3x a tiny mammal tail evolved flukes

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.

Figure 1. Moeritherium skeleton. Note the tiny, slender tail.

Figure 1. Moeritherium skeleton. Note the tiny, slender tail.

 

Figure 1. Prorastomus is a pro-sirenian with legs. All four feet remain unknown.

Figure 2. Prorastomus is a pro-sirenian with legs. All four feet remain unknown.

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.

Figure 2. Dusisiren, a manatee sister has a robust tail and presumably, flukes.

Figure 3. Dusisiren, a manatee sister has a robust tail and presumably, flukes.

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.

Figure 1. GIF animation of the Neoparadoxia (original image from Barnes 2013). It seems illogical that the tiny tail of a desmostylian like this would ever become the giant tail of a mysticete, while the giant hind limbs disappear into the torso, but phylogenetic analysis recovers just such a scenario. Many long-jawed desmostylians are known from cranial material only and these are likely to be those that had large tails and smaller hind limbs.

Figure 4. GIF animation of the Neoparadoxia (original image from Barnes 2013). It seems illogical that the tiny tail of a desmostylian like this would ever become the giant tail of a mysticete, while the giant hind limbs disappear into the torso, but phylogenetic analysis recovers just such a scenario. Many long-jawed desmostylians are known from cranial material only and these are likely to be those that had large tails and smaller hind limbs.

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).

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Figure 5. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

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.

Figure 2. Caperea, the pygmy right whale, is a much smaller sister to Eubalaena. Only the skeleton with the ribs angled back fits the stranded in vivo specimen and the skull is a better fit when it is slightly larger.

Figure 6. Caperea, the pygmy right whale, is a much smaller sister to Eubalaena. Only the skeleton with the ribs angled back fits the stranded in vivo specimen and the skull is a better fit when it is slightly larger.

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.

Figure 1. Rorqual evolution from desmostylians, Neoparadoxia, the RBCM specimen of Behemotops, Miocaperea, Eschrichtius and Cetotherium, not to scale.

Figure 7. Rorqual evolution from desmostylians, Neoparadoxia, the RBCM specimen of Behemotops, Miocaperea, Eschrichtius and Cetotherium, not to scale.

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.

Figure 2. The short-tailed tenrec, Hemicentetes. Other than size and tail length, this taxon shares a long list of traits with the basal whale, Maiacetus in figure 1.

Figure 8 The short-tailed tenrec, Hemicentetes. Other than size and tail length, this taxon shares a long list of traits with the basal whale, Maiacetus in figure 1.

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.

Figure 1. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Figure 9. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

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.


References
.researchgate.net/The_triple_origin_of_whales
wiki/Evolution_of_sirenians

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.