrae would have been filled with fatty marrow in life. They interpret this as meaning Basilosaurus lived predominantly on the sea surface, which is a horizontal plane...

I'd be curious if there actually is some sort of connection between living at the surface and lateral movement. Phocid seals typically move by lateral undulations of the spine (Adam, 2004), so I wonder if there were similar pressures at some point in the past. Despite their locomotion in the water pinnipeds locomote on land by flexing their spines vertically, and elephant seals are apparently flexible enough to literally bend over backwards. Could both forms of locomotion be possible to a degree? Buchholtz 2001 simply stated that the "torso undulated during locomotion" (clever) with a higher wave amplitude than any other whale, but due to the surprisingly low vertebral count* it couldn't have multiple undulations. She concluded that it was a slow swimming specialist in maneuvering, presumably meaning a lot of flexibility both laterally and vertically. Gingerich et al. 1990 noted that other faunal evidence pointed to Basilosaurus isis inhabiting shallow mangroves or seagrass. It is also worth pointing out that Gingerich had described 243 partial Basilosaurus skeletons as of 1990 (vs. 77 for other archaeocetes) in Zeuglodon Valley, so they do seem to have been rather common. Exactly what niche Basilosaurus occupied doesn't seem too clear, but whatever it did it seemed to have been successful for a while.

*She gave a figure of 60, but it may be 70 (see below). As mentioned previously, Lissodelphis has 86-89 and Lagenorhynchus albirostris has 91.


The size of these species is also something of a confusing matter. Basiloterus cetoides is normally stated to be somewhat larger than B. isis (Uhen, 1998), which is stated to be 16 m (52 feet) long (Gingerich et al. 1990). Judging by Kellogg 1936's statement that the largest heads were 1.5 meters long, the whale in his illustration should have measured 18 m (60 feet) in length, although sometimes a figure of 20 m (65 feet) is given (Naish 2004).

Basilosaurus cetoides, from Kellogg 1936. This illustration is still used in publications (e.g. Naish 2004), although appears to be a problem with it.

Gingerich et al. 1990 mention that B. isis has 7 more (rather long) vertebrae than B. cetoides and Zalmout et al. 2000 indicate three more vertebrae were present. Kellogg 1936 gave a count of 7 cervical, 15 thoracic, 15 sacral/lumbar and 21 caudal; Zalmout et al. gave it as 7 cervical, 18 thoracic, 20 sacral/lumbar and 25 caudal vertebrae. Kellogg based his assumption on non-overlapping vertebrae from two specimens, and the Gingerich studies based theirs off of three overlapping specimens. Zalmout et al. suggest that this was the formula for both of the later Basilosaurus species, so I modified Kellogg's illustration subsequently:

Fortunately, this bears a strong resemblance to the illustration of B. isis* from Gingerich et al. 1990 (which I saw afterwards), except for the more elongated thoracic region due to the presence of three more very long vertebrae. This version of B. cetoides is about 25% longer than before, going from 18 meters to 23 (75 feet). Then there are of course vague indications of 20 meter individuals or even vaguer references to 25 meter ones which apparently did not take the increased vertebral count into affect. There still is the possibility that B. cetoides actually was shorter, we just can't know for certain at the moment, but either way it was an incredibly large mammal. As for the weight, the fat Basilosaurus from the BBC's Walking with Beasts was said to be 18 meters long and 60 tonnes; Gingerich et al. 1990 pegged it at roughly 6 tonnes (3-12 in the confidence interval), which is far more bearable.

*Zalmout et al. mention that Ca-14 in B. cetoides is 32% longer the corresponding one in the B. isis specimen, but also 20% wider. Did very large Basilosaurids get even thinner proportionally?

I should probably discuss the notion at the Palaeos page that Basilosaurus wasn't a sideline, but a "mechanically necessary intermediate form". They seem to be working by the theory treating "Eocene Cetacea essentially as a single population moving through phylospace along a smooth morphocline" - which I find quite problematic. Since when does evolution happen like this? The existence of oddballs like the trunked mid-Eocene Makaracetus should be enough to discount this idea. Basilosaurus is said to represent a grade between Georgiacetus and Dorudon, although the page neglects the fact that Dorudontids and Basilosaurids co-existed. There are also lots of missing species, some of which (Zyghorhiza, Chrysocetus) make more plausible "transitional" forms. The fact that cetaceans supposedly went through a stage when they had very elongated non-interlocking vertebrae and then quickly reversed this seems problematic and pointless. I think with more information on Eocene whales, we don't have to use mental gymnastics to make something like Basilosaurus a transitional form.

There are some interesting points brought up aside from the dubious phylogeny. The page raises the possibility of pike-like locomotion, that is, using a long body as a rudder and implying that it may not have been very slow after all. The "rigid" thoracic region is curious, but I think in light of the extremely elongated vertebrae and the presence of only one wave in the body at the time it makes sense. The possibility that the forelimbs were used in locomotion is raised, but they seem proportionally very small to the body size. While these ideas are interesting, their appearance outside of peer-reviewed literature and logical oddities means that we should by no means discount Buchholz.

While probably not directly related to modern whales, Basilosaurus and Basiloterus represent a fascinating evolutionary experiment. Exactly how a 60+ foot long serpentiform whale worked, especially with such strange vertebrae, is still a fascinating matter. Since even living species such as Lissodelphis have bauplans that are strange and effective (i.e. poorly understood), working out how oddballs with no modern counterparts work must be exceedingly difficult. Our knowledge of fossil whales seems to be increasing quite rapidly, and we probably haven't heard the last of old Basilosaurus.

-Cameron

References:

Adam, Peter J. 2004. Monachus tropicalis. Mammalian Species. No. 747, p. 1-9.

Buchholtz, Emily A. 2001. Vertebrae osteology and swimming style in living and fossil whales (Order: Cetacea). J. Zool. Lond. 253, 175-190.

Gingerich, P. D., B. H. Smith, and E. L. Simons. 1990. Hind limbs of Eocene Basilosaurus isis: evidence of feet in whales. Science, 249: 154-157.

Gingerich, Phillip D. et al. 1997. Basilosaurus drazindai and Basiloterus hussaini, new Archaeoceti (Mammalia: Cetacea) from the middle Eocene Drazinda formation, with a revised interpretation of ages of whale-bearing strata in the Kirthar group of the Sulaiman range, Punjab (Pakistan). Contributions from the Museum of Paleontology, Vol. 30, No. 2, pp. 55-81.

Kellogg, Remington. A Review of the Archaeoceti. Carnegie Institution of Washington, Washington 1936.

Manning, Earl M. The Eocene/Oligocene Transition in Marine Vertebrates of the Gulf Coast Plain. From Greenhouse to Icehouse: The Marine Eocene/Oligocene Transition. Columbia University Press, 2003. Partially available: Here (p. 377)

Naish, Darren. 2004. Fossils explained 46: Ancient toothed whales. Geology Today, Vol 20, No 2.

Uhen, Mark H. 1998. Middle to Late Eocene Basilosaurines and Dorudontines. The Emergence of Whales. Partially available: Here

Uhen, M. D. 1999. New Species of Protocetid Archaeocete Whale, Eocetus wardii (Mammalia: Cetacea) from the middle Eocene of North Carolina. Journal of Paleontology Vol. 73, No. 3 pp. 512-528.

Zalmout, Iyad S. 2000. Priabonian Basilosaurus isis (Cetacea) from the Wadi Esh-Shallala formation: First Marine Mammal from the Eocene of Jordan. Journal of Vertebrate Paleontology 20(1):201–204.

Possible life appearance of an advanced basilosaurid (B. isis, maybe B. cetoides). The rounded flukes and dorsal/anal structures are speculative. The fineness ratio is 14 and the head could be up to 5 feet long.