Hippos prancing about underwater (Hippo-Ecomorph Rant #1)

The origin of most marine mammals invokes some imaginary semiaquatic transitional form. In the last decade, many of these transitional forms have actually been found, including the semiaquatic sirenian, Pezosiren portelli, which I had the honor of collecting with my master's advisor, Daryl Domning, in Jamaica at the end of my undergraduate studies and beginning of my MS in Anatomy at Howard University. Likewise, in 2001 two papers were published in the same week that described fossil ankle bones of whales that conclusively linked the origins of the Cetacea to the Artiodactyla. I remember that day, I think it was Sept 21, 2001, a Friday, because I was interviewing at Johns Hopkins that day. Shawn Zack (now at Marshall) picked me up at the train station and asked me if I had seen the paper by Thewissen and colleagues published on Thursday, and then said that there was another paper published that Friday by Gingerich and colleagues. Since then, the molecular phylogenetic assertions that hippos are the most closely related modern mammal to the Cetacea has gained much more backing, evn though these original papers in 2001 never inform us of what fossil group they are more closely aligned to. Some recent finds in Pakistan by Thewissen and colleagues have claimed an ancestor in an unusual group of terrestrial mammals known as raoellids, though further analysis by Geisler & Theodor calls some questions to exactly which fossil groups are related to cetaceans and which are not. Yet despite this rigor of questioning what are the real sister taxa to cetaceans, the message most in the public and media have gotten is that there is some connection to hippos. Unfortunately, the connection to hippos also leads some people to examine hippos as examples of what the origins of whales were like, even though the fossil record of hippos goes only into the Miocene and the understanding of whether these earliest forms were semiaquatic or not is still very rudimentary.

Hippos, in my opinion, should be considered the poster-child for how paleontologists take their overgeneralized views of modern animal biology/ecology and infer it for fossil organisms without much, if any fustification. I wont go into this too much further, as a hippo-ecomorph rant(s) is certainly already partly written and worth spending some time on.

But, in the hopes of starting the hippo discussion, how they might relate to cetacean origins and better understanding their roles as examples of the semiaquatic "transitional" mammal, and to relate to a very recent paper recently brought to my attention, it is worth discussing one key point, underwater locomotion in hippos.

A recent paper by Coughlin and Fish (2009) about underwater locomotion in the modern common hippo is an informative look at what many people often see as a "transitional state" for the terrestrial to aquatic transition. This paper is excellent, and in an elegant way really get to the heart of how hippo underwater locomotion is unique, and perhaps a good, or not so good, example of how transitional forms may have made it into the water. I am not sure the parallels they draw between hippos as raoellids, or Pezosiren portelli for that matter, are sufficiently supported, but they gently bring it up as a future avenue for research and should be commended for calling attention to the subject in a rigorous way.

The funny thing is, I have seen these sorts of results from a study of this sort many years ago in the work of a master's student from the University of Florida, Matthew Mihlbachler. In his work, notably done on a budget with a stopwatch and VHS recorder, Matt was able to come to much of the same conclusions as Coughlin and Fish (2009). Though his thesis entailed multiple aspects of the paleobiology of some fossil rhinos (which includes some interesting work about paleodemography) Mihlbachler has only published a small amount of this data in a paper about a fossil brontothere with curiously similar short limbs he named Aktautitan hippopotamopus (NOTE: hippopotamopus is not a typo, see the paper for the etymology).

The best part about these two studies is that they ultimately come to the same conclusion - when you look at hippo underwater locomotion, they are unique. Though there may be some parallels to be found in the hydrostasis controls found in either group someday (as Mihlbachler hints at with some data in his thesis), at present all we can say is that, if hippos are a good example of a transitional semiaquatic mammal (which I am not sure I would claim they are), then many of the characteristics we see in whales that lead from that negatively buoyant transitional form need to be further explored (although Sandy Madar has done some excellent work along these lines, as has Lisa Cooper).

All in all, hippos are an excellent source of information for trying to understand how large mammals might adapt to a life that involves more regular use of water, and in that way they may be good examples of the "transitional form" - BUT the notion that the ancestor of cetaceans was a hippo in the way they are today, or worse, the way we THINK they are today, is poorly conceived and unrealistic.