Welcome to another article on the Matamata Chelus fimbriatus. Yay!
In the previous episode we looked briefly at the Matamata’s long, thick neck and on a few aspects of Matamata evolution (a brief introduction to what the Matamata is, and where it lives, can be found here) [in the composite image shown above, the skull and neck is from digimorph while the photo of the live animal is from wikipedia]. This time, we’re going to look at the anatomy of the skull and hyoid skeleton (do stick with me here, it’s well worth it). I made the point previously that, despite being comparatively well-known as turtles (and animals) go, I just wasn’t able to find a good, illustrated description of the neck because such a thing doesn’t exist (though Hoffstetter & Gasc (1969) did at least figure the neck vertebrae). In contrast, the skull has been well described… and it’s surreal.
As you see from these diagrams [from Gaffney (1977)], it’s really broad across the palate, quadrates and otic capsules (in fact, the skull is about as broad as it is long), and very flattened overall, particularly in the facial region. Gaffney (1977) described the skull as looking like it had been “run over by a truck” (this in an otherwise technical description of cranial anatomy) [skull in left lateral view shown below (from Gaffney (1977)) The eye socket is the opening right down near the snout-tip at left]. Some authors have described the skull as ‘arrow-shaped’ (Lemell et al. 2010): the snout is pointed, and the skull as a whole is flattened and light due both to reduction or loss of some elements and to relatively small muscles. The head can thus be thrown forward at great speed, all the while producing only a small pressure wave. Incidentally, the strong convergent similarity that the Matamata skull has with that of another aquatic-feeding tetrapod – the Surinam toad Pipa pipa – has not been lost on some (Lauder & Shaffer 1993, Sanderson & Wassersug 1993, Claude et al. 2004).
Nasal bones are absent*, the maxillae are strongly reduced, jugal bars are absent (as is the case across Chelidae), the jaws are weak, and horny beak tissue is absent (meaning that this is a turtle devoid of a beak). Because the maxillae are really small, the ridge normally present along the edge of the upper jaw has (uniquely) moved to the palatine bones. The tongue is really small (we’ll see why in the next article). The dentaries are thin and weak compared to those of other turtles.
* In other chelids, the nasals are subrectangular bones located just posterior to the single bony nostril opening.
The hyoid bones of Chelus are enormous, heavily ossified and very distinctive [see image below, from Lemell et al. (2010)]: the apparatus as a whole is about two-thirds the length of the skull and, at its broadest point, about one-third of the skull’s width. Much of the space between the two halves of the lower jaw is filled up by the hyoid apparatus: there is a massive, vaguely cruciform hyoid body located along the midline (it has a prominent trough – the tracheal sulcus – running along most of its length), and two pairs of giant, finger-like processes (the brachial horns and their associated epibranchials) curve outwards and backwards from the anterior and posterior parts of the hyoid body. The tips of the brachial horns stretch wide apart when the jaws are opened wide, resulting in massive distension of the oesophagus. An also enormous hypoglossum – a streamlined plate of bone that looks something like an alien spacecraft – is located ventral to the hyoid body in between the rami of the lower jaws (Lemell et al. 2010).
The soft tissues associated with the jaws are also really interesting [see figures below; again from Lemell et al. (2010)]. The adductors (the muscles responsible for closing the jaws) are weak, but they’re very thin and cover quite a large area on the dorsolateral surface of the skull (the external adductor extends from the posterior orbital margin all the way up to the posterior margin of the skull). The front part of the external adductor is covered by a special folded flap of skin that opens out when the jaws are opened, and then functions as a sort of cheek that helps to make the open mouth into a tunnel. This structure has been called the ‘Mundplatte': it’s lubricated by a special layer of mucus (Lemell et al. 2010).
Further back on the head, the geniohyoid muscle (originating on the lateral and medial surfaces of the lower jaw, as well as from part of the hyoid apparatus) is enormous: combined with the brachiomandibular and coracohyoid muscles, it allows rapid, simultaneous downward and backward rotation of the hyoid skeleton and associated expansion of the throat.
So – that’s the anatomy…. but what does the Matamata do with all of these weird cranial and hyoid features? That’s what we’ll look at next. If you can’t wait until then, Lord Geekington covered Matamata anatomy and feeding behaviour back here.
For previous Matamata articles see…
- Matamata: turtle-y awesome to the extreme
- The familiar Matamata, known to us all since the 1700s, and its long, fat neck (matamatas part II)
And for previous Tet Zoo articles on turtles see…
- Giraffe-necked giant tortoises
- Giant African softshells – wow
- Gilbert White’s pet tortoise, and what is ‘grey literature’ anyway?
- The goat-eating hot water bottle turtles
- Hard-shelled sea turtles and a diet of glass
- Terrifying sex organs of male turtles
Refs – –
Claude, J., Pritchard, P., Tong, H., Paradis, E. & Auffray, J.-C. 2004. Ecological correlates and evolutionary divergence in the skull of turtles: a geometric morphometric assessment. Systematic Biology 53, 933-948.
Gaffney, E. S. 1977. The side-necked turtle family Chelidae: a theory of relationships using shared derived characters. American Museum Novitates 2620, 1-28.
Hoffstetter, R. & Gasc, J.-P. 1969. Vertebrae and ribs of modern reptiles. In Gans, C., Bellairs, A. d’A. & Parsons, T. (eds.) Biology of the Reptilia Volume 1. Academic Press (New York), pp. 201-310.
Lauder, G. V. & Shaffer, H. B. 1993. Design of feeding systems in aquatic vertebrates: Major patterns and their evolutionary interpretations. In Hanken, J. & Hall, B. K. (eds) The Skull. University of Chicago Press (Chicago), pp. 113-149.
Lemell, P., Beisser, C. J., Gumpenberger, M., Snelderwaard, P., Gemel, R., & Weisgram, J. (2010). The feeding apparatus of Chelus fimbriatus (Pleurodira; Chelidae) – adaptation perfected? Amphibia-Reptilia, 31, 97-107.
Sanderson, S. L. & Wassersug, R. 1993. Convergent and alternative designs for vertebrate suspension feeding. n Hanken, J. & Hall, B. K. (eds) The Skull. University of Chicago Press (Chicago), pp. 37-112.