Prorastomus/Pezosiren: when sirenians still had legs

Nothing heretical today.
We haven’t looked at any sirenians yet. And this one adds one more taxon to the LRT.

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

Figure 1. Prorastomus (or is this Pezosiren) is a pro-sirenian with legs. All four feet remain unknown. Elements from Pezosiren are also shown.

Prorastomus sirenoides (Owen 1855; Middle Eocene, 40 mya; 1.5m in length; Fig. 1) and Pezosiren are basal sirenians with four legs, a short tail and more teeth. They nest with the recenly extinct dugong, Dusisiren, in the large reptile tree (LRT, 1006 taxa).

Figure 2. Sirenian skulls, including Dusisiren, Prorastomus, and Eotheroides.

Figure 2. Sirenian skulls, including Dusisiren, Prorastomus, and Eotheroides. Note the loss of many teeth in Dusisiren.

Compared to its phylogenetic predecessor,
Moeritherium, Prorastomus/Pezosiren demonstrates the reduction in sacral vertebrae, the reduction in the cranial crest and the enlargement of the tail (what little is known). Pezosiren portelli (Domning 2001) is a related genus

According to Domning 2001
“Modern seacows (manatees and dugongs; Mammalia, Sirenia) are completely aquatic, with flipperlike forelimbs and no hindlimbs. Here I describe Eocene fossils from Jamaica that represent nearly the entire skeleton of a new genus and species of sirenian—the most primitive for which extensive postcranial remains are known. This animal was fully capable of locomotion on land, with four well-developed legs, a multivertebral sacrum, and a strong sacroiliac articulation that could support the weight of the body out of water as in land mammals. Aquatic adaptations show, however, that it probably spent most of its time in the water. Its intermediate form thus illustrates the evolutionary transition between terrestrial and aquatic life. Similar to contemporary primitive cetaceans3, it probably swam by spinal extension with simultaneous pelvic paddling, unlike later sirenians and cetaceans, which lost the hindlimbs and enlarged the tail to serve as the main propulsive organ. Together with fossils of later sirenians elsewhere in the world, these new specimens document one of the most marked examples of morphological evolution in the vertebrate fossil record.”

Domning DP 2001. The earliest known fully quadrupedal sirenians. Nature. 413 (6856): 625–627. online.
Owen R 1855.
 On the fossil skull of a mammal (Prorastomus sirenoïdes, Owen) from the island of Jamaica. The Quarterly Journal of the Geological Society of London 11:541-543.
Self-Sullivan C 2006. Evolution of the Sirenia.


3 thoughts on “Prorastomus/Pezosiren: when sirenians still had legs

  1. What do you mean by Moeritherium being a “predecessor” of Prorastomus or Pezosiren? It’s not even old enough for that, before we even get to its being a proboscidean and all.

    Why do you give Prorastomus only two lower incisors, but five lower premolars and four molars like it’s some kind of Kielantherium? And isn’t the upper canine, too, on the wrong side of the pmx-mx suture?

    Prorastomus (or is this Pezosiren)

    Of course it’s Pezosiren. No such complete skeleton of Prorastomus is known.

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