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Because caecilians are important

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Mexican burrowing caecilian (Dermophis mexicanus), by Franco Andreone and from calphotos.berkeley.edu (licensed under Creative Commons Attribution-Share Alike 2.5 Generic license).

This is the 200th article at Tet Zoo ver 3 – thanks, pass the champagne, donation cheque etc. (hint hint). The plan is to produce a lengthy introspective-type article that includes links to all the content that’s appeared on Tet Zoo ver 3 so far. Alas, I just haven’t been able to find time for that and it’ll have to wait until conference season is over. Until then, here’s a short article about caecilians (to long-time Tet Zoo readers, it’s a section from one of the longer caecilian articles that appeared on ver 2 back in 2008). I so need to cover caecilians at length, once again. And in time I will.

All extant caecilians (pronounce it ‘say-see-lee-un’) are long-bodied, limbless, superficially worm-like amphibians with reduced eyes. [Image of Dermophis above from the CalPhotos database.] Caecilians are predominantly fossorial (adapted for burrowing), subterranean animals, although some are aquatic or semiaquatic and some terrestrial species have aquatic larvae. Limb girdles are entirely absent and skin folds (annulae) encircle the body. Despite their mostly subterranean habits, some caecilian species are visually striking, being blue, purple, green, bright orange, yellow, or with bright yellow lateral stripes. Others are decorated with blotches, or have a head that’s lighter in colour than the body. Poison glands are present (some species may even be quite toxic to humans), so it’s likely that the bright colours are aposematic.

Brightly coloured dermophiid caecilian Schistometopum thomense, with babies. This distinctive species is endemic to islands in the Gulf of Guinea. Photo (c) R. A Nussbaum.

While the smallest species (like the caeciliid Idiocranium russelli from Cameroon) are mature at just 70 mm, the largest – the very slender-bodied Caecilia thompsoni from Colombia – reaches an incredible 1.5 m [C. thompsoni shown in image below; photo by Taran Grant].

Caecilia thompsoni, photo (c) Taran Grant. There's no scale, but you can clearly see that this is a very very long animal.

In the members of some caecilian clades (like the South American rhinatrematids), a short tail is present and the mouth is located at the front of the head; in others, a tail is absent and the mouth is situated further back, slung underneath the head. Among the weirder features of caecilians are their short, sensory tentacles. Unique to the group, these are paired structures that emerge from cavities on either side of the snout, between the eye and nostril. The tentacles are typically easy to see, even in the smallest species. Derived from the tear duct, extrinsic eye muscles and other orbital structures, the tentacles are connected to the vomeronasal organs and presumably allow the animals to test their environment for sensory data.

Excellent diagrams showing skull anatomy and jaw musculature of (from top to bottom) rhinatrematid, ichthyophiid, caeciliid and scolecomorphid caecilians. Caecilians are unique in that they have co-opted various throat muscles as jaw-closers. From Nussbaum (2000).

The caecilian skull is generally bullet-shaped, robust, thick-boned and with strongly adhering skin. Their recurved teeth are sometimes sharp-keeled and bicusped. The eyes of caecilians are sometimes visible beneath the skin and set within bony sockets (e.g., Ichthyophis), are sometimes hidden beneath the bones of the skull (e.g., Scolecomorphus), and are sometimes completely absent (e.g., Boulengerula) [UPDATE: not correct. See comments].

While the tentacle is often located close to the nostril and some distance from the eye, the eye and tentacle are close in position in some species. Scolecomorphids, containing only Scolecomorphus and Crotaphatrema, are unique to equatorial Africa and have a particularly large tentacular opening located near the tip of the snout, ahead of the under-slung mouth. The close position of the eye and tentacle mean that they’ve become connected: in its resting position, the eye is located beneath the lateral surface of the skull, but full extrusion of the tentacle causes the eye to move out of the skull and down the tentacle (O’Reilly et al. 1996). An area of the tentacle lacking in pigmentation presumably allows light to reach the retina. Scolecomorphids are the only tetrapods that can deliberately move their eyes out of their skulls. Scolecomorphids are also unusual in lacking a stapes.

Aquatic Typhlonectes, foraging near the bottom of an aquarium. These caecilians are often sold in the pet trade as 'rubber eels' or 'black eels'. Photo by Neil Phillips.

It’s mostly assumed that caecilians are generalist predators of soil-dwelling invertebrates such as earthworms, ants and termites – animals termed ‘soil ecosystem engineers’ by ecologists. However, it’s also been argued that some caecilians are specialists (preying specifically on termites, earthworms or beetle pupae), or even detritivores, ingesting leaf humus and other plant fragments (Hebrard et al. 1992).

That last proposal would be radical given that amphibians are essentially all carnivorous as adults*. Alas, a later study showed that soil and detritus recovered from the gut contents of the species concerned (the east African caeciliid Boulengerula taitana) actually represented the gut contents of its earthworm prey (Gaborieau & Measey 2004). Small vertebrates including frogs, lizards, burrowing snakes (Presswell et al. 2002) and possibly rodents sometimes fall prey to the larger species. Caecilians themselves are preyed upon by burrowing snakes, fish (Gazola Da Silva et al. 2007), and by introduced animals like chickens, pigs and the tenrecs that have been introduced to the Seychelles.

* Yes yes, I am aware of the exceptions (and have written about them on Tet Zoo: herbivory in sirens, frugivory and even leaf-eating in frogs etc.).

Caecilians occur throughout the humid tropical regions of the world: in South America, equatorial eastern and western Africa, and tropical Asia including the Philippines and the western Indo-Australian archipelago. They also inhabit the Seychelles. The absence of caecilians from central equatorial Africa and Madagascar is odd and we should consider the possibility that they might await discovery in these areas. Generally assumed to be rare and highly elusive, some studies have shown that at least some caecilian species can be abundant if searched for (Measey 2004), and in fact present in sufficient numbers to be (presumably) ecologically significant, especially so given their predation on soil ecosystem engineers (Jones et al. 2006).

Caecilians and Tet Zoo have history. Here's the ZSL's Helen Meredith talking about the caecilian material covered on Tet Zoo as of 2009. Photo by Darren Naish.

There is, of course, loads and loads and loads more than needs to be said about caecilians, some of which has been covered here on Tet Zoo before. Don’t worry, this additional data will be covered here (again) in time, in updated form – there is lots of exciting news from the world of caecilians!

For previous Tet Zoo coverage of caecilians, see…

Refs – -

Gaborieau, O. & Measey, G. J. 2004. Termitivore or detritivore? A quantitative investigation into the diet of the east African caecilian Boulengerula taitanus (Amphibia: Gymnophiona: Caeciliidae). Animal Biology 54, 45-56.

Gazola Da Silva, F. F., Mott, T., Garey, M. V., & Vutule, J. R. S. 2007. Chthonerpeton viviparum Parker & Wettstein, 1929 (Amphibia, Gymnophiona, Typhlonectidae) in Paraná state, Brazil and the first record of predation of this species by Hoplias malabaricus (Bloch, 1794) (Actinopterygii, Erythrinidae). Pan-American Journal of Aquatic Science 2, 261-262.

Hebrard, J., Maloiy, G. & Alliangana, D. 1992. Notes on the habitat and diet of Afrocaecilia taitana (Amphibia, Gymnophiona). Journal of Herpetology 26, 513-515.

Jones, D. T., Loader, S. P. & Gower, D. J. 2006. Trophic ecology of east African caecilians (Amphibia: Gymnophiona), and their impact on forest soil invertebrates. Journal of Zoology 268, 117-126.

Measey, G. J. 2004. Are caecilians rare? An east African perspective. Journal of East African Natural History 93, 1-21.

Nussbaum, R. A. 2000. Caecilians. In Cogger, H. G., Gould, E., Forshaw, J., McKay, G. & Zweifel, R. G. (consultant eds) Encyclopedia of Animals: Mammals, Birds, Reptiles, Amphibians. Fog City Press (San Francisco), pp. 492-499.

O’Reilly, J. C., Nussbaum, R. A. & Boone, D. 1996. Vertebrate with protrusible eyes. Nature 382, 33.

Presswell, B., Gower, D. J., Oommen, O. V., Measey, G. J. & Wilkinson, M. 2002. Scolecophidian snakes in the diets of south Asian caecilian amphibians. Herpetological Journal 12, 123-126.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. John Harshman 9:55 am 08/25/2013

    I have never, ever seen a caecilian in the wild. How do you go about finding them? Oddly (at least to me) I’ve seen more free-living nematodes than caecilians.

    On pronunciation: say-see-lee-un? I would think see-silly-un. There seems to be a controversy surrounding the pronunciation of “ae”. I’ve heard three versions, mostly at the end of family names “-idae”. The most common is the (British) public-school “ee” as in “beet”. But I also hear “ay” as in “bay” and, rarely “i” as in “bite”. The last would be the real Latin pronunciation, but I’ve always supposed that public-school Latin is the standard. It all appears to depend on what your advisor said.

    Link to this
  2. 2. naishd 10:12 am 08/25/2013

    Having heard quite a few people use the term (mostly thanks to the ZSL conference of 2009), my impression is that people mostly say ‘suh-see-lee-un’, not necessarily ‘say-see-lee-un’ as I said above.

    As for finding them in the field… it seems to be a matter of going to the right places and either digging holes, or lifting up the right sorts of objects.

    Darren

    Link to this
  3. 3. Heteromeles 4:01 pm 08/25/2013

    Didn’t Simon and Garfunkel write a song about caecilians?

    Link to this
  4. 4. Heteromeles 7:01 pm 08/25/2013

    Actually, an off-topic but serious question: are there any secondarily terrestrial tetrapods? By this I mean, is there anything whose ancestors were terrestrial, then aquatic, then became terrestrial again?

    If I recall correctly, this is thought to have happened with monocots among plants, where the oldest monocots were aquatic plants, but most are (secondarily) terrestrial. The ancestors of the monocot clade were terrestrial.

    The reason I’m asking here is that I wonder if some caecilian is secondarily terrestrial, or at least secondarily fossorial.

    Link to this
  5. 5. Gigantala 7:07 pm 08/25/2013

    In Madagascar at least, we could assume eradication due to early Palaeogene drying.

    Link to this
  6. 6. John Harshman 8:01 pm 08/25/2013

    One theory of the origin of snakes is that they went through a marine phase. This is probably linked to those phylogenetic analyses showing mosasaurs to be the sister group of snakes.

    There are of course other theories and other cladograms.

    Link to this
  7. 7. Heteromeles 8:43 pm 08/25/2013

    Of course, my thought after I typed this was something like “D’oh Pristichampsus.” Slow Sunday, I guess. Thanks for the note about snakes, John. That would be before or after the blindsnakes split off?

    Link to this
  8. 8. Squiddhartha 9:34 pm 08/25/2013

    Heteromeles, I made the same Simon and Garfunkel joke on Facebook. :)

    Link to this
  9. 9. Michael_Blonde 9:54 pm 08/25/2013

    There’s also echidnas. By the way, is there any particular reason why leglessness evolved so many times among squamates?

    Link to this
  10. 10. llewelly 10:08 pm 08/25/2013

    Speculative zoology question.

    Suppose no squamates had survived the end Cretaceous, but caecilians had. (Unlikely, but bear with me.)

    Call this period the caecilizoic.

    What snake-like niches might caecilians expand into ?
    What snake-like niches would be more likely taken up by crocodyliforms?

    What varanoid-like niches might caecilians expand into ?
    What varanoid-like niches would be more likely taken up by crocodyliforms?

    And so on.

    (Actually, given the past diversity of crocodyliforms , perhaps this would be better called the crocozoic. Maybe the crocodyliforms need to go extinct for the caecilizoic to happen.)

    A related question – are there any caecilians which live in environments where the soil is not moist most of the year?

    Not really related – there are caecilians whose young feed on their mother’s shed skin (covered in two of the past tet zoo caecilian articles linked above). Is this behavior derived from or related to the fact that the fetii of many viviparous caecilians feed on special cells in the oviduct during gestation?

    Link to this
  11. 11. llewelly 10:23 pm 08/25/2013

    ” By the way, is there any particular reason why leglessness evolved so many times among squamates?”

    Squamates are scrumptious, and thus need to do a lot of hiding. It’s a very efficient shape for hiding underground, in rockpiles, or in undergrowth. Works fairly well for swimming too.

    Alternatively … it’s a terrible shape for warm endotherms, due to the high surface area to mass ratio, so birds and mammals are not likely to evolve such a shape. (But, there are moles, mole rats, golden moles, ferrets … none of which are quite legless.)

    On the other hand, what about crocodyliforms? Why no legless crocodyliforms?

    Link to this
  12. 12. ectodysplasin 12:03 am 08/26/2013

    @Heteromeles:

    The reason I’m asking here is that I wonder if some caecilian is secondarily terrestrial, or at least secondarily fossorial.

    Very doubtful. The only aquatic caecilians are the typhlonectids, and they’re relatively deeply nested within caecilian phylogeny.

    Link to this
  13. 13. ectodysplasin 12:16 am 08/26/2013

    @llewelly:

    Squamates are scrumptious, and thus need to do a lot of hiding. It’s a very efficient shape for hiding underground, in rockpiles, or in undergrowth. Works fairly well for swimming too.

    I’m not convinced that hiding is sufficient for an animal to spend such a significant portion of its life underground, and as a hypothesis does not really account for numerous other morphological and behavioral innovations that seem to evolve over and over again in elongate-bodied fossorial species (not just squamates, but also caecilians, some salamander species, and some extinct problematic taxa). Long story short (and I can expand on this if you’d like) head-first burrowing in these taxa seems to be associated with exploitation of burrowing arthropod prey and sheltering from environmental stress, particularly heat and water stress.

    Link to this
  14. 14. Andreas Johansson 1:13 am 08/26/2013

    fetii

    Fetuses. Or, if you really have to use the Latin plural, fetus, indistinguishible from the singular in writing.

    (Pet peeve: Latin plurals only ever end in double -ii if there’s an -i- in the singular already.)

    Link to this
  15. 15. Pristichampsus 2:20 am 08/26/2013

    In an episode of “River Monsters”, Jeremy Wade puts out bait for the scavenging catfish, and instead hauls up a load of Typhlonectes, who were apparently scavening the bait. He kept saying that they were Swamp Eels, but I’m pretty sure they were Typhlonectes.

    Link to this
  16. 16. Perisoreus 4:20 am 08/26/2013

    Well, if you go with traditional latin, it would be Kaey-kee-lee-uns (or something like that, I’m not really good as transcribing into english, mainly because that language doesn’t have any consistent rules for pronunciation at all).

    If you’re not that accurate but still somewhat traditional, at least the first c should be pronounced as a “k” as indicated by the closed vocal “a” by which it is followed. If you contract the “a” and the “e” to an “æ” (slightly more open than the “e” in “hell” and slightly more closed than the “a” in “than”) is up to you.

    In the end, I think, it doesn’t really matter. Firstly, because English is one of the worst languages to transcribe latin correctly (okay, maybe apart from !Xhosa), and secondly because no one cares as long as you can make yourself comprehensible. Caecilians matter, their pronunciation not so much.

    Link to this
  17. 17. naishd 4:38 am 08/26/2013

    Thanks for all these great comments. I’ll be quiet for a while since I’m at the SVPCA meeting in Scotland. But…

    The Jeremy Wade issue referred to above (Pristichampsus’s comment # 15) was brought to my attention a while back by Markus Bühler. It’s pretty amazing, and check out this email exchange…

    Me to caecilian expert David Gower…
    “You’ve probably heard of, or seen, the excellent (but ichthyologically-themed) TV series River Monsters. In one episode – devoted to the Amazonian caniduru asu – Jeremy Wade uses a large dead fish (a pacu, I think) as bait. He seems to succeed in capturing a large number of typhlonectids – so far, I’ve only seen the German edition of the show. It’s here’s on youtube – the segment with the apparent caecilians is at 37:50. Have a look and let me know what you think… [link to the video]. Those sure look like caecilians to me. I have never heard of caecilians scavenging from vertebrate carcasses before – is this behaviour on record? If not, it should probably be written up. I’m hoping this is not old news – sorry if it is.”

    Dave Gower to me…
    “Yes, I had seen that before and yes they are caecilians so well spotted. Probably Potomotyphlus kaupi. People (including us) have caught aquatic caecilians using dead fish etc – it works very well. It is in the literature in bits and pieces. I meant to try and contact the Jeremy Wade show after seeing it to try and find out the locality. Do either of you know him? I had the joy of first seeing this in the US at Ron Nussbaum’s house, also with Mark Wilkinson. We kept rewinding it and re-playing it because we couldn’t stop laughing at his reaction.”

    So, aquatic caecilians scavenge on big dead vertebrates. Wow.

    Darren

    Link to this
  18. 18. David Marjanović 9:55 am 08/26/2013

    and are sometimes completely absent (e.g., Boulengerula).

    Nope. Boulengerula boulengeri has lensless eyes that are covered by the skull roof and are extremely tiny: 0.13 mm wide and 0.06 mm deep (they’re not even spherical).

    Wake, M. H. 1985. The comparative morphology and evolution of the eyes of caecilians (Amphibia, Gymnophiona). Zoomorphology 105: 277–295.

    as cited by:

    Himstedt, W. 1996. Die Blindwühlen. Technically volume 630 of Die neue Brehm-Bücherei. Westarp Wissenschaften.

    I fear Himstedt’s little book (paperback, A5 format, 160 pages in total) is the only general book about caecilians. It covers everything from anatomy over distribution to behavior and ecology.

    the tenrecs that have been introduced to the Seychelles

    what

    The absence of caecilians from central equatorial Africa and Madagascar is odd and we should consider the possibility that they might await discovery in these areas.

    Central Africa, sure; but their absence from Madagascar may not be odder than their absence from Australia/New Guinea.

    I’ve always supposed that public-school Latin is the standard

    What makes you think there’s a standard? :-)

    Of course, my thought after I typed this was something like “D’oh Pristichampsus.”

    Pristichampsus is only descended from semiaquatic = amphibious crocodylians, not from aquatic ones.

    That would be before or after the blindsnakes split off?

    Long before.

    are there any caecilians which live in environments where the soil is not moist most of the year?

    Not that I know of.

    !Xhosa

    No ! there; the xh is the aspirated lateral click.

    link to the video

    “This video is not available. We’re sorry. :-/ “

    Link to this
  19. 19. ectodysplasin 10:24 am 08/26/2013

    If pronunciation of the word “caecilian” gives you too much grief, just call them “gymnophionans.” Problem solved.

    Link to this
  20. 20. Hai~Ren 10:50 am 08/26/2013

    Based on their current distribution, I would have thought that caecilians had Gondwanan origins, but Eocaecilia doesn’t seem to fit that. Unless caecilians were once distributed all over Pangaea, but the most recent common ancestor of all the extant caecilians was from Gondwana.

    Any evidence that some of the caecilians endemic to the Seychelles may have rafted over, instead of having been stranded by plate tectonics?

    Link to this
  21. 21. Heteromeles 11:27 am 08/26/2013

    @David: True, but echidnas are also descended from semi-aquatic ancestors. Aside from (possibly) snakes, it appears there aren’t any truly secondarily terrestrial vertebrates. That somewhat begs the question, because there aren’t a lot of truly secondarily aquatic animals (as opposed to amphibious vertebrates) to begin with.

    Link to this
  22. 22. Dartian 11:48 am 08/26/2013

    David:
    “Die neue Brehm-Bücherei”

    This, by the way, is also generally speaking an excellent book series; recommended for anyone who can read German.

    Heteromeles:
    echidnas are also descended from semi-aquatic ancestors. Aside from (possibly) snakes, it appears there aren’t any truly secondarily terrestrial vertebrates

    What’s your definition of ‘aquatic’? If the echidnas’ platypus-like ancestors aren’t sufficiently aquatic for you, then I suppose that there indeed aren’t any secondarily terrestrial vertebrates.

    If, however, we apply slightly less strict criteria for what qualifies as ‘aquatic’ then I suspect that the pygmy hippopotamus should qualify. Also, there are several ‘waterbird’ lineages that have produced effectively terrestrial taxa; for example, the Cape Barren goose and the Hawaiian goose among extant anatids, and arguably the takahe (and a few other species) among the rails.

    Link to this
  23. 23. StupendousMan 12:52 pm 08/26/2013


    If pronunciation of the word “caecilian” gives you too much grief, just call them “gymnophionans.” Problem solved.

    Problem solved? I think not. How do you pronounce the initial consonant in “gymnophionans?” Socrates would have used a hard “g”, modern speakers probably favor a soft “g”. Which is correct?

    Link to this
  24. 24. Heteromeles 12:53 pm 08/26/2013

    @Squiddhartha: As for the song, since it’s late August, shouldn’t we, erm, nerd-source some taxon-appropriate lyrics to Simon and Garfunkel’s tune?

    For example:

    Caecilians, you’re breaking my heart
    You’re shaking my confidence daily
    Oh, caecilians, I’m down on my knees
    I’m digging you free, please don’t go
    Please don’t go

    Fishing up in the Amazon near caecilians
    And I had no clue (had no clue!)
    Pulled a long line in too late,
    and found Potomotyphlus
    had chewed up my bait.

    Surely someone can do better than that.

    Link to this
  25. 25. ectodysplasin 2:07 pm 08/26/2013

    @StupendousMan:

    Problem solved? I think not. How do you pronounce the initial consonant in “gymnophionans?” Socrates would have used a hard “g”, modern speakers probably favor a soft “g”. Which is correct?

    They also made Socrates drink hemlock for being a public annoyance.

    Link to this
  26. 26. ectodysplasin 2:15 pm 08/26/2013

    By the way, in terms of secondarily-terrestrial groups, direct-developing plethodontids probably count.

    Link to this
  27. 27. BrianL 4:08 pm 08/26/2013

    Regarding eyelessness: If these caecilians don’t count, are there any tetrapods that are naturally well and truly eyeless? Moles, fossorial rodents, olms and the like wouldn’t count as they still have vestigial eyes.

    Link to this
  28. 28. Gigantala 5:41 pm 08/26/2013

    Marsupial moles have their eyes covered by skin and reduced to miniscule lenses.

    Link to this
  29. 29. Dartian 12:54 am 08/27/2013

    Brian:
    are there any tetrapods that are naturally well and truly eyeless?

    No.

    Link to this
  30. 30. David Marjanović 10:42 am 08/27/2013

    Based on their current distribution, I would have thought that caecilians had Gondwanan origins, but Eocaecilia doesn’t seem to fit that. Unless caecilians were once distributed all over Pangaea, but the most recent common ancestor of all the extant caecilians was from Gondwana.

    Exactly – Eocaecilia is far outside the crown-group. The one other known stem-caecilian, Rubricacaecilia from the beginning of the Cretaceous of Morocco, is from Gondwana and is closer to the crown-group.

    Any evidence that some of the caecilians endemic to the Seychelles may have rafted over, instead of having been stranded by plate tectonics?

    Nope. The Seychelles are a real continent that broke off of India in the early Paleocene.

    They also made Socrates drink hemlock for being a public annoyance.

    Thread won.

    By the way, in terms of secondarily-terrestrial groups, direct-developing plethodontids probably count.

    Oh yes, very good point! Lunglessness looks like an adaptation to living in cold mountain streams.

    Link to this
  31. 31. Jenny Islander 12:32 pm 08/27/2013

    The first impression this amateur gets is that, tiny as they are, these critters have extremely robust skulls. They look a lot like fossils of bigger amphibians from the Coal Age, actually. Can I make the inference that the bigger animals spent a lot of time with their muzzles in the dirt?

    Link to this
  32. 32. ectodysplasin 2:23 pm 08/27/2013

    @Jenny Islander,

    >The first impression this amateur gets is that, tiny as they are, these critters have extremely robust skulls. They look a lot like fossils of bigger amphibians from the Coal Age, actually. Can I make the inference that the bigger animals spent a lot of time with their muzzles in the dirt?

    Not likely. What is important to understand is that the simplification of the tetrapod skull from that of our fishy ancestors was not an instantaneous process. Heavily ossified dermal skulls were commonplace in fish for a variety of reasons, likely associated either with some combination of defense, bouyancy, calcium storage, and developmental constraint (in other words, the genetic blueprinting hadn’t quite become sophisticated enough to start specifying where to put dermal bone, and instead was still stuck putting dermal bone everywhere). This persisted throughout many Paleozoic tetrapods even up into the Triassic.

    We know that most of these animals were not burrowers because their skulls are not otherwise specialized for burrowing. Burrowing requires a very specific type of skull; you need a snout that shovel-shaped or bulldozer-shaped to push soil out of the way, you need various specializations of the ear for hearing underground, and you need to squeeze your skull into a sort of elongate structure so you have all the head you need in a relatively small cross-sectional space. You also need strong structural supports within the skull itself that protect against compressional forces; these are usually derived from the cartilages of the braincase, primarily the sphenoid and ethmoid regions. Furthermore, burrowers are often pretty small compared to their non-burrowing relatives.

    This isn’t the case for any of the large Paleozoic tetrapods, and is only really the case for a single group of small Paleozoic tetrapods, a group of animals called ‘microsaurs’ (in quotes because we’re not certain yet whether they form a natural group). Some analyses have suggested that ‘microsaurs’ may actually be stem-caecilians, but newer analyses reject this idea, and instead suggest that ‘microsaurs’ are convergent with caecilians in some key ways but are not closely related. ‘Microsaurs,’ by the way, are super-small, as the name would suggest. Many of the species have skulls no longer than a centimeter or two long.

    Link to this
  33. 33. ectodysplasin 3:42 pm 08/27/2013

    @David,

    >Exactly – Eocaecilia is far outside the crown-group. The one other known stem-caecilian, Rubricacaecilia from the beginning of the Cretaceous of Morocco, is from Gondwana and is closer to the crown-group.

    Yes, and the importance of these huge gaps in caecilian evolution cannot be understated. Eocaecilia is definitely a stem-caecilian (see Hillary Maddin’s recent paper in PLoSONE) but beyond some anatomy of the braincase and palate, it’s very difficult to interpret in terms of both other caecilians and in terms of general early tetrapod diversity. More work by Hillary Maddin (she’s kinda the world expert of caecilian morphology at this point) suggests that caecilians probably do fall out within temnospondyls and likely had ancestors with a tympanic ear….but whether they fall out within amphibamids (as do salamanders and frogs) or whether they fall out more basally within temnospondyls is unclear because there’s such a huge gap in morphology.

    Basically, there are two vertebrate groups that are still total enigmas in terms of origins, and that’s turtles and caecilians. So caecilians are serious business.

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  34. 34. Dartian 6:01 am 08/28/2013

    David:
    Thread won.

    Yeah, good on you ectodysplasin! Socrates may have been a great philosopher, but he was arguably also the inventor of trolling. ;)

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  35. 35. David Marjanović 6:40 am 08/28/2013

    So caecilians are serious business.

    I agree with that part. :-)

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  36. 36. David Marjanović 9:12 am 08/28/2013

    If, however, we apply slightly less strict criteria for what qualifies as ‘aquatic’ then I suspect that the pygmy hippopotamus should qualify.

    Also, the dwarf hippos of Crete are thought to have undertaken seasonal migrations between the plains and the mountains.

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  37. 37. Dartian 9:18 am 08/28/2013

    David:
    Also, the dwarf hippos of Crete are thought to have undertaken seasonal migrations between the plains and the mountains.

    …and at least some of the extinct hippos of Madagascar seem to have been mostly terrestrial as well. All in all, hippos don’t seem to be very strongly committed to remaining (semi-)aquatic.

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  38. 38. BrianL 10:19 am 08/28/2013

    I think it’s worth noting that *Hexaprotodon* hippos (regardless if you include the modern pygmy hippo in that genus or not) would seem to have generally been more terrestrial than *Hippopotamus* too. With this in mind, it might be worth pondering if hippopotamids may not ancestrally be less semi-aquatic with our modern perception skewed by the perhaps uncharacteristically amphibious *Hippopotamus* being the more common and better-known species of the two living. Given the terrestrial habits of Mediterranean and Madagascar species of the genus, *H. amphibius* could even be atypically amphibious within its genus. I’ve read that some of its extinct relatives, like *H. gorgops* ‘out-amphibioused’ the modern species, though. Even so, I don’t know if the difference would have been that extreme so it might be safe to assume that *amphibius* is indeed near the extreme end of the aquatic range of hippopotamids, rather than in the ‘normal’ part of the spectrum for the clade throughout its history.
    Is there anybody knowledgeable about fossil hippos who could comment on this?

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  39. 39. Perisoreus 11:13 am 08/28/2013

    Dartian:

    Yeah, good on you ectodysplasin! Socrates may have been a great philosopher, but he was arguably also the inventor of trolling.

    Or at least, trolling the trolls. I wonder if it paid out for him. Seems Plato was all too uptight for being an acceptable successor in this respect, but Diogenes did quite well.

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  40. 40. ectodysplasin 11:44 am 08/28/2013

    By the way, on the subject of secondarily terrestrial species, it might be worth considering mekosuchine crocodiles, e.g. Mekosuchus and Quinkana. Their ancestors were likely somewhat more aquatic than those of pristichampsines, and a lot of derived mekosuchines were actually quite a bit more terrestrialized than Pristichampsus.

    Speaking of, anyone know if there’s any plans in the works to recover aDNA from any mekosuchine material?

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  41. 41. Dartian 11:59 am 08/28/2013

    Brian:
    Given the terrestrial habits of Mediterranean and Madagascar species of the genus, *H. amphibius* could even be atypically amphibious within its genus.

    In principle at least, it’s quite possible that hippos have been somewhat more terrestrial during their evolutionary history than we might tend to think. But the ancestors of those hippos on the Mediterranean islands and Madagascar were almost certainly more aquatic than their descendants; they must have reached those islands by swimming.

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  42. 42. BrianL 12:29 pm 08/28/2013

    @Dartian:

    You’re right about that, of course but note that I wasn’t advocating that those island hippos were primarily terrestrial. Their ancestors were undoubtedly aquatic but that does not neccessarily mean they were equal to *H. amphibius* in that regard. Deer and elephants prove that being a good swimmer is really enough and that you don’t need to be primarily aquatic to colonise islands if you’re a large herbivore. Let’s be clear that I’m not advocating terrestrial hippos invading Madagascar here: I merely want to point out that there’s a sliding scale of amphibiousness and that modern hippos may be at a more extreme position on this scale than many or even most of their fossil relatives were. The (as far as I know) absence of fully aquatic hippos may be worth pointing out here too. The modern hippo may not be that behaviorally typical of hippopotamids in general. One could wonder if that might even tie in to its survival in the face of the Quaternary extinction of its relatives (except the pygmy hippo, of course). Note that I’m not advocating this, just pointing out that there might be something to it.

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  43. 43. ectodysplasin 1:18 pm 08/28/2013

    Worth pointing out that those same Mediterranean islands also were home to dwarf elephants and other Afro-Asian megafauna, so I’m not 100% certain you’d have to propose that those hippos were aquatic, especially when it comes to basal members of the genus Hippopotamus and the entire genus Hexaprotodon.

    Remember, if they were too aquatic, you wouldn’t expect to see island dwarfism and other signs of genetic isolation.

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  44. 44. gentle lemur 4:32 pm 08/28/2013

    I had a shock this morning as I was driving down the M56: I was overtaken by a van with lettering on its side in a rather exotic font. I read it as CAECILIAN!
    When I looked again I saw that it said CAECILLIAN – Google tells me that it’s a firm of plumbers from Dukinfield, so I trust that the driver had an appropriate number of limbs and functional eyes.

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  45. 45. Heteromeles 5:10 pm 08/28/2013

    Aren’t fully aquatic hippos called whales?

    There are two real problems with fully aquatic hippos: drought and dugongs. The advantage hippos have is that they can feed outside rivers, and, if needs must, they can walk away from a drying river to find some place else to live. The issue with dugongs is that they may have excluded hippos from the fully aquatic herbivore niche in the few perennial river systems where they could have conceivably evolved.

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  46. 46. ectodysplasin 6:27 pm 08/28/2013

    Cetes + Raoellids are somewhat closely related to hippos, but it would not be correct to call cetes “fully aquatic hippos.”

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  47. 47. Jerzy v. 3.0. 7:33 pm 08/28/2013

    Wasn’t Hippopotamus gorgops more aquatic than the living hippo? Actually living Nile hippos mostly feed on dry land at night, they are less aquatic than people think.

    About secondarily terrestrial – Moeritherium? New Zealand snipes?

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  48. 48. Jerzy v. 3.0. 7:34 pm 08/28/2013

    BTW, apparently Jurassic Park 3D is popular in cinemas now. How much it aged in 20 years, with naked velociraptors and wrinkle-skinned sauropods… You may write something about it, compare All Yesterdays book.

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  49. 49. Heteromeles 9:05 pm 08/28/2013

    So what makes H. gorgonops more aquatic? The only feature I can find is that its eyes were more elevated than those of the modern H. amphibius. Oddly, it appears that there’s a bit of a correlation between the size of the hippo and the relative elevation of the eyes (the bigger the beast, the higher its eyes are elevated). One can therefore wonder whether H. gorgonops was as aquatic as claimed.

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  50. 50. Dartian 12:31 am 08/29/2013

    ectodysplasin:
    those same Mediterranean islands also were home to dwarf elephants and other Afro-Asian megafauna

    Actually, no. The ancestors of those Mediterranean pygmy elephants almost certainly were neither African nor Asian. In all likelihood they evolved from the European straight-tusked elephant Palaeoloxodon antiquus. As for the pygmy hippos, their ancestral species more likely than not also came from Europe. It’s less clear which species that was, however – there were two(!) species of hippo living in mainland Europe in the Pleistocene: the endemic European hippopotamus Hippopotamus antiquus, and later on also the extant common hippo Hippopotamus amphibius (my bet is on antiquus).

    In fact, off the top of my head I can’t think of any genuinely African-derived mammal taxa present on these Mediterranean islands we’re talking about here. Which species did you have in mind?

    basal members of the genus Hippopotamus and the entire genus Hexaprotodon”

    These insular pygmy hippos weren’t phylogenetically closely related to the extant West African pygmy hippos – they were all true Hippopotamus (although a few workers would prefer to place at least one of the Malagasy species in another genus), and not particularly basal members of this taxon either (see above). Regarding pygmy hippo nomenclature, it’s probably best to reserve Hexaprotodon for the fossil species and put the extant West African species back in Choeropsis as long as the monophyly of Hexaprotodon is in doubt.

    if they were too aquatic, you wouldn’t expect to see island dwarfism and other signs of genetic isolation

    Why not? Evolution is not irreversible. Fully terrestrial kangaroos may become arboreal and fully arboreal chameleons may become terrestrial. And strongly-flying birds such as anatids and pigeons may become completely flightless on oceanic islands. Why wouldn’t a ‘fully’ aquatic hippopotamus that made it to an isolated, competitor-free island start to exploit its terrestrial ecological niches instead?

    Heteromeles:
    The issue with dugongs is that they may have excluded hippos from the fully aquatic herbivore niche in the few perennial river systems where they could have conceivably evolved.

    Dugongs are actually almost exclusively marine; they rarely if ever swim up rivers (like manatees do). As long as the hippos stayed in their freshwater habitats I don’t see much ecological overlap between these two groups.

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  51. 51. John Scanlon FCD 9:17 am 08/29/2013

    “anyone know if there’s any plans in the works to recover aDNA from any mekosuchine material?”

    The Australian environment doesn’t preserve ancient DNA, as far as I know (Alan Cooper talked about this at a meeting in 2005, and I haven’t heard different since). New Cal and Fiji probably aren’t any better, but the material is much younger there so I suppose it’s worth a try with M. inexpectatus and Volia. Unfortunately there’s nothing post-Miocene in NZ (where DNA lasts quite well).

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  52. 52. Heteromeles 10:20 am 08/29/2013

    @Dartian: were ancestral dugongs marine? It’s rather interesting that sirenians are afrotherians, but there’s no obvious rivers system they could have evolved in, unlike the Amazon, which does support manatees year-round (although they starve during part of the year). If sirenians are basally marine, this suggests pretty strongly that African rivers weren’t and aren’t a great place for purely aquatic mammalian herbivores. This in turn might help explain why hippos are amphibious.

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  53. 53. Yodelling Cyclist 11:27 am 08/29/2013

    I think people maybe overlooking the West African Manatee. Not sure it has any baring on hippo evolution, but historically it was abundant in the river systems of west Africa.

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  54. 54. ectodysplasin 11:40 am 08/29/2013

    @John Scanlon

    The Australian environment doesn’t preserve ancient DNA, as far as I know (Alan Cooper talked about this at a meeting in 2005, and I haven’t heard different since). New Cal and Fiji probably aren’t any better, but the material is much younger there so I suppose it’s worth a try with M. inexpectatus and Volia. Unfortunately there’s nothing post-Miocene in NZ (where DNA lasts quite well).

    That’s a pity.

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  55. 55. Dartian 11:53 am 08/29/2013

    Heteromeles:
    If sirenians are basally marine, this suggests pretty strongly that African rivers weren’t and aren’t a great place for purely aquatic mammalian herbivores.

    You asked some good questions there. As for whether sirenians are originally marine/coastal, I don’t know the answer (and perhaps it’s really not even possible to tell), but my guess would be yes.

    The reason for this is that on a geological time scale freshwater habitats such as rivers and lakes are, especially when compared with the sea, ephemeral. The odd exception such as Lake Baikal notwithstanding, they typically last for just a couple of million years or so (or perhaps not even that) before they shrink/dry out, or (in the case of rivers) drastically change their course.

    What this means if you’re a large-bodied, fully aquatic mammal (i.e., one that cannot move over land at all) is that you’re very vulnerable to extinction when the environment changes drastically for the worse (e.g., during a period of local or global drought). A hippo (or, for that matter, a crocodylian) can leave its river or its lake and seek out another body of water, but a sirenian or a cetacean usually will not have that option. The sea, on the other hand, will never dry out. This is probably the reason why obligately aquatic tetrapods have evolved many times in the marine environment, but only rarely have they evolved in, or even colonised, freshwater habitats.

    Yodelling Cyclist:
    I think people maybe overlooking the West African Manatee.

    We aren’t (or at least I am not). It’s just not really relevant for this discussion because the manatee is a newcomer in Africa; it may have arrived there as recently as in the Pleistocene (Darren has an excellent old Tet Zoo post called “When manatees crossed the Atlantic” – look it up if you haven’t already). In other words, for geographical reasons, manatees haven’t really had the chance to influence hippo evolution at all.

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  56. 56. Dartian 11:59 am 08/29/2013

    All right, a preemptive self-correction:
    The sea, on the other hand, will never dry out.

    OK, so the (pre-)Mediterranean did dry out once. But that’s pretty much the only real exception.

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  57. 57. Heteromeles 1:22 pm 08/29/2013

    @Dartian: thanks. IIRC, the Mediterranean has dried out at least five times. Probably why mermaids didn’t evolve there :P

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  58. 58. Jerzy v. 3.0. 5:12 pm 08/29/2013

    on a geological time scale freshwater habitats such as rivers and lakes are, especially when compared with the sea, ephemeral.

    Lakes maybe, but rivers are long-lived. Nile is allegedly 35 my old, Amazon even older and may even be the half of an ancient Gondwanan river. Certainly freshwater dolphins and fish managed to evolve!

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  59. 59. Yodelling Cyclist 8:00 pm 08/29/2013

    Currently kicking myself, you are correct. I would point out that really, the planet earth isn’t a great place for purely aquatic herbivorous tetrapods. Certainly turtles and sirenians exist, but compared with the variety of predatory groups that have evolved (and the varietybthat has evolved within those groups) I would suggest that this list is quite short. Even including marginal cases – marine iguanas, thalassocnus (anything else?) doesn’t make this a vast list.

    What is the evolutionary history of seagrass, btw? I assume it evolved in the Cretaceous alongside gram insides on land, but I have really no idea.

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  60. 60. Christopher Taylor 8:39 pm 08/29/2013

    What is the evolutionary history of seagrass, btw? I assume it evolved in the Cretaceous alongside gram insides on land, but I have really no idea.

    The earliest known seagrasses are from the Cretaceous, but it seems that they may not have become widespread until much later (Miocene or so). The seagrass fossil record is apparently not great.

    I’m not sure if I understood your assumption correctly, but seagrasses are not closely related to true grasses. Seagrasses belong to a few different lineages of Alismatales, the same monocot lineage that includes aroids and pondweed.

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  61. 61. Dartian 12:36 am 08/30/2013

    Heteromeles:
    the Mediterranean has dried out at least five times

    Five?! Which were the other four times when it happened?

    Jerzy:
    rivers are long-lived

    You missed the part in my comment where I said that they may drastically change their course over long periods of time. There has surely been a large river (or, more correctly, a river system) in the northern part of South America for a very long time, but that doesn’t mean that it would have been identical to the current-day Amazon. In fact, the extraordinary biodiversity that’s so characteristic of the modern Amazon River bears testimony to its previous range contractions and changes. If the river’s course had stayed pretty much the same for tens of millions of years we should expect it to have less biodiversity, not more. All those countless species of tetras, catfish, cichlids, etc. are the result of stretches of river being cut off and become isolated for long periods of time – and then come back into contact again*.

    * Side note: What’s true of the Amazon River specifically is also true of freshwater habitats generally. For example, an astonishing 40% of all extant fish species live in freshwater – even though fresh water bodies comprise only about 0.01% of the Earth’s water supply (Lundberg et al., 2000)! The reason for this (as I said before) is that freshwater ecosystems are much less stable than marine ones on geological timescales. Which means more opportunities for speciation to occur.

    Going back to mammals: small fish may easily survive such range contractions, but for large aquatic mammals it’s much more difficult; a population of them that ends up trapped and isolated in a tiny lake is not likely to survive for very long. The evolutionary odds are heavily against them, so to speak.

    Still, a few lineages such as the extant river dolphins have been able to beat those odds (and mind you, I didn’t say that evolution of large obligately aquatic freshwater tetrapods is impossible, I just said that it’s rare). As for those river dolphins, however, it’s good to keep in mind the possibility that the extant taxa aren’t necessarily all that ancient – and they may not have become adapted to living in freshwater until relatively recently. Indeed, one species of ‘river’ dolphin, the La Plata dolphin Pontoporia blainvillei is actually more at home in saltwater.

    Reference:
    Lundberg, J.G., Kottelat, M., Smith, G.R., Stiassny, M.L.J. & Gill, A.C. 2000. So many fishes, so little time: an overview of recent ichthyological discovery in continental waters. Annals of the Missouri Botanical Garden 87, 26-62.

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  62. 62. CS Shelton 5:22 am 08/30/2013

    gentle lemur @44- I LedOL. :-)

    jerzy@48- I know! It’s crap. I want my feathery Jurassic Park update now. Also, as ancient organic material from the cores of large dinosaur bones have proven to disintegrate even in perfect isolation from the environment, the amber idea is nuked. Back to time travel hokum for the dinosaur grab. Or far-future designer genetics, in which case, why dino shapes? Why not even fancier fake animals? Nevermind…

    Blonde@9- I tried clumsily to offer an idea on that question once, and Marjanovic totally not-parsimonious’d my ass. The most zealously correct expert around is most likely right, but here was my uneducated guess anyway:

    I took the echolocation of widely separated mammals as my guide. Apparently bats and whales have modification in the same region of their DNA, arrived at independently. I took this to mean that some animals, by the structure of their genes, may be more able to come across certain types of adaptations. So maybe the branches of, say, Lepidosauria that have repeatedly reduced or lost limbs may have a structure to their HOX genes (or ?) that makes that type of adaptation easier for them to arrive at than it is for mammals. I think if echolocators outside of Mammalia proved to have their adaptation in the same region of DNA, that would sink the hypothesis by suggesting it’s just the only way to echolocate.

    Aside from one sketchy extinct mole*, no mammal has ever gone totally limbless. It just doesn’t make sense to me.

    *I call it sketchy because a lot of fossils are missing parts and I only ever saw it mentioned in a Dougal Dixon book, so… ?

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  63. 63. Heteromeles 11:48 am 08/30/2013

    @Dartian: The Messinian salinity crisis was basically a stuttering event from 5.96 to 5.33 million years (I’m taking this from Wikipedia, because I’m too lazy to go find the old book). What I read years ago was that the Mediterranean had dried partially or completely perhaps five separate times, due to things like climate change, freshwater inflow, etc. According to the, ahem, omniscient Wikipedia, the Messinian crisis was probably a single long, complex event. Since this is based on numeric models, I’m not sure where the truth lies, but there are multiple layers of evaporites separated by sediments, so it wasn’t a simple event.

    Aside from the evidence of stromatolites in the sediments, which is kind of cool, it’s worth noting that the hippos, elephants, et al. probably walked to the present-day islands as the Mediterranean dried down, got stranded on them as mountain refugia (aka “sky islands”) as the Mediterranean sink heated up and became inimical (think Dead Sea in an oven on steroids), then were truly isolated when the sea came back. There’s no reason to think they swam that distance.

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  64. 64. David Marjanović 1:31 pm 08/30/2013

    You asked some good questions there. As for whether sirenians are originally marine/coastal, I don’t know the answer (and perhaps it’s really not even possible to tell), but my guess would be yes.

    I’ll just mention Pezosiren.

    As for those river dolphins, however, it’s good to keep in mind the possibility that the extant taxa aren’t necessarily all that ancient – and they may not have become adapted to living in freshwater until relatively recently.

    Miocene marine relatives are known for at least some of them.

    Aside from one sketchy extinct mole*, no mammal has ever gone totally limbless. It just doesn’t make sense to me.

    The reason seems to be that, as alluded to in comment 11, an endotherm can’t become too long and thin. Weasels already have a metabolism twice as high as expected for a mammal their size, just to keep warm*. Limbs are still not known for the lipotyphlan you mention (I forgot its name, I think it starts with Hetero-), but too little is known of the skeleton to confirm their absence; I predict they were there.

    * Source forgotten. :-(

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  65. 65. Dartian 1:40 pm 08/30/2013

    Heteromeles:
    the hippos, elephants, et al. probably walked to the present-day islands as the Mediterranean dried down, got stranded on them as mountain refugia

    No, because the timing doesn’t match. The Messinian Salinity Crisis happened in the Miocene, ending some 5 MYA. The Palaeoloxodon elephants from which the Mediterranean pygmy elephants evolved hadn’t yet evolved by then. In other words, the elephants, at least, must have got to those islands much later (i.e., in the Pleistocene). I’d guess that the hippos and the deer did so too.

    (The cave goat Myotragus on the Balearic Islands is a different matter; its ancestor almost certainly did get marooned on the mountain tops when the Mediterranean Basin was re-flooded.)

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  66. 66. Jerzy v. 3.0. 2:12 pm 08/30/2013

    As for those river dolphins, however, it’s good to keep in mind the possibility that the extant taxa aren’t necessarily all that ancient – and they may not have become adapted to living in freshwater until relatively recently.

    Well, they are separate families – so pretty long.

    Apparently bats and whales have modification in the same region of their DNA

    Would be interesting to see genome of shrews and seals which also partially use echolocation. BTW – genome of a fruit bat, which doesn’t use echolocation, was sequenced – it would be interesting to see if it has these echolocation-related genes.

    Lepidosauria that have repeatedly reduced or lost limbs

    I think losing limbs is more related to “wriggling” mode of movement of lizards and snakes. Much of propulsion is provided by abdomen and tail swinging back and forth and pushing against the ground, the legs are more for stabilizing the body from moving too much. I think it may be Darren himself who explained it once.

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  67. 67. Heteromeles 4:07 pm 08/30/2013

    Note that some humans have learned to effectively echolocate, and AFAIK, they aren’t mutants.

    As for limbless mammals, there are a bunch of problems. It isn’t just SA/V, it’s how they lose that heat. Soil’s cold, and it conducts heat better than air does. This isn’t a huge problem for an ectotherm, because it’s already adapted for operating its body at the soil temperature. It is a huge problem for an animal that’s hotter than the surrounding soil, because it’s going to be constantly losing heat. Good fur will help this to some extent, but then you run the risk of overheating if the fur is too good, or dying if the fur gets dirty. This may be why naked mole rats are naked and (AFAIK) quasi-ectothermic.

    Then there’s the anatomy thing. Mammals, in general, oscillate their torsos very nicely in the up and down direction. It’s the basis for everything from the mammalian gallop to the dolphin’s stroke to twerking. There’s far fewer mammals that move by oscillating side to side. Seals are the only thing I can think of offhand, and even there, it’s only the lumbar spine that’s moving side to side. There may be some variation of Carrier’s Constraint that would come in, too, and make it hard for a mammal oscillating side to side to breathe. It’s worth noting that, on land, seals don’t undulate like snakes, they hump like caterpillars, up and down just as one would expect, even though it’s inefficient and not what they do in the water when they’re holding their breaths.

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  68. 68. Christopher Taylor 6:32 pm 08/30/2013

    Well, [the river dolphins] are separate families – so pretty long.

    Yes, they’re well-separated phylogenetically, but what was meant by ‘recent’ was that they might not have become restricted to fresh water until recently. It seems likely that the modern river dolphins represent relicts of families whose marine members have gone extinct.

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  69. 69. Dartian 1:29 am 08/31/2013

    what was meant by ‘recent’ was that they might not have become restricted to fresh water until recently. It seems likely that the modern river dolphins represent relicts of families whose marine members have gone extinct.

    Exactly. Thanks, Christopher, for explaining my point more clearly than I did.

    Incidentally, this post is about caecilians but now, almost 70 comments later, we’re discussing river dolphins. Yet another example of epic thread drift. Gotta love Tet Zoo! :)

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  70. 70. Hai~Ren 9:07 am 08/31/2013

    The supposedly limbless insectivore (eulipotyphlan?) is Proterix.

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  71. 71. BrianL 9:42 am 08/31/2013

    Thanks Hai-Ren! As soon as the beastie was mentioned I could neither remember nor find its name. Turns out it really wasn’t that difficult a name to remember…

    Anyhow, I suppose Heteromeles gave a pretty good explanation for why it would be disadvantageous for a mammal (or other endotherm) to go limbless on land. This does beg the question if limblessness in water would be biologically impossible. I know living marine mammals all use their forelimbs but would it be possible for another, hypothetical mammal to actually be limbless and marine?

    Also, whenever limbs are lost in endotherms they are generally the hindlimbs. The only examples I can think of an endotherm truly losing its forelimbs are moa. Are there any other examples, not counting examples where arms may be/have been functionally non-existent but still present such as in abelisaurs, elephantbirds, kiwi, hesperornithids and casuariids?

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  72. 72. naishd 1:32 pm 08/31/2013

    Just back from Edinburgh SVPCA meeting and about to start catching up with everything. Just wanna say…. 71 comments?? What the hell? (smiley).

    Darren

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  73. 73. Heteromeles 6:49 pm 08/31/2013

    Too bad we got side-tracked from discussing how caecilians were going to conquer Syria and then…the world!

    Personally, I think turning a tear duct into a sensory tentacle is one of those incredibly weird things that has not been adequately exploited by Hollywood. Imagine a were-caecilian, for example.

    As for limbless marine endotherms, I suspect the issue is stability. Theoretically, they could evolve secondary fins off of their vertebrae (or hammerheads, for that matter), but modifying limbs into flippers appears to be ontogenetically simpler.

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  74. 74. Christopher Taylor 7:08 pm 08/31/2013

    The only examples I can think of an endotherm truly losing its forelimbs are moa. Are there any other examples, not counting examples where arms may be/have been functionally non-existent but still present such as in abelisaurs, elephantbirds, kiwi, hesperornithids and casuariids?

    Some Australian skinks of the genus Lerista, the sliders, lack forelimbs.

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  75. 75. Heteromeles 7:27 pm 08/31/2013

    Technically, don’t pythons and other primitive snakes fall into the “lacking all trace of forelimbs” category of tetrapod?

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  76. 76. BrianL 4:38 am 09/1/2013

    @Christopher Taylor and Heteromeles:
    Note that I meant endotherms without front limbs. Hence, skinks and snakes wouldn’t count.

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  77. 77. David Marjanović 8:41 am 09/1/2013

    The supposedly limbless insectivore (eulipotyphlan?) is Proterix.

    *facepalm* Yes!

    I know living marine mammals all use their forelimbs but would it be possible for another, hypothetical mammal to actually be limbless and marine?

    This is where the surface/volume ratio strikes back, I guess: an active swimmer that doesn’t need stabilizing fins probably has to be snake-shaped. Basilosaurus, a huge tropical animal with (therefore) a low surface/volume ratio, is probably the closest we’ll get.

    Also, whenever limbs are lost in endotherms they are generally the hindlimbs.

    Well, all of the examples are mammals, so I can’t tell if that’s about endothermy.

    The only examples I can think of an endotherm truly losing its forelimbs are moa. Are there any other examples

    Nope.

    Personally, I think turning a tear duct into a sensory tentacle is one of those incredibly weird things

    It is – but it may have happened twice: over part of its length, the tear duct was laterally open (as far as the skull is concerned anyway) in at least one burrowing “microsaur”, presented by Jason Anderson at the SVP meeting 2 years ago (IIRC).

    Link to this
  78. 78. Heteromeles 1:49 pm 09/1/2013

    Worth pointing out that a paper on Proterix is available as a pdf at http://digitallibrary.amnh.org/dspace/handle/2246/2525

    Also worth pointing out that they compare it to Scutisorex, and don’t spend a lot of time talking about any putative limb reductions.

    Link to this
  79. 79. John Scanlon FCD 2:36 pm 09/1/2013

    As for limbless marine endotherms, I suspect the issue is stability. Theoretically, they could evolve secondary fins off of their vertebrae (or hammerheads, for that matter), but modifying limbs into flippers appears to be ontogenetically simpler.

    I used to assume that the flukes of whales and sirenians were derived from the fleshy bits of the hindlimbs, dissociated from what’s left of the pelvic skeleton and shifted posteriorly. This seemed parsimonious (even knowing that some whales occur with atavistic external hindlimbs), but then we saw those photos of the dolphin with small but definitely flipper-like hindlimbs and apparently normal flukes.

    Link to this
  80. 80. ectodysplasin 4:46 pm 09/1/2013

    @David,

    It is – but it may have happened twice: over part of its length, the tear duct was laterally open (as far as the skull is concerned anyway) in at least one burrowing “microsaur”, presented by Jason Anderson at the SVP meeting 2 years ago (IIRC).

    2 years ago, Anderson presented new data on Cardiocephalus ontogeny. Nothing on the nasolacrymal duct as far as I remember. There’s a somewhat complex opening of the nlc into the orbit, but nothing deeply embayed in the same way that you see in caecilians. There also emphatically isn’t the secondary olfactory nerve nucleus and branch that you see in caecilians.

    Link to this
  81. 81. ectodysplasin 4:48 pm 09/1/2013

    @BrianL:

    Note that I meant endotherms without front limbs. Hence, skinks and snakes wouldn’t count.

    Some skinks are endotherms.

    Link to this
  82. 82. Heteromeles 6:36 pm 09/1/2013

    @79: Not quite what I mean, but dorsal fins and flukes are associated with the spine, if not spinous processes.

    No, the ontogenetic problem is a) non-serpentine bodies need some sort of stabilization; b) mammals don’t seem to do serpentine very well (probably something about that pesky diaphragm and lumbar region), and c) there’s got to be simultaneous limb loss and fin/fluke/hammerhead development in parallel so that the end result is a limbless mammal with stabilizing structures that aren’t limbs. That’s kind of, erm, hard, although one can imagine it happening.

    Link to this
  83. 83. Christopher Taylor 8:45 pm 09/1/2013

    Some skinks are endotherms.

    Still, reading comprehension failure on my part.

    Link to this
  84. 84. StockyOne 10:13 pm 09/1/2013

    Oh Caecilian, your breaking my heart, your shaking’ my confidence baby… (sorry, Simon and Garfunkel)

    Link to this
  85. 85. Heteromeles 10:21 pm 09/1/2013

    @84: forgot to reference 3, 8, and 24 above.

    Link to this
  86. 86. StockyOne 10:31 pm 09/1/2013

    @85: Thanks. I didn’t read through the comments before posting.

    Link to this
  87. 87. David Marjanović 2:16 am 09/2/2013

    I used to assume that the flukes of whales and sirenians were derived from the fleshy bits of the hindlimbs, dissociated from what’s left of the pelvic skeleton and shifted posteriorly. This seemed parsimonious

    …From a development biology point of view, it’s extremely hard to imagine.

    2 years ago, Anderson presented new data on Cardiocephalus ontogeny. Nothing on the nasolacrymal duct as far as I remember.

    It definitely wasn’t Cardiocephalus.

    And “lacrima” is Latin. There’s no y in it. If you want the Greek word instead, that’s “dákryma” with delta.

    Link to this
  88. 88. Heteromeles 5:54 pm 09/2/2013

    @86: Not a problem. I’m still grumbly that the extremely lame effort I posted in #24 hasn’t been bettered. Come on, people. Surely it can’t be that hard to write songs about caecilians. Can it?

    Link to this
  89. 89. Jerzy v. 3.0. 5:45 pm 09/4/2013

    Sure:
    Let’s drink a round for a caecilian,
    which ate a whole desmostylian;
    let’s drink a round for Scolecomorphus,
    which ate a whole fish called Gomphus,
    …and so on.

    Link to this

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