Ontogeny of the palatoquadrate and adjacent lateral cranial wall of the endocranium in prehatching Alligator mississippiensis (Archosauria: Crocodylia)

The purpose of this article is to gain insight into the ossification sequence of the palatoquadrate and the adjacent lateral cranial wall of prehatching Alligator mississippiensis, a process about which there is almost no published information. Results were obtained by studying serial histological sections of the series of ontogenetic stages and enlarged wax‐plate models of several stages. The cartilage of the palatoquadrate starts to ossify endochondrally in the quadrate portion of the pars pterygoquadrata palatoquadrati in Stage 6A. In this stage, a bone, called the lamina palatoquadrati anterior here, appears at and close to the anteromedial wall of the cartilaginous pterygoid portion of the pars pterygoquadrata. The lamina palatoquadrati anterior ossifies in membrane. Later in ontogeny, the lamina palatoquadrati anterior spreads into the cavum epiptericum and sheathes the posterior portion of the trigeminal ganglion laterally. The jaw adductor muscles insert at the outer surface of the lamina palatoquadrati anterior. The lamina palatoquadrati anterior is a new structure not previously recorded in crocodylians or any other Recent reptile. The topology, mode of ossification, and functional anatomy of the lamina palatoquadrati anterior correspond to those of the membranous ossification of the alisphenoid of marsupials. Another bone, called the lamina prootici anterior here, spreads in membrane from the anterolateral wall of the prootic portion of the otic capsule into the prootic fenestra, above the trigeminal ganglion. The lamina prootici anterior represents a structure not recorded previously in crocodylians. It contributes to the orbitotemporal braincase wall. J. Morphol. 262:644–658, 2004. © 2004 Wiley‐Liss, Inc.

The first description of the ontogeny of the skull of Alligator mississippiensis was undertaken by Parker (1883). Subsequently, several authors described various portions of the embryonic skull of A. mississippiensis and other crocodylians (summarized in Bellairs and Kamal, 1981, and references therein;Klembara, 1991Klembara, , 1993Klembara, , 2001). An ontogenetic series of cleared and stained skulls of A. mississippiensis was described recently by Rieppel (1993a). In describing the ossification of the palatoquadrate and the lateral braincase wall, Rieppel (1993a, p. 309) stated that: "Ossification of the quad-rate starts in the middle portion of the shaft, and was first observed after 38d. As it progresses, it renders investigation of the ossification in the lateral braincase wall increasingly difficult and eventually impossible." Apart from this passing remark, no other data are available to elucidate comprehensively the ossification of the palatoquadrate and the adjacent lateral cranial wall either in A. mississippiensis or in other crocodylians.
Crocodylians are generally considered to lack an epipterygoid in adults (DeBeer, 1937;Mü ller, 1967;Iordansky, 1973;Bellairs and Kamal, 1981;Rieppel, 1988Rieppel, , 1993b. As a consequence, the conditions in lizards and rhynchocephalians have mostly been considered in attempts to homologize the reptilian epipterygoid with the mammalian alisphenoid (Goodrich, 1930;Barry, 1965;Romer, 1976;Presley and Steel, 1976;Presley, 1981;Bellairs and Kamal, 1981;Gardiner, 1982;Rieppel, 1988;Maier, 1987Maier, , 1989Maier, , 1993. The aims of this article are: 1) description of the prehatching ontogeny of the palatoquadrate and the structures of adjacent lateral cranial wall in Alligator mississippiensis; 2) comparison of the cranial structures of A. mississippiensis studied here with those in lizards, rhynchocephalians, and mammals; 3) functional interpretation of new ontogenetic components of the palatoquadrate and orbitotemporal braincase wall of A. mississippiensis; and 4) interpretation of the ontogenetic components of the quadrate of A. mississippiensis.
The terms laterosphenoid and pleurosphenoid are used in the sense of Bellairs and Kamal (1981) throughout.

MATERIALS AND METHODS
The following transversely sectioned ontogenetic stages of Alligator mississippiensis, arranged according to head lengths, were used (FS stages after Ferguson [1985] The specimens are from Dade County, Florida, Everglades National Park (collector: Dr. J.A. Kushlan). Information from Stages 02, 01A, 1A, 4A, 6B, 7A, 8A, and 9B was used to build enlarged wax-plate models. Sections were examined and photographed using a WILD M8 stereomicroscope.

RESULTS
In this section, the structure and the ossification of the palatoquadrate during prehatching development of Alligator mississippiensis is described. The endochondral ossification of the palatoquadrate begins at Stage 6A approximately in the middle portion of the pars pterygoquadrata palatoquadrati. As well as this ossification, a bone is present at the anteromedial wall of the palatoquadrate and in the mesenchyme immediately adjacent to it. This bone, participating in the formation of the adult quadrate, is referred to here as the lamina palatoquadrati anterior. From the anteromedial wall of the palatoquadrate, the lamina palatoquadrati anterior spreads in membrane into the cavum epiptericum. During the early growth of the lamina palatoquadrati anterior, the cartilaginous palatoquadrate is still not resorbed in the region from which the lamina palatoquadrati anterior extends.
In the following, the growth of the lamina palatoquadrati anterior and the endochondral ossification of the cartilaginous palatoquadrate are described in anteroposterior sequence in five prehatching stages. The relationships of these two components to each other and to the neighboring lateral braincase wall and soft structures are also described.

Stage 02
The palatoquadrate has a massive pars pterygoquadrata with a posteriorly extending otic process (Fig. 1). The ventral end of the pars pterygoquadrata is fused with the posteriormost portion of Meckel's cartilage. From the central portion of the anteromedial wall of the pars pterygoquadrata, the pterygoid process extends anteromedially. The anterior portion of the pterygoid process is a mediolaterally narrow and anteroposteriorly elongate plate. Its continuation into the pars pterygoquadrata is dorsoventrally narrow.

Stage 01A
The palatoquadrate is a triangular plate composed of the pars pterygoquadrata and two distinct processes: processus pterygoideus palatoquadrati and processus oticus palatoquadrati ( Fig. 2A). The pars pterygoquadrata is a massive and dorsoventrally oriented plate. Its ventral articular termination is massive. The pterygoid process is long and plate-like. Its narrow continuation into the pars pterygoquadrata can still be observed. The otic process is long.
A long rod of cartilage is interposed between the pila antotica and the anterior part of the canalicular portion of the otic capsule ( Fig. 2A). The anterior end of the rod lies very close to the medial wall of the anteriormost end of the pars pterygoquadrata. Fur- Fig. 1. Alligator mississippiensis. Palatoquadrate and adjacent endocranium of Stage 01A in left anterolateral view. Otoccipital region of skull is strongly posteroventrally flexed. ot.ca, otic capsule; pi.aot, pila antotica; tr.com, trabecula communis. ther anteriorly, it continues in the form of the procartilaginous tissue (Fig. 2B). Its anteriormost end lies immediately above the anterior portion of the trigeminal ganglion. Thus, the maxillary and the mandibular rami of the trigeminal nerve run ventrally from the base of the rod, but laterally to it. The ophthalmic ramus of the trigeminal nerve lies medial to the procartilaginous base of the rod (Fig. 2B). This long rod, placed in the prootic fenestra, is called the columella prootica here, and probably corresponds to the ascending process of the palatoquadrate in other Recent reptiles (see below).

Stages 1A-4A
The pars pterygoquadrata is slightly larger anteroposteriorly, thinner mediolaterally, and more concave medially (Klembara, 1991). The pterygoid process is thin and long and its root protrudes slightly medially. The columella prootica is represented by a slender dorsoventrally prolonged rod of cartilage interposed between the dorsal portion of the pila antotica, the posterior section of the taenia marginalis, and the anterodorsal part of the canalicular portion of the otic capsule. The columella prootica is already substantially shorter than it is in Stage 01. In Stage 2A, the columella prootica is still in the form of an independent cartilage. In Stage 3A, it starts to fuse with the antotic pila and the taenia marginalis. In Stage 4A, the anterior portion of the pars pterygoquadrata palatoquadrati extends into the wedge-like anterodorsal process (Klembara, 1991). The columella prootica is partly fused with the adjacent portions of the pila antotica and the taenia marginalis, and extends above the dorsal wall of the canalicular portion of the otic capsule. In later stages the columella prootica fuses completely with the pila antotica and the taenia marginalis. A: Palatoquadrate and adjacent endocranium of Stage 02A in left lateral view. Otoccipital region of skull, palatoquadrate, and ascending process are posteroventrally flexed; hence, columella prootica lies almost horizontally. Line x-y indicates level of transverse section presented in (B). B: Transverse section through head of the same stage showing position of ophthalmic ramus (V 1 ) of trigeminal nerve relative to anterior (ventral) portion of columella prootica. Scale bar ϭ 1 mm. c.Mc, cartilago Meckeli; col.po, columella prootica; g.tr, trigeminal ganglion; n.V 1 , ramus ophthalmicus of trigeminal nerve; ot.ca, otic capsule; pi.aot, pila antotica; PQ, palatoquadrate; s.io, septum interorbitale; ta.mar, taenia marginalis.

Stage 5
A membrane (dense tissue layer) spreads between the inner surface of the palatoquadrate on one side and the upper portion of the antotic pila and the anterolateral wall of the otic capsule on the other side. The upper portion of the antotic pila is fused with the prootic portion of the otic capsule in this stage. This dense membrane corresponds with its position to the sphenoobturatory membrane of sauropsids (cf. Hopson and Rougier, 1993), and in this membrane two ossifications described below spread: one from the palatoquadrate and one from the prootic portion of the otic capsule. A strip of condensed mesenchyme lies in this layer, close to the inner wall of the anterior portion of the pars pterygoquadrata palatoquadrati (Fig. 3A). At the level of this condensed mesenchyme, the inner wall of the pars pterygoquadrata protrudes slightly medially and is lined with intact perichondrium. This condensed mesenchyme partly sheathes the lateral wall of the trigeminal ganglion, which lies mostly in the medial part of the cavum epiptericum; the root of the trigeminal ganglion lies in the prootic fenestra. The cartilage of the palatoquadrate is not resorbed.

Stage 6A
The palatoquadrate cartilage starts to ossify endochondrally approximately in the mid-length of the pars pterygoquadrata palatoquadrati. The lamina palatoquadrati anterior appears at the surface of the inner wall of the anterior portion of the pars pterygoquadrata and at the anteromedial surface of the base of the pterygoid process (l.pq.a, Figs. 3B, 4). The anterior portion of the pars pterygoquadrata is called the pars pterygoidea of the pars pterygoquadrata palatoquadrati here. This is an important landmark, since 1) the pterygoid process extends from it; 2) the basal portion of the columella prootica is placed closely to it; and 3) the lamina palatoquadrati anterior grows from it (Fig. 4). It is further distinguished from the posterior portion, the pars quadrata of the pars pterygoquadrata palatoquadrati articulating with the articular portion of Meckel's cartilage. From the pterygoid portion the lamina palatoquadrati anterior spreads in membrane medially into the cavum epiptericum. At its anterior end, the lamina palatoquadrati anterior extends slightly ventromedially and partly separates the jaw adductor muscles from the trigeminal ganglion. Further posteriorly, the lamina palatoquadrati anterior covers the trigeminal ganglion from the dorsolateral side. The cartilage of the pterygoid portion is not resorbed in the section where the lamina palatoquadrati anterior is formed. However, it is partially resorbed at its outer surface where it starts to ossify perichondrally producing an osseous ridge abutting against the quadratojugal. The lamina palatoquadrati anterior also spreads posteriorly and extends slightly behind the level of the trigeminal ganglion.
Here it terminates as a short extension reaching immediately ventrolaterally to the anteriormost level of the canalicular part of the otic capsule. The upper wall of the Eustachian tube system, interposed between the lateral cranial wall and the palatoquadrate, reaches below this posterior tip of the lamina palatoquadrati anterior.
Immediately posteriorly to the lamina palatoquadrati anterior, the cartilage of the middle portion of the pars pterygoquadrata is resorbed and a large fenestra is formed (cf. conditions in adult crocodylians in Iordansky, 1973). A strip of cartilage separates this fenestra from a deep otic incisure (Fig. 4). The quadrate portion of the pars pterygoquadrata is slightly hollowed as a consequence of the incipient pneumatization. Its articular portion is cartilaginous. The otic process is cartilaginous.

Stage 7A
The anteriormost section of the lamina palatoquadrati anterior spreads in membrane ventromedially and nears the dorsolateral wall of the trigeminal ganglion, at the level of the root of its mandibular ramus. Further posteriorly, the lamina palatoquadrati anterior broadens mediolaterally. Its dorsolateral portion extends dorsally, almost reaching the upper margin of the pterygoid portion of the pars pterygoquadrata, which is not resorbed here (Fig. 3C). Most of the anterior section of the pterygoid process of the pars pterygoquadrata is cartilaginous. The lamina palatoquadrati anterior borders the hollow for the anterior portion of the tympanic diverticulum (a part of the Eustachian tube system). The ventral half of the pterygoid portion is resorbed. In its posterior section, the lamina palatoquadrati anterior sheathes the adjacent posterior portion of the trigeminal ganglion, laterally enclosing the latter almost completely in the prootic fenestra ( Fig. 3D).
Slightly anteriorly to the hollow for the tympanic diverticulum, bony trabeculae appear at and close to the perichondrium of the ventrolateral wall of the anteriormost portion of the canalicular part of the otic capsule. Such trabeculae spread for a short distance into the space of the prootic fenestra above the trigeminal ganglion. Thus, a short osseous anteroventral extension from the prootic is formed. This bone ossifies in membrane, in the same mode as that of lamina palatoquadrati anterior. It is called the lamina prootici anterior (l.po.a, Fig. 3D) here. This bone has not been recorded in any crocodylian previously.
The lamina palatoquadrati anterior is clearly distinguishable from the partially resorbed cartilage of the pterygoid portion of the pars pterygoquadrata. Behind the lamina palatoquadrati anterior, the ventrolateral margin of the pars pterygoquadrata is already largely perichondrally ossified and a narrow bony lamella extends from the margin ventromedially. The otic process is cartilaginous.
tached to the posteriormost portion of the dorsolateral wall of the pterygoid. Further posteriorly, the lamina palatoquadrati anterior is prolonged dorsoventrally and forms the dorsolateral wall of the trigeminal ganglion (Figs. 5B, 6B). Here, the jaw adductor muscles insert to the ventrolateral and dorsolateral walls of the lamina palatoquadrati anterior.
Further posteriorly, the lamina palatoquadrati anterior becomes gradually more massive, filling completely the space of the cavum epiptericum between the cartilaginous pterygoid portion of the pars pterygoquadrata and the pila antotica (Fig.  5C). The lamina palatoquadrati anterior sheathes the posterior portion of the trigeminal ganglion laterally. The ganglion is almost completely enclosed in the prootic fenestra: it is positioned between 1) the base of the antotic pila dorsally, 2) the upper margin of the prefacial commissure ventrally, 3) the partially perichondrally ossified pterygoid process ventrolaterally, 4) the lamina palatoquadrati anterior laterally, and 5) the dura mater medially. The ventral half of the pterygoid portion of the pars pterygoquadrata is perichondrally and endochondrally ossified. The temporalis portion of the jaw adductor muscles is wedged between this pterygoid portion and the lamina palatoquadrati anterior (Fig. 5C).
Further posteriorly, the lamina palatoquadrati anterior borders the hollow for the anterior portion of the tympanic diverticulum. The hollow divides the pterygoid portion of the pars pterygoquadrata lying laterally from the dorsoventrally prolonged lamina palatoquadrati anterior lying medially (Fig. 5D). The lamina prootici anterior is a plate-like extension from the anteroventral wall of the prootic portion of the otic capsule spreading in the membrane (thickened tissue) and com-pletely filling the gap of the prootic fenestra dorsolaterally to the trigeminal ganglion. The anterior end of the lamina prootici anterior lies in the lateral portion of the prootic fenestra, in the level of the lateral surface of the antotic pila. Posteriorly, the lamina prootici anterior is continuous with the ventromedial margin of the anteriormost portion of the canalicular part of the otic capsule and the posterior margin of the antotic pila (Figs. 5D-F, 6A). The lamina prootici anterior lies within the membrane which is joined with the inner wall of the lamina palatoquadrati anterior. The membrane extends laterally to the trigeminal ganglion. The lamina palatoquadrati anterior abuts very tightly the ganglion and the lamina prootici anterior from the lateral side ( Fig.  5D-F).
The fenestra positioned immediately behind the posterior level of the lamina palatoquadrati anterior is elongated dorsoventrally (Fig. 6). The bony lamella extending ventrally from the pars pterygoquadrata forms a ventrolateral wall of the section of the Eustachian tube system lying between this lamella and the cochlear portion of the otic capsule (Fig. 6). Most of the palatoquadrate is already ossified, but the otic process, the pterygoid process, and the anterodorsal portion of the pterygoid portion of the pars pterygoquadrata are all cartilaginous.

Stage 9B
The anterior section of the lamina palatoquadrati anterior is a large plate closely attached to the lateral wall of the antotic pila which is, with the exception of its dorsal portion, already ossified (Fig. 7). The relationships of the lamina palatoquadrati anterior to all neighboring structures are basically the same as in the preceding stage. The pterygoid process remains cartilaginous; however, the pterygoid portion of the pars pterygoquadrata and the root portion of the pterygoid process are already substantially ossified. The quadrate portion of the pars pterygoquadrata is substantially endochondrally and perichondrally ossified and hollow (for part of the Eustachian tube system). The otic process extends dorsally to the middle ear cavity and lies anterolaterally and dorsally to the parotic crest; its posterodorsal portion is cartilaginous.

Columella Prootica and the Ascending Process of the Palatoquadrate
Parker (1883) described a short ascending process extending anteromedially from the root of the pterygoid process of the palatoquadrate in Alligator mississippiensis and Crocodylus palustris. Shiino (1914) found a similar small process in the same position in Crocodylus porosus (his C. biporcatus). According to Shiino (1914), the mandibular branch of the trigeminal nerve runs lateral to this ascending process. Klembara (1991) did not find such an ascending process in any ontogenetic stage of A. mississippiensis. However, in a previous study I reported a dorsoventrally prolonged rod of cartilage in Stage 1A of A. mississippiensis situated in the uppermost part of the prootic fenestra and suggested that it may represent an ascending process of the palatoquadrate, although it is not connected with the palatoquadrate cartilage itself (Klembara, 1991). Such a rod of cartilage was not recorded by Parker (1883) and Shiino (1914), but Kesteven (1957) recorded a rod of cartilage with an expanded base in a similar position in C. porosus. Besides a rod of cartilage, I described a protrusion present at the inner wall of the base of the pterygoid process of the palatoquadrate (i.e., at a similar place as Parker's and Shiino's ascending process) in some ontogenetic stages of A. mississippiensis and suggested that it could represent a basal process of the palato-quadrate (Klembara, 1991, figs. 3A, 15E). I suggested (Klembara, 1991), therefore, that this protrusion is probably that interpreted by Parker (1883) and Shiino (1914) as the ascending process.
As described above, in an even smaller specimen of Alligator mississippiensis (Stage 01) there is a long rod of cartilage, columella prootica, lying in the region of the prootic incisure (Fig. 2). The columella prootica is much longer than that in Stage 1A and its base lies close to the medial wall of the anterior margin of the pterygoid portion of the pars pterygoquadrata. It does not reach the base of the pterygoid process (i.e., the place where the ascending process of Parker [1883] and Shiino [1914] is located), but it lies slightly dorsally to it. The ophthalmic ramus of the trigeminal nerve lies medially to the procartilaginous anterior end of the columella prootica. Because in the even smaller Stage 02 this rod of cartilage is not developed, it is plausible that it develops as an independent element. In Recent reptiles, in which the ascending process is present, the ascending process of the palatoquadrate is a more or less long rod of cartilage lying close to the prootic incisure. Besides this ascending process, there is no other rod-like structure present in the given region of the endocranium during ontogeny of Recent reptiles (Bellairs and Kamal, 1981). Therefore, it is highly probable that the columella prootica of A. mississippiensis is the secondarily detached ascending process of the palatoquadrate and corresponds to the ascending process of the palatoquadrate of other Recent reptiles. This interpretation is further supported by the fact that the ventral portion of the columella prootica preserves its close vicinity to the body of the palatoquadrate and its lateral position to the ophthalmic ramus of the trigeminal nerve (Fig.  2) as in other Recent reptiles (Bellairs and Kamal, 1981;Rieppel, 1993b).

Cavum Epiptericum in Alligator mississippiensis
The cavum epiptericum has been defined by Gaupp (1900Gaupp ( , 1902Gaupp ( , 1905 as an extracranial space situated laterally to the primary cranial wall of the orbitotemporal region (represented by the pila antotica, adjacent cartilages, and membranes), medially to the ascending process of the palatoquadrate cartilage (epipterygoid when ossified) and dorsally to the basipterygoid process of the basisphenoid. It contains the trigeminal and facial ganglia, and is traversed by their branches, vena capitis, and orbital (or stapedial) artery. According to de Beer (1937), the cavum epiptericum cannot be said to be demarcated in crocodylians, because the ascending process (sensu Parker, 1883) is very small.
As described above, although the ventral end of the columella prootica in Alligator mississippiensis lies laterally to the ophthalmic branch of the trigeminal nerve as in lizards, its dorsal portion lies very close to the structures of the lateral cranial wall, and relatively early in ontogeny it fuses with them. Hence, strictly taken, the conclusion of de Beer (1937) is correct. However, as shown below, the pterygoid portion of the pars pterygoquadrata palatoquadrati of A. mississippiensis corresponds to the pterygoid portion of the epipterygoid in Sphenodon punctatus and possibly to the basalmost portion of the epipterygoid in lizards. Therefore, the cavum epiptericum in A. mississippiensis is also related to the epipterygoid portion of the palatoquadrate, and well demarcated, although 1) the columella prootica is not an independent ossification in the adult, and 2) the pterygoid portion of the pars pterygoquadrata is not an independent ossification, but it is included in the quadrate of the adult (see below).
Later in ontogeny, the space of the cavum epiptericum of Alligator mississippiensis immediately dorsolaterally to the trigeminal ganglion is filled with the lamina palatoquadrati anterior, and the corresponding portion of the trigeminal ganglion is almost completely enclosed laterally in the prootic fenestra. The trigeminal ganglion lies at the level of the antotic pila dorsally and the prefacial commissure ventrally. These conditions largely differ from those in other Recent reptiles and are in some respects more similar to those in mammals (see below).
According to Gardiner (1982), the cavum epiptericum is absent in crocodylians, as well as in mammals and birds, and the epipterygoid is incorporated into the braincase wall. Gardiner (1982) considered the pleurosphenoid of crocodylians and birds, the structure generally regarded (Goodrich, 1930) to be the ossification of the primary braincase wall (ossified pila antotica), and the alisphenoid of mammals to be homologous structures. Similar views were also expressed by Parker (1883) and Kesteven (1957). As demonstrated above, the columella prootica of Alligator mississipiensis probably represents the ascending process of the palatoquadrate. If this interpretation is correct, then the pleurosphenoid of crocodylians cannot be considered to be the epipterygoid (see also below).

Comparison of the Palatoquadrate in Sphenodon punctatus, Lizards and Alligator mississippiensis
The palatoquadrate cartilage ossifies into two bones in most tetrapods: the epipterygoid and the quadrate (Stadtmü ller, 1936;Romer and Parsons, 1977).
In the embryo of the Recent rhynchocephalian Sphenodon punctatus, the palatoquadrate consists of a large pars pterygoquadrata, an ascending process, and a pterygoid process (Howes and Swinnerton, 1901) (Fig. 8A). In the adult of S. punctatus, the epipterygoid consists of a dorsoventrally broad pterygoid portion of the pars pterygoquadrata, an anteroposteriorly broad ascending process, and a very short pterygoid process (Ver-sluys, 1936;Werner, 1962Werner, , 1963. However, a strip of cartilage persists in adults of S. punctatus and separates the epipterygoid bone from the large quadrate bone (Versluys, 1936;Haas, 1973;pers. obs.).
In the fully formed chondrocranium of lizards, the palatoquadrate (or pterygoquadrate sensu Bellairs and Kamal, 1981) consists of a narrow pars pterygoquadrata, a long ascending process, and a relatively short pterygoid process (Fig. 8B). There is also a basal process present which, however, becomes detached from the palatoquadrate early in ontogeny and forms an independent basipterygoid meniscus interpost between the processus basipterygoideus and the pterygoid. During ontogeny, the ascending process ossifies into the columnar epipterygoid (Parker, 1880), whereas most of the quadrate portion of the pars pterygoquadrata ossifies into the quadrate. This implies that, contrary to the conditions in Sphenodon punctatus, the pterygoid process and the cartilage connecting it and the ascending process with the quadrate portion (i.e., the whole pterygoid portion and anterior part of the quadrate portion of the pars pterygoquadrata) are resorbed. The situation I have just described for the palatoquadrate and its ossifications in lizards is based on the anatomy of the palatoquadrate in Lacerta agilis Gaupp (1900) which Rieppel (1993b, p. 354) considered "the paradigmatic condition with which to compare the fate of the element in all other reptiles." However, the composition of the quadrate of Alligator mississippiensis is more similar to the condition of S. punctatus with two exceptions: 1) the ascending process and 2) the strip of cartilage connecting the epipterygoid and the quadrate in S. punctatus are absent in A. mississippiensis. In the embryo of A. mississippiensis the pars pterygoquadrata is a large plate. Two processes extend from it: a pterygoid process extending anteroventrally and a long otic process projecting posteriorly (Fig. 8C). In the hatchling of A. mississippiensis the palatoquadrate is ossified as one element (only the anteriormost portion of the pterygoid process and the posterior tip of the otic process remain cartilaginous), and it is not divided into epipterygoid and quadrate bones. This does not mean, however, that the bone called the quadrate in the hatchling (or adult) of A. mississippiensis does not contain any of the embryonic epipterygoid components of other Recent reptiles. The quadrate of A. mississippiensis ossifies from the following embryonic palatoquadrate components (Fig. 8C): i) The pterygoid portion of the pars pterygoquadrata and the pterygoid process (a presumed basal process (see above) ossifies within the posterior portion of the pterygoid process and has no relation to the other portions of the palatoquadrate). Both components are also involved in the adult epipterygoid of S. punctatus; however, they are resorbed during ontogeny in lizards. In S. punctatus and lizards, unlike the situation in A. mississippi-ensis, the epipterygoid contains the embryonic ascending process of the palatoquadrate. ii) The quadrate portion of pars pterygoquadrata. This is large and completely ossified in S. punctatus. In lizards, the narrow anterior part of the pars quadrata is resorbed and the large posterior part of the quadrate portion ossifies into the quadrate. iii) The lamina palatoquadrati anterior. This structure (i.e., membranous portion of the quadrate in adult A. mississippiensis) is absent in S. punctatus and lizards.
From the above, it also follows that the quadrate in lizards and Sphenodon punctatus, on the one hand, and the quadrate in Alligator mississippiensis (and probably in all Recent crocodylians), on the other, are formed from different constituent elements during development.

Topological and Functional Interpretation of the Lamina Palatoquadrati Anterior
The lamina palatoquadrati anterior participating in the formation of the palatoquadrate in Alligator mississippiensis is a new structure not observed before in crocodylians. It has not previously been recorded in any other Recent reptile. The only tetrapods in which a similar bone is present are mammals.  Howes and Swinnerton, 1901;Werner, 1962) and adult (after Versluys, 1936;Haas, 1973). B: A lizard (after Bellairs and Kamal, 1981). C: Alligator mississippiensis (based on Stage 6A and 11A; approximate position of columella prootica based on Stages 01A and 1A). Cartilage is designated by rings, remaining portions are bone. Line x-y in lizard and A. mississippiensis indicates level of strip of cartilage interposed between epipterygoid and quadrate bones in adult S. punctatus. Note that bone called quadrate in A. mississippiensis involves pterygoid portion of epipterygoid as present in S. punctatus (including cartilaginous pr.pt.pq). col.po, columella prootica; EPT, epipterygoid; "EPT," portion of quadrate developing from elements forming parts of epipterygoid in S. punctatus and lizards; l.pq.a, lamina palatoquadrati anterior; pr.asc.pq, processus ascendens palatoquadrati; pr.ot.pq, processus oticus palatoquadrati; pr.pt.pq, processus pterygoideus palatoquadrati; QU, quadrate.
In mammals, the structures representing the remnants of the original palatoquadrate are: the ala temporalis and its ascending process and the pterygoid process (de Beer, 1937;Presley and Steel, 1976;Presley, 1981;Maier, 1987Maier, , 1993. During ontogeny, a bone spreads from the dorsal surface of the ala temporalis and the ascending process into the sphenoobturatory membrane (periosteal or appositional bone in the sense of Maier [1987Maier [ , 1993; membranous ossification at the ala temporalis [in early ontogenetic stages] or membranous portion of the alisphenoid [in adults] called here). In marsupials, the membranous ossification at the ala temporalis spreads laterally to the trigeminal ganglion, thus forming the secondary cranial wall. The membranous ossification at the ala temporalis encloses the trigeminal ganglion in the cavum trigeminale and protects it from the action of the chewing muscles (Maier 1987(Maier , 1993. According to Maier (1987Maier ( , 1993, the membranous ossification at the ala temporalis forms a mechanical support for the orbitoparietal cartilage, which undergoes mechanical stress exerted by the temporalis muscles during suckling and swallowing in early postnatal stages of marsupials.
There are several surprising similarities in the development of the structures in question when the conditions in mammals (especially marsupials, Maier, 1987Maier, , 1993 and Alligator mississippiensis are compared: 1. The ala temporalis of mammals corresponds to the pterygoid portion of the pars pterygoquadrata in Alligator mississippiensis from which the pterygoid process extends and to which the basal portion of the columella prootica lies very closely. This similarity should not detract attention from the fact that 1) in A. mississippiensis only the course of the ophthalmic branch of the trigeminal nerve can be directly related to the columella prootica, and 2) the pterygoid and quadrate portions of the pars pterygoquadrata of A. mississippiensis form one unit, while the quadrate portion is already detached in mammals and lies in the middle ear region. 2. The location where the lamina palatoquadrati anterior arises in A. mississippiensis (i.e., the pterygoid portion of the pars pterygoquadrata) corresponds to the location of origin of the membranous portion of the alisphenoid in mammals (i.e., at the ala temporalis).

Both the lamina palatoquadrati anterior in A.
mississippiensis and the membranous ossification at the ala temporalis in mammals ossify in membrane. 4. The lamina palatoquadrati anterior forms and spreads in membrane before the cartilaginous components, to which it is attached, begin to ossify, a condition similar to that of therian mammals (Presley, 1981;Maier, 1987).
5. The lamina palatoquadrati anterior spreads dorsally reaching the level of the taenia marginalis; in a similar way, the membranous ossification at the ala temporalis reaches the orbitoparietal commissure in marsupials (Maier, 1987). 6. The lamina palatoquadrati anterior spreads dorsolaterally to the posterior portion of the trigeminal ganglion, thus protecting it from the action of the jaw adductor muscles. The ventral margin of the lamina palatoquadrati anterior lies between the ophthalmic and the maxillary rami of the trigeminal nerve medially and the mandibular ramus laterally. It may be hypothesized that if the lamina palatoquadrati anterior grew further ventrally, it would completely divide the maxillary and mandibular rami. The membranous ossification at the ala temporalis arises in the exactly corresponding position in marsupials and monotremes, between the ala temporalis and the ascending process (Maier, 1989(Maier, , 1993 or at the inner surface of the ala temporalis (Gaudin et al., 1996). The membranous ossification at the ala temporalis divides the maxillary ramus lying medially from the mandibular ramus lying laterally (Maier, 1993, fig. 12.8.) to it. 7. The lamina palatoquadrati anterior sheathes the posterior portion of the trigeminal ganglion laterally and encloses it in the space of the fenestra prootica. The trigeminal ganglion, therefore, lies very close to the structures of the primary cranial wall (the pila antotica dorsally and the commissura prefacialis ventrally). In mammals, the trigeminal ganglion also fills the original cavum epiptericum (cavum trigeminale) completely and bulges medially into the primary lateral cranial wall (Maier, 1989).
In order to infer the phyletic origin of the secondary cranial wall in mammals from the reptile level of organization, the conditions in lizards have been used for comparisons (Presley and Steel, 1976;Maier 1987Maier , 1993. On the basis of these comparisons, and other detailed anatomical data, Maier (1987Maier ( , 1993 concluded that the epipterygoid of lizards is homologous with the alisphenoid of mammals. As stated above, the epipterygoid of lizards probably represents only the ossified long ascending process of the palatoquadrate. If so, the ascending process (ϭepipterygoid) in lizards does not correspond to the whole alisphenoid in mammals, but only to the ascending process of the ala temporalis (cf. also Jarvik, 1980). The ala temporalis and the pterygoid process of mammals correspond to the pterygoid portion of the pars pterygoquadrata and the pterygoid process in Alligator mississippiensis. However, both pterygoid portion of the pars pterygoquadrata and pterygoid process are resorbed during the growth of the palatoquadrate in lizards (Bellairs and Kamel, 1981); only the basalmost portion of the epipterygoid of lizards may include a part of the pterygoid portion of the pars pterygoquadrata of Sphenodon punctatus or A. mississippiensis. Furthermore, the lamina palatoquadrati anterior is formed at the pterygoid portion of the pars pterygoquadrata of A. mississippiensis. In a similar way, the membranous portion of the alisphenoid is formed at the ala temporalis in mammals. It is concluded that the alisphenoid of mammals corresponds only in part to the epipterygoid in lizards: the similarity is only in the presence of the ascending process in both groups. The similarities in the construction and the ossification of the pterygoid portion of the pars pterygoquadrata in A. mississippiensis and the alisphenoid in mammals, together with the position of the trigeminal ganglion within the cavum epiptericum, show that the conditions in A. mississippiensis are structurally very similar to the conditions in mammals, especially marsupials.
As mentioned, Maier (1987Maier ( , 1993 concluded that the membranous portion of the alisphenoid originated as a functional adaptation for a rapid strengthening of the secondary cranial sidewall of the braincase in connection with suckling and swallowing in early postnatal stages of marsupials. The formation of the lamina palatoquadrati anterior in Alligator mississippiensis is obviously not connected with such feeding requirements; however, the lamina palatoquadrati anterior seems to have the same function: it protects the posterior portion of the huge trigeminal ganglion dorsolaterally, thus building a new partial sidewall both for the ganglion and for adjacent elements of the primary cranial wall.

Topological and Functional Interpretation of the Lamina Prootici Anterior
The lamina prootici anterior, contributing to the closure of the braincase sidewall in the region of the fenestra prootica, is a new structure not recorded before in crocodylians.
Within Recent reptiles, a bone with a similar position and development pattern has been recorded in lizards, amphisbaenians, and snakes. Gaupp (1906) described a plate-like process extending forward from the prootic in Lacerta agilis. This ossification spreads in the membranous lateral wall. This structure has been described in various lizards under various names (crista alaris, Oelrich, 1956;superior anterior process, Jollie, 1960; alar process or crista alaris, Rieppel, 1981Rieppel, , 1993b. This structure, the anterior extension of the prootic, is a feature typical of autarchoglossan lizards (Gauthier, 1982). In the burrowing skink Acontias meleagris, the anterior extension of the prootic lies dorsolaterally to the trigeminal ganglion (Brock, 1941).
Within amphisbaenians, Zangerl (1944) reported a separate ossification, called the pleurosphenoid, immediately dorsal to the trigeminal incisure in Leposternon microcephalum. However, Rieppel (1981) was unable to confirm this pleurosphenoid to be a separate ossification in his specimens of Trogonophis wiegmanni and Amphisbaena alba. May (1978) found a continuous extension of the otic capsule from the wall of the anterior semicircular canal in Leposternon microcephalum and he designated it the "prootic process." Recently, Montero et al. (1999) studied the embryonic development of the skeleton of Amphisbaena darwini heterozonata. They found that in this amphisbaenian the main body of the anterior process of the otic capsule is preformed in cartilage and concluded that this process cannot be homologized with the alar process of the prootic in lizards, which is a membranous ossification of the otic capsule. They conclude (Montero et al., 1999, p. 23), "If such a dermal ossification existed in A. darwini, we would expect it to represent a small addition to the pleurosphenoid process." From these conflicting findings, it appears that further studies of more complete ontogenetic series of various amphisbaenian taxa are needed in order to follow the process of ossification of the structure in question.
In snakes, a structure found in a similar position to that in lizards and amphisbaenians is also present (Bellairs and Kamal, 1981). It develops as a ventral membranous extension of the prootic. During ontogeny it fuses with the dorsolateral outgrowth of the basal plate spreading in the sphenoobturatory membrane called the laterosphenoid (Rieppel, 1993b). This compound bone lies in the prootic incisure, encloses the trigeminal ganglion laterally, and separates the maxillary and mandibular rami of the trigeminal nerve. This ventral membranous extension of the prootic may be compared with the anterior extension of the prootic in Acontias meleagris (Bellairs and Kamal, 1981, p. 188).
The above results show that the anterior prootic lamina of Alligator mississippiensis, like the anterior membranous extension of the prootic of lizards and the ventral membranous extension of the prootic of snakes (conditions in amphisbaenians remain incompletely resolved), 1) ossifies in membrane, and 2) contributes to the lateral braincase wall in the region of the fenestra prootica, dorsolaterally to the trigeminal ganglion.
A bone in a partially similar position to the lamina prootici anterior of Alligator mississippiensis has been also found in Ornithorhynchus anatinus by Watson (his O. paradoxus, 1916). Watson (1916) interpreted it as an anterior process of the periotic which spreads in the sphenoobturatory membrane. This process (anterior lamina of the periotic in the sense of Hopson, 1964) lies posteriorly and dorsolaterally to the trigeminal ganglion. The anterior lamina, called also lamina obturans or os obturans, was recorded by Kuhn (1971) in Tachyglossus aculeatus and by Zeller (1989) in O. anatinus as an independent bony lamella ossifying within the sphenoobturatory membrane which fuses with the periotic in later ontogenetic stages. Contrary to the lamina prootici anterior, the lamina obturans is relatively larger and it spreads laterally and lateroventrally to the posteriormost portion of the trigeminal ganglion. However, both the lamina prootici anterior and the lamina obturans contribute to the orbitotemporal region of the braincase wall immediately anteriorly to the otic capsule, although in the case of the lamina obturans much more substantially. Presley and Steel (1976) and Presley (1981) concluded that there is no fundamental distinction between the structure of the lateral braincase wall in monotremes and therians, i.e., that the bone ossifying within the sphenoobturatory membrane, the anterior lamina of the periotic (lamina obturans) and the membranous portion of the alisphenoid, is always the same structure. The difference is only that this ossification fuses in ontogeny and phylogeny with the periotic in monotremes and with the ala temporalis in therians respectively. Zeller (1989) found both ossifications in Ornithorhynchus anatinus: lamina obturans and a small periosteal outgrowth from the ala temporalis. The problem of the homology of these two ossifications was last discussed by Hopson and Rougier (1993). They concluded that the anterior lamina of the periotic on one side and the membranous portion of the alisphenoid on another side are two different structures. They showed that in carnivorous Triassic cynodonts and Early Jurassic tritheledontids, the epipterygoid is expanded dorsally, indicating the presence of the membranous component. They hypothesized that the anterior end of the prootic ossification of the otic capsule probably included also a membranous component. Hence, Hopson and Rougier (1993) considered these structures to be developmentally and phylogenetically different.
As demonstrated above, the development of topologically corresponding structures and ossifying in membrane, the lamina prootici anterior and the lamina palatoquadrati anterior may also be studied in Alligator mississippiensis. The lamina prootici anterior corresponds in position to the anterior lamina of the periotic in non-therian mammals, and the lamina palatoquadrati anterior corresponds in position to the membranous portion of the alisphenoid in therians. Both the lamina prootici anterior and the lamina palatoquadrati anterior of A. mississippiensis are ontogenetically different structures, as surmised by Hopson and Rougier (1993) for the anterior lamina of the periotic and the membranous portion of the alisphenoid in Triassic cynodonts and mammals.
The topological correspondence of the lamina palatoquadrati anterior of Alligator mississippiensis and the membranous portion of the alisphenoid of marsupials on one side, and of the lamina prootici anterior of A. mississippiensis and the lamina anterior (lamina obturans) of non-therian mammals on the other side, all ossifying in membrane, is surprising, because the synapsids and the archosaurs are two different clades. The question of the possible homology of the compared structures in both clades remains open and its solution is beyond the scope of this article. It is possible to conclude, however, that the topologically corresponding structures are involved in the protection of the huge trigeminal ganglion and the partial enclosing of the orbitotemporal braincase wall in both clades.