I found the following results
recovered from the large reptile tree (LRT, 1709+ taxa) to be particularly fascinating given the apparent illogic of developing a robust swimming tail with flukes from an tiny ancestral tail barely able to act as a ‘flap’.
You might remember
earlier we looked at the reversal of teeth in the lineage of odontocetes (toothed whales), reversing step-by-step to a simple cone from the typical complex, multi-cusped molar of a tree shrew.
Likewise in toothed whales, but not exactly correlated,
the tail also experienced a reversal, becoming longer and more robust after derivation from the tiny speck of a tail in tenrec ancestors.
With that introduction
here are the three times the tail has elongated and grown horizontal flukes in placental mammals:
1 – Manatee tail evolution
The terrestrial Moeritherium-like ancestors of today’s aquatic manatees and dugongs had a long torso and tiny tail, distinctly unlike the robust tail with flukes found in today’s Sirenia (Figs. 1–3). Prorastomus (Fig. 2) is a transitional taxon having a more robust tail. Procavia, the living hyrax, has an even smaller tail than these taxa and is more primitive.
The splitting in two of ancestrally longer caudal vertebrae (or the increase in caudal number while reducing each caudal vertebral length) appears to be the method employed by evolution to create a longer, more robust tail in manatees and their ancestors.
2 – Mysticete tail evolution
Neoparadoxia (Fig. 4), a desmostylian ancestor of modern toothless (baleen) whales, likewise had a tiny tail, similar to that of its hippo-like ancestors, useless for propulsion.
The re-elongation of the tail in mysticete ancestors is not (yet) documented in transitional fossils, which is one factor in keeping this bit of evolution a secret, even from whale experts. Nevertheless, the rest of the anatomy is enough to nest these two former clades together into one clade. Here the number of tail vertebrae does not increase so much as the robust morphology of each one (Figs. 5–7).
The apparent length of the tail is enhanced by the disappearance of the hind limbs and the pelvis in mysticetes and other completely aquatic mammals.
Behemotops and Miocaperera fossils (Fig. 7) do not presently preserve tail vertebrae. These transitional taxa are the ones most likely to transition to reduced legs and a robust tail. It is also apparent that these taxa are ancestral to rorquals, while Desmostylus (Fig. 5) is ancestral to right whales… which means 4x a tiny mammal tail evolved flukes.
3 – Odontocete tail evolution
The elongation of the torso and tail in the ancestors of odontocete (toothed) whales is better preserved in the fossil and extant record.
Here, starting with the tiny tail found in Hemicentetes (Fig. 8), the tail elongates in Indohyus and Leptictidium (Fig. 9) to become the swimming organ used in Pakicetus and fully aquatic toothed whales.
Since a long, robust tail is already in the gene pool,
a placental mammal can redevelop a long, robust tail from not much of one.