First of all, one issue that has long been debated is whether flight evolved before location, or if echolocation predated flight, or if the two features evolved "in tandem." Onychonycteris has distinct features that confirm the "flight first" hypothesis. First of all, its forelimbs are "clearly capable of powered flight," with a manus typical of known bats, a strong clavicle, distinctively shaped rib cage, keeled sternum, and faceted scapular fossa.
While the wings of this bat are familiar-looking, the ears are much different from modern bats. The cochlea is smaller, as is the orbicular apophysis on the malleus. Also, the stylohyal bone shows no evidence of the extended cranial tip that facilitates echolocation. If you're not too familiar with bat anatomy, the take-home message from this is that Onychonycteris lacked not just one but several features that are crucial for a functional echolocation system.
Although Onychonycteris appears to have been fully flighted, its forelimbs do show one extremely conspicuous difference from modern bats. It has claws on every digit, in contrast to modern bats, which have lost their claws on digits II-IV (except for a few of the fruit bats, which do have a claw on digit II). The wings have low aspect ratios, and the authors suggest that they would have been best adapted for a style of flight similar to extant mouse-tailed bats: undulating fluttering alternating with periods of gliding. This would be appropriate for a basal member of a flying clade, because gliding has often been proposed as an intermediate stage preceding more sophisticated flight. (See here for a great overview of chiropteran flight mechanics).
The hindlimbs were also unique. Although Onychonycteris could fly, it also retained significant quadrupedal mobility. I thought this figure from the paper was absolutely beautiful, could you ask for a better intermediate? The top cluster represents known bats (both modern and extinct), the bottom cluster is terrestrial mammals, and the two dots in the middle are the two known Onychonycteris specimens. (Note: full credit to the paper for the figure, I don't want to get "in trouble" for posting it, but I really thought it seemed worth sharing).
Speaking of limbs, the Onychonycteris fossil includes the calcar! This is significant, many previous bat fossils have not had a calcar, but it was uncertain as to whether this was simply due to incomplete preservation or lack of the structure during life. The calcar supports the uropatagium, the membrane between the hind leg and tail. This tells us that even the most basal known bat had a uropatagium. The authors suggest that the uropatagium initially surved as an airfoil to aid in flight, but that after echolocation evolved the structure was handily preadapted to aid in capture of insects and other flying prey. Speaking of prey, the dentition indicates that Onychonycteris consumed mostly insects.
So, to summarize all of the reasons this is a truly exciting find: it helps to solve some of the big mysteries about bat evolution. Now we know that (1) basal bats did have uropatagiums, (2) that flight predates echolocation, that (3) claws and (4) quadrupedal agility persisted after the evolution of functional wings, and (5) that gliding was included in the primitive forms of flight, indicating a "top down" route to evolution (as opposed to ground-up). And (6) insectivory appears to be the ancestral condition for bats. We still don't know how nocturnal these animals were, eye size would be a good indicator but the skulls were too damaged to get good orbital measurements.
This announcement made my week. It will definitely be interesting to see if more specimens are recovered from the site in Wyoming, hopefully some with completely intact skulls? I'd be interested to know more about their reproductive biology and behavioral ecology, what was a typical litter size? Were they tree or cave roosters, solitary or colonial? Those things are hard to tell from fossils, but one can always hope...
Anyway, this is a great week for bat fans!
Simmons, N. B., K. L. Seymour, J. Habersetzer, and G. F. Gunnell. 2008. Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451: 818-22.
Although Onychonycteris appears to have been fully flighted, its forelimbs do show one extremely conspicuous difference from modern bats. It has claws on every digit, in contrast to modern bats, which have lost their claws on digits II-IV (except for a few of the fruit bats, which do have a claw on digit II). The wings have low aspect ratios, and the authors suggest that they would have been best adapted for a style of flight similar to extant mouse-tailed bats: undulating fluttering alternating with periods of gliding. This would be appropriate for a basal member of a flying clade, because gliding has often been proposed as an intermediate stage preceding more sophisticated flight. (See here for a great overview of chiropteran flight mechanics).
The hindlimbs were also unique. Although Onychonycteris could fly, it also retained significant quadrupedal mobility. I thought this figure from the paper was absolutely beautiful, could you ask for a better intermediate? The top cluster represents known bats (both modern and extinct), the bottom cluster is terrestrial mammals, and the two dots in the middle are the two known Onychonycteris specimens. (Note: full credit to the paper for the figure, I don't want to get "in trouble" for posting it, but I really thought it seemed worth sharing).
Speaking of limbs, the Onychonycteris fossil includes the calcar! This is significant, many previous bat fossils have not had a calcar, but it was uncertain as to whether this was simply due to incomplete preservation or lack of the structure during life. The calcar supports the uropatagium, the membrane between the hind leg and tail. This tells us that even the most basal known bat had a uropatagium. The authors suggest that the uropatagium initially surved as an airfoil to aid in flight, but that after echolocation evolved the structure was handily preadapted to aid in capture of insects and other flying prey. Speaking of prey, the dentition indicates that Onychonycteris consumed mostly insects.So, to summarize all of the reasons this is a truly exciting find: it helps to solve some of the big mysteries about bat evolution. Now we know that (1) basal bats did have uropatagiums, (2) that flight predates echolocation, that (3) claws and (4) quadrupedal agility persisted after the evolution of functional wings, and (5) that gliding was included in the primitive forms of flight, indicating a "top down" route to evolution (as opposed to ground-up). And (6) insectivory appears to be the ancestral condition for bats. We still don't know how nocturnal these animals were, eye size would be a good indicator but the skulls were too damaged to get good orbital measurements.
This announcement made my week. It will definitely be interesting to see if more specimens are recovered from the site in Wyoming, hopefully some with completely intact skulls? I'd be interested to know more about their reproductive biology and behavioral ecology, what was a typical litter size? Were they tree or cave roosters, solitary or colonial? Those things are hard to tell from fossils, but one can always hope...
Anyway, this is a great week for bat fans!
Simmons, N. B., K. L. Seymour, J. Habersetzer, and G. F. Gunnell. 2008. Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature 451: 818-22.
2 comments:
It really is a great paper. Now I want to restore it, butmy knowledge of bat anatomy is scant (to the say the least). If I were to look at a modern-day example for general proportions and overall look, who should I look at?
still wondering why they didn't log transform their data...
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