MIRROR IMAGE COMPARISON O F UPPER AND LOWER JAWS IN PRIMITIVE TETRAPODS

ALFRED SHERWOOD ROMER H a w a r d Universitv. Cambridge. Massachusetts

TWO FIGURES

I n the higher vertebrate classes upper and lower jaws are not particularly comparable in structure. Work on lower tetrapods, however, leads to a realization that in primitive amphibians and even in many early reptiles upper and lower jaws are essentially mirror images of each other. The outer surface of the lower jaw is comparable, in the number, na- ture and position of the bony elements present, with the cheek region of the skull; the inner surface of the jaw is closely comparable in its pattern to the primitive tetrapod palate.

This comparison is apparent in the inspection of the early amphibians shown in figures 1 and 2 (many other genera could well have been substituted for those illustrated). I n making this comparison the rostral ” elements of the skull- premaxillae and nasals-are disregarded, since it is generally agreed that the primitive condition was one in which the jaws did not extend forward beyond the nares; the rostral region is still in a state of flux in crossopterygians and even in many early amphibians, clearly indicating that forward jaw extension was a late innovation.

On the outer surface of both upper and lower jaws (fig. 1) the main tooth-bearing element A (maxilla or dentary) is a prominent and conspicuous member. In the form figured the maxilla is somewhat smaller than the dentary; in some cases, however, it is larger and extends back, as does the dentary to gain a contact with element E. In both upper and lower jaws we find, adjacent to the maxilla o r dentary, and

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176 ALFRED S H E R W O O D ROMER

continuing back to the articular region, a series of four other dermal elements, B to E, including in the upper jaw the lacrimal, jugal, squamosal and quadrato-jugal, and in the lower jaw splenial, post-splenial, angular and surangular. Elemeiits B and C are adjacent to the main tooth-bearing element ; D is always large, forms much of the rounded poster- ior angle of the jaw and is in contact along this margin with element J in both upper and lower jaws. E tends to be rela- tively small, forms the posterior portion of the dental margin

Fig. 1 Above, cheek region of the carboniferous labyrinthodont amphibian, Orthosaurus ; skull outline completed in dashed lines. Below, external vietv of lower jaw of the same. Both a f te r Watson (1926, figs. 7, 10). I n upper jaw: A, maxilla; B, lacrimal; C, jugal; D, squan~osal; E, quadrato-jugal; F, quadrate. I n lower jaw: A, dentary; B, splcnial; C, postsplenial; D, angular; E, surangular; F, articular.

of the jaw, and is tightly braced against the articular element I?.

hfarked differences are to be found only as regards the “peripheral” margins of elements B and C. This is to be expected because of their relationships to other structures. Below, B and C merely form part of the lower border of the jaw; in the upper jaw the corresponding elements are modified

COMPARISON O F J A W S I N TETRAPODS 177

in coririection with articulations with the bones of the skull roof and the presence of the orbit.

This comparable arrangement of external elements might be attributed to pure chance were not the similarities re- inforced by a more fundamental mirror image comparison of the inner surfaces of the jaws (fig. 2) . In each case the margin consists of a narrow tooth-bearing band formed by element A, behind which element E (and sometimes C in the upper

Fig. 2 Above, loft side of palate of the carboniferous labyrinthodont Baphetes, the premaxilla in dashed outline and the braincase omitted. Below, inner side of the le f t mandible of the related genus Orthosaurus; teeth omitted. Both after Watson (1926, figs. 11, 4A). In palate: A, E, F’as in figure 1; G, prevomer; H, palatine; I, ectopterygoid; J, pterygoid; K, subtemporal fossa. I n lower jaw, E, A, F as in figure 1; G, H, I, three eoronoids; J, preartieular; K, meckeliaii fossa.

jaw) continue back to the cartilage bone F forming the articular surface-quadrate in the upper jaw, true articular element in the lower. In either case a large fossa (K) is present posteriorly, the subtemporal fossa in the upper jaw, meckelian fossa in the lower. In the lower jaw a dental fora- men, carrying blood vessels and nerve, enters posteriorly, typically piercing element J and (in some cases at least) enter-

178 ALFRED SHERWOOD ROMER

ing the fossa between E and F. In the upper jaw there is frequently found a quadrate foramen entering the fossa pos- teriorly between elements E and F. The functions of this foramen are unknown but must surely have been similar, in part at least, to those of the lower jaw foramen.

Internal to element A is a longitudinal series of three bones, G, H, and I, comvrising the coronoid series in the lower jaw and in the upper termed prevomer (or vomer?), palatine and ectopterygoid. These three are tooth-bearing in all primitive forms. In both upper and lower jaws they carry a shagreen of tiny “denticles.” In the upper jaw each bone also typically bears in primitive amphibians, a large fang and a pit for the replacing tooth. In the lower jaw these are absent in amplii- bians; but in the crossopterygians, from which (in a broad sense) tetrapods surely arose, similar fangs and adjacent pits are present on the lower jaw series also (cf. Watson, ’26, fig. 38). Element G differs somewhat. in the two jaws, for in tlie upper jaw it extends forward into the “sub-rostral” region and meets its fellow of tlie opposite side.

The innermost element, J , is, in both cases, very large and long; this is the pterygoid in the upper jaw, the pre- articular in the lower. The pterygoid commonly bears teeth ; the prearticular may do so, but less frequently. Both are intimately associated posteriorly with the respective articu- lar elements, and appear to have arisen in connection with the primary palato-quadrate bar ; both form a deep medial bouiidary for fossa K and are connected with element D ; both extend well forward inside of the series G, H, and I. At the margins farthest from the tooth row, however, the upper and lower elements differ in their middle and anterior por- tions, for the prearticular of the lower jaw is simply connected with the external elements a t the lower margin of the jaw, while the pterygoid enters into the articulation of upper jaw with braincase.

Upper and lower jaws are thus seen to be similar to olie another both externally and internally in t l l e arrangement

COMPARISON O F JAWS I N TETRAPODS 179

and character of the bony elements present. Demonstration of this mirror image relationship proves didactically useful. We have, however, no adequate explanation f o r the presence of these structural similarities. Certain points may be in- terpreted on functional grounds ; it is, for example, reasonable to expect in both jaws a single large elcmcnt forming a base for the margnial tooth row. But no such explanation will account for most of the other common features; proof that they are not of functional importance is given by the fact that later tetrapods tend to modify them greatly. Similarity may be due to inheritance from some simple and primitive an- cestral stage, but our knowledge of the piscine ancestors of the tetrapods is very inadequate. Perhaps the presence of similar adult features may be the result of similar conditions and forces operating ontogenetically in both upper and lower jaws ; but unfortunately embryos of extiiict paleozoic amphi- bians are not available,

LITERATURE CITED

WATSON, D. M. S. 1926 Avoniaii Lecture-The Evolution and Origin of the Amphibia. Philos. Trans. Roy. SOC., London, Ser. B., vol. 214, pp. 189-257.