CS Feibel 1994 Lethaia 26:359-366
Freshwater stingrays from the Plio-Pleistocene of the Turkana Basin, Kenya
and Ethiopia

The fossil freshwater stingray from the Turkana Basin is redescribed &
reassigned to Dasyatis africana (Arambourg) on the basis of extensive new
collections. The ray apparently evolved into an endemic freshwater species,
derived from a stock which entered the Turkana Basin from the Indian Ocean
at c 1.9 Ma.
At that time, the ancestral Omo River system flowed through a major lake and
exited to the SE. A fluvial corridor, termed the Turkana River, connected
the Turkana Basin with the Indian Ocean. Once established in the basin, the
rays flourished & persisted for over half a million years. Their extinction
has been placed subsequent to 1.3 Ma, and likely reflects the changing
environmental & tectonic conditions recorded in subsequent strata. The
fluvial corridor which formed the route of migration into the Turkana Basin
has important implications for modern African biogeography as well as that
of the past.
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GRADUAL VERSUS PUNCTUATED EQUILIBRIUM EVOLUTION IN THE TURKANA BASIN
MOLLUSCS: EVOLUTIONARY EVENTS OR BIOLOGICAL INVASIONS?
Bert Van Bocxlaer, Dirk Van Damme & Craig S. Feibel 2007
doi:10.1111/j.1558-5646.2007.00296.x

A running controversy in evolutionary thought was Eldredge & Gould's PE
model, which proposes long periods of morphological stasis interspersed with
rapid bursts of dramatic evolutionary change. One of the earliest & most
iconic pieces of research in support of PE is the work of Williamson on the
Plio-Pleistocene molluscs of the Turkana Basin. Williamson claimed to have
found firm evidence for 3 episodes of rapid evolutionary change, separated
by long periods of stasis in a high-resolution sequence. Most of the
discussions following this report centered on the topics of (eco)phenotypy
vs genotypy, and the possible presence of preservational & temporal
artifacts. The debate proved inconclusive, leaving Williamson's reports as
one of the empirical foundations of the paradigm of PE. Here we
conclusively show Williamson's original interpretations to be highly flawed.
The supposed rapid bursts of punctuated evolutionary change represent
artifacts resulting from the invasion of extra-basinal faunal elements in
the Turkana palaeo-lakes during wet phases well known from elsewhere in
Africa.

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Stiffening the Stingray Skeleton - An Investigation of Durophagy in
Myliobatid Stingrays (Chondrichthyes, Batoidea, Myliobatidae)
AP Summers 2000 J.Morphol.243:113-126

The stingray family Myliobatidae contains 5 durophagous (hard prey
specialist) genera & 2 planktivorous genera.
A suite of morphological features makes it possible for the hard-prey
specialists to crush mollusks & crustaceans in their cartilaginous jaws:
1) flat pavement-like tooth plates set in an elastic dental ligament;
2) multiple layers of calcified cartilage on the surface of the jaws;
3) calcified struts running through the jaws;
4) a lever system that amplifies the force of the jaw adductors.
Examination of a range of taxa reveals that the presence of multiple layers
of calcified cartilage, previously described from just a few spp, is a
plesiomorphy of Chondrichthyes.
Calcified struts within the jaw (trabecular cartilage) are found only in
myliobatids, incl.the planktivorous Manta birostris.
In the durophagous taxa, the struts are concentrated under the area where
prey is crushed, thereby preventing local buckling of the jaws.
Trabecular cartilage develops early in ontogeny, and does not appear to
develop as a direct result of the stresses ass.x feeding on hard prey.
A "nutcracker" model of jaw function is proposed : the restricted gape,
fused mandibular & palato-quadrate symphyses, & asynchronous contraction of
the jaw adductors function to amplify the closing force by 2-4 times.