Crack open just about any recent popular overview of evolution (namely Why Evolution is True, The Greatest Show on Earth, and Evolution: What the Fossils Say and Why it Matters) and somewhere inside you will find a string of skeletal whales. Starting with either Indohyus or Pakicetus, the illustration will feature a graded series of forms that connect modern whales with their terrestrial ancestors. A caveat may be included in the text to say that we cannot be absolutely sure that each included genus gave rise to the next, but the general idea is that the evolution of whales occurred in a gradual, linear fashion through a series of intermediate stages.
Such iconography is not entirely wrong. We know that living whales are the descendants of land-dwelling ancestors that lived about 55 million years ago and the widely-published sequence roughly documents how the ancestors of living whales were adapted to life at sea. But by only being concerned with connecting living whales to their early ancestors, however, we run the risk of suggesting that such illustrations accurately depict the entire evolutionary history of whales. They most certainly do not.
Earlier this year I wrote about a redescription of the early whales Kutchicetus and Andrewsiphius by fossil whale experts Hans Thewissen and Sunil Bajpai. Neither of these forms fit along the neat continuum presented by Coyne, Dawkins, and Prothero in their books. Instead Kutchicetus and Andrewsiphius were part of a radiation of long-snouted, otter-like whales called remingtonocetids which lived alongside forms taken to represent a transitional stage to living whales such as Rodhocetus and Maiacetus. Indeed, if you could travel back about 48 million years ago to the beaches and nearshore environments of what is now northern Pakistan you would encounter a diversity of early whales inhabiting a range of habitats.
A paper just published in the latest issue of the Journal of Vertebrate Paleontology contributes further evidence that whale evolution was not set along a unilinear path. The new research, conducted by Lisa Cooper, Hans Thewissen, and S.T. Hussain, focuses on the early part of whale evolution, from the time of Pakicetus to Ambulocetus. Contrary to what the popular illustrations present, there were many species of semi-aquatic whales living alongside one another for millions of years./p>
The focus of the new study is a particularly rich fossil site in the Kuldana
Formation in northern Pakistan which contains deposits between about 48 and 40 million years old. From bottom to top the formation records the incursion of marine environments into earlier freshwater habitats, and the diversity of early whales shifted along with these changes. (In fact, a number of early whales were first described from fossils found in this formation.) The level of most relevance to the new paper, however, is a limestone bed indicative of a freshwater habitat just a bit older than the earliest known fossils of Ambulocetus. Within these limestone layers are many teeth of early whales, including some that represent two new species of Pakicetus.
The restored, partial upper tooth row of a pakicetid whale made from a composite of teeth from multiple species. The top row is the view of the teeth looking down, and the bottom row is a view from the side. M=Molar, P=Premolar, C=Canine. (From Cooper et al., 2009)
When the genus Pakicetus was established in 1981 it was recognized that there were two species of the early whale. There was Pakicetus inachus, the remains of which had spurred the initial description, and Pakicetus attocki, which was represented by teeth originally attributed to another whale. Now Cooper and colleagues have added two more species on the basis of dental evidence; Pakicetus calcis and Pakicetus chittas. Of particular interest, though, is that the fragments of both species were found in geologically younger rocks than Pakicetus inachus and Pakicetus attocki. Rather than being found at the bottom of the Kuldana Formation they were found near the top, not far below the level from which Ambulocetus was exhumed, suggesting that several species of Pakicetus lived alongside each other for a long span of time.
A restoration of the skull of Remingtonocetus, a close relative of Attockicetus. From “Whale Origins as a Poster Child For Macroevolution” in BioScience.
But Pakicetus was not the only genus of early whale to be found in the limestone layers. The identification is still tentative, but there were also teeth from a relative of Kutchicetus called Attockicetus which was originally described by Thewissen and Hussain in 2000. The initial description was made on the basis of bones found in marine sediments of a slightly younger age than those that yielded Ambulocetus, so if the new assignment is correct is extends the temporal range of Attockicetus down the geological column. This means that it would have overlapped with the newly recognized Pakicetus species.
A stratigraphic map of the Kuldana Formation in Pakistan. The levels at which the remains of early whales have been found is indicated on the right. (From Cooper et al., 2009)
At first this might seem rather dull, but it has important implications for how we understand the evolution of early whales. Looking at the stratigraphic map of the Kuldana Formation from bottom to top it is apparent that Pakicetus inachus, Pakicetus attocki, and its close relatives Nalacetus and Icthyolestes all lived in freshwater habitats in the area at about the same time. There had obviously been a radiation of pakicetid whales early in the history of the group.
By the time the relatively more recent limestone layer was being laid down, however, the area was inhabited by a different assemblage of semi-aquatic whales. Not only were there two different species of Pakicetus (Pakicetus calcis and Pakicetus chittas), but also the long-snouted remingtonocetid Attockicetus. The fact that the remains of Ambulocetus are found not far above this layer suggests that this portion of the formation is from a time when early whales were diversifying from the earlier Pakicetus-type form. Some of those, such as Ambulocetus, can be slotted in to represent a transitional form between Pakicetus and living whales, while others (i.e. Attockicetus) cannot.
This pattern contrasts with the traditional image of constant change between forms along a straight line. Instead there appears to have been an early diversification of which few forms survived (primarily Pakicetus) but did not change very much. Yet this group of survivors formed the basis for a later radiation of forms that were adapted to spend more time in the water, with Pakicetus overlapping with some early members of this second radiation before going extinct. Unfortunately the record from this area is not yet complete enough to identify direct ancestors and descendants, but the overall pattern accords more closely with the pattern predicted by punctuated equilibrium than the popular view of whale evolution as a straightforward march to the sea.
Cooper, L., Thewissen, J., & Hussain, S. (2009). New Middle Eocene Archaeocetes (Cetacea:Mammalia) from the Kuldana Formation of Northern Pakistan Journal of Vertebrate Paleontology, 29 (4), 1289-1299 DOI: 10.1671/039.029.0423