The Origin and Evolution of Bats Part 2

Earlier we looked at basal bats and their closest outgroups. That entry from several years ago has proven to be a weekly and annual favorite among blog posts here at the pterosaur heresies. Part 3 on this subject was posted here.

Today we’ll do the same with newly arranged graphics (Figs. 1,2) principally matching the non-bat, Ptilocercus, to the basal bats, Onychonycteris and Icaronycteris (Fig. 1). I’m surprised I never did this before because the results are illuminating.

Figure 1. Ptilcercus (above) and Icaronycteris (below), sister taxa in the origin of bats.

Figure 1. Ptilcercus (above) and Icaronycteris (below), sister taxa in the origin of bats. Click to enlarge. Despite the similarities of these two, the differences in dent ion and the size of the manus kept scholars from comparing these two taxa directly with one another. Ptilocercus is also close to the flying lemur, which is why its dentition is more like the flying lemur.

In figure 1 the similarities are striking:

  1. skull, torso, pelvis and tail have similar shapes
  2. ribs are flat in both
  3. radius is longer than the humerus in both.
  4. ulna is reduced distally, to no more than one third the width of the radius (as in bats).
  5. carpus rotates posterolaterally in both
  6. the ability to spread the digits so widely that digits 1 and 5 oppose one another by 180º
  7. first manual digit is somewhat thumb-like, able to grasp objects.
  8. tibia longer than femur in both
  9. ankles are more flexible in both. The astragalus and calcaneum move away from stacked one upon the other to more of a  side-by-side configuration.
  10. Pedal digits 2 – 5 are equal in length and their metatarsals follow suit. The pedal unguals also deepen

Now let’s examine the differences. In the bat:

  1. cervicals are more gracile
  2. clavicle is longer and the scapula is larger (for large pectoral flight muscles)
  3. lumbar region is longer
  4. tail is shorter
  5. entire forelimb is longer, especially the hand
  6. hand is webbed
  7. The tibial malleolus (lateral distal process), which restricts ankle rotation in most mammals is not present in bats
  8. tarsals of bats are smaller, the penultimate phalanges are longer and the unguals are larger. Better to hang inverted.
  9. medial digit of the foot loses its ability to oppose the other pedal digits
  10. Onychonycteris develops a new bone arising from the ankle which helps frame the uropatagium.
  11. some bats use echolocation for prey capture
Figure 2. Selected details of Ptilocercus and Onychnycteris.

Figure 2. Selected details of Ptilocercus and Onychnycteris. The spreading of the metacarpals is a synapomrophy.

Remember
Ptilocercus has different teeth because it is more closely related to Cynocephalus, the flying lemur (Fig. 3), which is also not too distant from bats. Despite the appearance of extradermal membranes in dermopterans, it appears that those were obtained convergently in bats.

Figure 3. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats.

Figure 3. Cynocephalus, the flying lemur, shares many traits with Ptilocercus and basal bats. Note the distally reduced ulna.

Take another look at the bat family tree (Fig. 4).
Ptilocercus
is not another tree shrew, like Tupaia. Ptilocercus is a miniature civet. Tupaia is in the lineage of rabbits. DNA evidence (Tsagkogeorga et al. 2013) supports this tree topology with bats arising from carnivores, like civets.

Figure 2. Bat evolution and origins from the Carnivora/Viverridae. They are sisters to the Pen-tailed tree shrew and colugos among living taxa. Protictis is an extinct outgroup taxon from the Paleocene.

Figure 4. Bat evolution and origins from the Carnivora/Viverridae. They are sisters to the Pen-tailed tree shrew and colugos among living taxa. Protictis is an extinct outgroup taxon from the Paleocene.

The origin of flight
and flapping in bats continues to be a vexing problem. An earlier hypothesis based on current behavior remains unsatisfying.

Interesting YouTube video
on bat cooling in the tropics here. Yes they flap gently to generate a self-directed breeze, but they also lick themselves for evaporative cooling.

Interesting YouTube video on bat flight here.

More tomorrow.

References
Jepsen GL, MacPhee RDE 1966. Early Eocene bat from Wyoming. Science 154 (3754): 1333–1339. doi:10.1126/science.154.3754.1333. PMID 17770307.
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Simmon NB, Seymour KL, Habersetzer J, Gunnell GF 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451 (7180): 818–21. doi:10.1038/nature06549. PMID 18270539.
Tsagkogeorga G, Parker J, Stupka E, Cotton JA, Rossiter SJ 2013. Phylogenomic analyses elucidate the evolutionary relationships of bats (Chiroptera). Current Biology 23 (22): 2262–2267.

wiki/Icaronycteris
wiki/Onychonycteris
wiki/Ptilocercus

Advertisements

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 )

Twitter picture

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

Facebook photo

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

Google+ photo

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

Connecting to %s