Ontogeny of the partial secondary wall of the otoccipital region of the endocranium in prehatching Alligator mississippiensis (Archosauria, Crocodylia)

The ontogeny of the posterior otic and anterior occipital portions of the neural endocranium of prehatching Alligator mississippiensis was investigated by reconstruction from sectioned material. In Stage 6 of this species, in which the endochondral ossification of the otoccipital region of the neural endocranium is only in its very early stage, two bony outgrowths—laminae—are present at the external wall of the posterior portion of the neural endocranium. The anterior lamina arises from the external surface of the basal plate at the level of the posterior margin of the subcapsular process; the posterior lamina arises from the external surface of that portion of the pila occipitalis that forms the posteroventral wall of the metotic fissure. During ontogeny, both laminae lying in the anteroposterior sequence ossify in membrane, fuse together, grow laterodorsally, and fuse with the lateral wall of the lateral semicircular canal and the crista parotica. This lamina forms a new, secondary wall enclosing the posterior section of the otic capsule and contains the large external jugular foramen (or foramen vagi) in its basal portion. The laminae, designated lamina juxtaotica anterior and posterior (lamina juxtaotica when fused together), have not been recorded previously in crocodylians and are absent in all other Recent reptiles. From the functional point of view, the juxtaotic lamina 1) forms the margins of the external jugular foramen, and 2) forms the floor of the posterior section of the Eustachian tube. In birds, the structure called the metotic cartilage, which arises in ontogeny as an independent element, has a similar position as the juxtaotic lamina. However, the two structures differ in their developmental origins and their relation to the Eustachian tube and the ramus hyomandibularis of the facialis nerve. Moreover, there is no external jugular foramen in birds. J. Morphol. © 2005 Wiley‐Liss, Inc.

Knowledge of the cranial development of the Recent crocodylians helps not only to understand their own cranial bauplan, but also to learn the similarities and differences relative to the taxa of the related clades. Several authors concluded that the development of certain cranial structures of crocodylians is more closely comparable with the conditions in birds than with those in other Recent reptiles (e.g., Baird, 1970, as to middle ear structures). Such conclusions stimulated many authors to study the development of various structures of various crocodylian taxa.
Bellairs and Kamal (1981, and references therein) summarized previous articles and described various portions of the embryonic skull of Alligator mississippiensis. Later, Klembara (1991) described the skull of several early ontogenetic stages of A. mississippiensis and several particular cranial structures in later ontogenetic stages of A. mississippiensis (Klembara, 1993(Klembara, , 2001(Klembara, , 2004. The ossification sequence of the skull bones of cleared and stained embryos of A. mississippiensis has been described recently by Rieppel (1993). Only a few of these publications focus on the ontogeny of the later developmental stages of the otoccipital portion of the crocodylian endocranium: Parker (1883, A. mississippiensis, Crocodylus palustris), Shiino (1914, C. porosus), DeBeer (1937, C. biporcatus), partially Simonetta (1956, A. mississippiensis), Mü ller (1967, C. cataphractus), Iordansky (1973, C. niloticus), Bellairs and Kamal (1981, A. mississippiensis), and Rieppel (1993, A. mississippiensis). However, in none of these articles is the detailed ontogeny of the individual bones or structures of the otoccipital region of the neural endocranium documented, and none of the structures presented here have been recorded previously.
It is known that there is a large foramen jugulare externum (Mü ller, 1967) or foramen vagi (Iordansky, 1973) in the confluent opisthoticϩexoccipital of the adult crocodylians that is divided in its depth into medial and lateral canals: 1) the medial canal extends into the cerebral cavity and is traversed by the glossopharyngeal and vagus nerves, and 2) the lateral canal extends into the tympanic cavity and is traversed by the ramus communicans of the sympathetic nerve, which connects the hyomandibular branch of the facialis nerve with the glossopharyngeal nerve. However, nothing is known about the developmental origin of the margins of the external jugular foramen and partial secondary wall of the adjacent wall of the otic capsule.
The aims of this article are: 1) to present a detailed description of the ontogeny of the lateral wall of the posterior portion of the otoccipital region of the neural endocranium in the prehatching Alligator mississippiensis; and 2) to compare several structures of the posterior portion of the otoccipital region of the neural endocranium in prehatching A. mississippiensis with topologically similar structures in several tetrapods exhibiting a similar general morphology in this region of their endocrania.

MATERIALS AND METHODS
The following transversely sectioned ontogenetic stages of Alligator mississippiensis, arranged by head length, were used (FS, stages after Ferguson [1985]

RESULTS
I describe the ossification of the outer wall of the posterior portion of the otic capsule and adjacent occipital portion of the neural endocranium during the prehatching ontogeny of Alligator mississippiensis (Fig. 1). Surprisingly, this wall is not preformed by cartilage, but it is formed by two bony outgrowths-laminae arising relatively late in ontogeny from the ventrolateral wall of the still almost completely cartilaginous neural endocranium. The laminae lie in an anteroposterior sequence and fuse together during growth, by which the posterior lamina is larger and grows more extensively in the anterodorsal direction. These laminae are referred to as the anterior juxtaotic lamina (lamina juxtaotica anterior) and the posterior juxtaotic lamina (lamina juxtaotica posterior) here and, when fused together, the bone is designated the juxtaotic lamina (lamina juxtaotica) (Figs. 2-9).

Stage 5
The otic capsule, basal plate, and pila occipitalis are fully cartilaginous, and no traces of resorption of the cartilage are present. Only a thickened layer of condensed mesenchyme is situated in the places where the future bones, the anterior juxtaotic lamina and the posterior juxtaotic lamina, appear.

Stage 6A
The endochondral ossification of the otoccipital portion of the neural endocranium is still not observable. Only sheets of perichondral ossifications are present at the external and internal surfaces of the occipital arch (pila occipitalis) (Figs. 2, 3B). The anterior juxtaotic lamina appears at and close to the outer wall of the basal plate, ventral to the posterior portion of the subcapsular process and immediately lateral of the anteriormost foramen of the hypoglos- sal nerve (Figs. 2,3A). In this stage the cartilages of the basal plate and of the base of the subcapsular process are continuous. The anterior juxtaotic lamina is a tiny plate slightly extending laterodorsally at its anterior portion. As a whole, the anterior juxtaotic lamina lies below the otic capsule at the level of the stapes, and mediad of the posterior section of the Eustachian tube (Fig. 3A).
The posterior juxtaotic lamina is a much larger and substantially more robust plate in comparison with the anterior juxtaotic lamina. It extends from the external surface of that portion of the pila occipitalis which forms the floor of the posterior section of the metotic fissure (or jugular foramen). Here it is joined with the sheet of the perichondral ossification that lines this portion of the pila occipitalis (Figs. 2, 3B). The posterior juxtaotic lamina extends anteriorly and dorsally, and forms an almost vertical wall covering the corresponding section of the metotic fissure laterally, including the nerves emerging from it (Fig. 3C). The free anterior margin of the posterior juxtaotic lamina lies slightly posterior to the posterior margin of the subcapsular process (Fig. 2). The dorsal margin of the posterior juxtaotic lamina reaches the level of the ventral wall of the perilymphatic foramen (Figs. 2, 3C).

Stage 7A
The anteriormost portion of the anterior juxtaotic lamina extends laterodorsally and anteriorly from the cartilaginous endocranial wall and underlies the posterior section of the Eustachian tube (Fig. 4). Further posteriorly, the anterior juxtaotic lamina becomes more massive and its posterior margin is joined with the sheet of the perichondral ossification of the basal plate. As a whole, the anterior juxtaotic lamina is a plate extending dorsolaterally from the ventrolateral wall of the neural endocranium; its anterior and dorsal margins are free (Fig. 4).
The anterior end of the posterior juxtaotic lamina already extends more anteriorly in comparison with the preceding stage and ends freely immediately posterior to the anterior juxtaotic lamina (Fig. 4A). Further posteriorly, the posterior juxtaotic lamina forms a convex plate and its ventral margin forms the posterolateral wall of the still incompletely bordered external jugular foramen. The dorsal margin of the posterior juxtaotic lamina reaches the level of the ventral wall of the lateral semicircular canal. Between the anterior juxtaotic lamina and the posterior juxtaotic lamina, a narrow free space still remains (Fig. 4A).

Stage 8A
The anteriormost margin of the anterior juxtaotic lamina is notched by the internal carotid artery from the ventral side (Fig. 5). As a whole, the anterior juxtaotic lamina is a massive plate extending from the ventrolateral wall of the endocranium, from a position immediately below the posterior portion of the subcapsular process. The cartilage of the endocranium from which the anterior juxtaotic lamina extends is still not resorbed (Fig. 6A). The anterior juxtaotic lamina forms a floor of the posterior section of the Eustachian tube extending from the ventral portion of the tympanic cavity here. The posteriormost section of the subcapsular process is partially ossified (Fig. 6B). Here the anteriormost end of the posterior juxtaotic lamina underlies the ventrolateral surface of the anterior juxtaotic lamina and both bones are fused together, thus forming the juxtaotic lamina; the line of fusion of both portions is still partially visible. As a result of this fusion, the lateral wall of the external jugular foramen (or foramen vagi) is formed. Below this posteriormost portion of the anterior juxtaotic lamina and immediately anterior to the external jugular foramen, a huge sympathetic ganglion is located in a shallow excavation (Fig. 6B). Further posteriorly, the posterior portion of the juxtaotic lamina forms an almost vertical convex plate that encloses the sympathetic, glossopharyngeal, and vagus nerves and the metotic fissure from the lateral side. The glossopharyngeal nerve passes through the metotic fissure in its anterior section (Fig. 6B) and leaves it immediately posteriorly to the posterior margin of the subcapsu-  (Fig. 6C). The vagus nerve emerging from the metotic fissure passes through the foramen positioned between this osseous lamina forming its dorsal margin and the basal plate (ventrolateral wall of the metotic fissure) forming its ventral margin. Posteriorly, the osseous lamina is joined with the adjacent lateral cartilaginous wall of the otic capsule.
Further posteriorly, the ventral portion of the juxtaotic lamina is fused with the basal plate, thus forming the posterior margin of the external jugular foramen. The posteriormost wall of the juxtaotic lamina is fused with the perichondrally and endochondrally ossifying anterior portion of the pila occipitalis and with the anterior wall of the crista parotica, with the perichondrally ossifying ventral wall of the lateral semicircular canal and with the otic capsule.

Stage 9B
The anteriormost portion of the juxtaotic lamina is deeply notched by the arteria carotis interna (Fig.  7). The juxtaotic lamina is a stout convex plate that reaches the ventral wall of the lateral semicircular canal dorsally (Figs. 7, 8A). Further posteriorly, at the level of the exit of the vagus nerve from the metotic fissure, there is a thin osseous bridge interconnecting the ventral margins of the posterior portion of the juxtaotic lamina positioned laterally and the osseous lamina lying medially, thus enclosing the posterolateral extension of the Eustachian tube (Fig. 8B). The osseous lamina is fused here with the ventrolateral wall of the otic capsule. The posteromedial extension of the Eustachian tube is floored by the anterior portion of the juxtaotic lamina ventrally and by the basal plate dorsally (part of the future exoccipitalϩopisthotic) (Fig. 8B). This anterior portion of the juxtaotic lamina forms the anteromedial margin of the external jugular foramen (Figs. 7, 8B). The posterolateral margin of this foramen is formed by the ventral margin of the posterior portion of the juxtaotic lamina. The large ganglion of the sympathetic nerve lies in the shallow excavation immediately anterior to the external jugular foramen. Posteriorly and posterodorsally, the juxtaotic lamina is fused with the endochondrally ossified anteromedial wall of the pila occipitalis and the parotic crest.
As a whole, the juxtaotic lamina is a large osseous wall covering the posterior portion of the partially endochondrally ossified otic capsule from the lateral side (Fig. 7). The anterior margin of the juxtaotic lamina is slightly incised posteriorly in its midlength; its dorsal margin lies at the level of the posterior end of the fenestra ovalis and its anteroventral margin reaches the anterior end of the latter structure. The juxtaotic lamina is fused with the following portions of the partially ossified neural endocranium: the basal plate anteroventrally, the pila occipitalis posteroventrally and posteriorly, the crista parotica posteriorly, and the lateral semicircular canal dorsomedially. From the outside the jux-taotic lamina covers the following structures in the anteroposterior sequence: 1) the posterior section of the Eustachian tube ventrally, and 2) the metotic fissure, the glossopharyngeal, vagus, and sympathetic nerves, and the posterior section of the perilymphatic foramen laterally.

Stages 10A and 11A
In Stage 10A the ossification of the juxtaotic lamina and the individual components of the neighboring lateral cranial wall is more substantial. The anteriormost end of the juxtaotic lamina reaches the posteriormost portion of the basitemporal portion of the already progressively ossified parabasisphenoid (see also Klembara, 1993), by which the juxtaotic lamina slightly overlaps the latter structure (Fig.  9A). Here the juxtaotic lamina abuts the quadrate laterally. The arteria carotis interna is almost completely surrounded by bone that is fused with the ventral surface of the already ossified subcapsular process. Further posteriorly, the juxtaotic lamina forms a continuous external semicircular wall of the posterior portion of the otic capsule and encloses the posterior extension of the Eustachian tube (Fig. 9B). Besides the progress in ossification, no other changes occur in the formation of the juxtaotic lamina relative to the preceding stage.
It is readily observable in Stage 11A (hatching) that the juxtaotic lamina is a part of the confluent opisthoticϩexoccipital. Most of the length of the anterior margin of the juxtaotic lamina is firmly sutured with the quadrate. A short section of the anteroventral margin of the juxtaotic lamina has a suture with the basitemporal and a short anteromedial margin of the juxtaotic lamina is sutured with the basioccipital.

DISCUSSION
The anterior juxtaotic lamina and the posterior juxtaotic lamina, when fused to form a single juxtaotic lamina, represent structural components in the otoccipital region of the skull of Alligator mississippiensis that have not been recorded previously in any crocodylian. The juxtaotic lamina forms the secondary external wall of the posterior portion of the otoccipital moiety of the neural endocranium, and its ventralmost portion participates in the bordering of the external jugular foramen. This is highly surprising because such a juxtaotic lamina is absent in any other Recent reptile and, consequently, no comparisons are possible in this context. However, the cartilago metotica, a structure in a similar position, is present in birds.

Metotic Cartilage, Subcapsular Process, and Juxtaotic Lamina
The metotic cartilage is a plate-like element that develops lateral of the otoccipital region of the endocranium. It was first described in Anas platyrhynchos f. domestica by Sonies (1907) and was later found in various birds (for review, see Mü ller, 1961Mü ller, , 1963Toerien, 1971). Weber (1990) considered this element to be an autapomorphic character of birds.
The metotic cartilage arises in ontogeny mostly as an independent element (i.e., Sonies, 1907;DeBeer and Barrington, 1934;May, 1962;Macke, 1969;Toe-Fig. 7. Alligator mississippiensis. Otoccipital region of neural endocranium of Stage 9B in right lateral view. Vertical lines indicate position of transverse sections in Figure 8A,B. n.V 1 , ramus ophthalmicus, n.V 2 , ramus maxillaris, n.V 3 , ramus mandibularis of trigeminal nerve. rien, 1971). However, in Spheniscus demersus it develops as an outgrowth of the otic capsule (Crompton, 1953), and in Pyromelana orix it arises as anterior and medial elements that fuse together later in ontogeny (Engelbrecht, 1958). In Podiceps cristatus, the metotic cartilage arises as an isolated mesenchymal anlage positioned lateral of the canalicular part of the otic capsule (Toerien, 1971) or slightly ventral to it (Frank, 1954). The chondrification of the anlage proceeds from its center. The metotic cartilage lies at the level of the jugular foramen, laterally encloses the posterior half of the foramen perilymphaticum, and forms a floor of the cavum tympani (May, 1962). Anteriorly, the metotic cartilage reaches the level of the stapes. During growth the ventral and posteroventral margins of the metotic cartilage fuse with the basal plate and pila occipitalis and the dorsal margin fuses with the lateral wall of the lateral semicircular canal and the crista parotica. The fusion of the metotic cartilage with the basal plate and pila occipitalis is in such a manner that it separates the glossopharyngeal and vagus nerves into separate foramina in some birds (DeBeer and Barrington, 1934). The ramus hyomandibularis of the facialis nerve runs in its dorsoventral course anteriorly and anteromedially to the metotic cartilage (Lang, 1956, fig. 5;Mü ller, 1961Mü ller, , fig. 4, 1963 fig. 10; May, 1962, fig. 9). A cavum metoticum is enclosed between the metotic cartilage and the primary lateral endocranial wall (Lang, 1956;Mü ller, 1963;Macke, 1969). In hatchlings and adults, this space houses the recessus (or sinus) tympanicus inferior.
According to DeBeer and Barrington (1934), the metotic cartilage arises in ontogeny from four cranial ribs in Anas platyrhynchos f. domestica. This conclusion was supported by Slabý 's (1958) observations in Phalacrocorax carbo and Larus ridibundus. However, other authors, such as Crompton (1953), Frank (1954), Engelbrecht (1958, and Macke (1969), have not confirmed the conclusion of DeBeer and Barrington (1934). Mü ller (1961) considered the metotic cartilage to be a lateral enlargement of the pila occipitalis.
According to DeBeer and Barrington (1934), the metotic cartilage is homologous with the so-called subcapsular process of crocodylians. DeBeer and Barrington (1934:457) stated: "The crocodile possesses a subcapsular process (Shiino, 1914) which extends forwards from the occipital arch beneath the auditory capsule, with which, however, it does not fuse"; and illustrate this condition in their fig. 25A, comparing it with the condition in sparrows in their fig. 25B.
The subcapsular process was first described by Shiino (1914) in the fully formed chondrocranium of Crocodylus porosus (his C. biporcatus) and later in two ontogenetic stages of Alligator mississippiensis (Klembara, 1991). The subcapsular process is a plate-like structure that extends dorsally from the lateral margin of the parachordal portion of the basal plate, immediately anteriorly to the pila occipitalis (Figs. 1,2,3A,4,5,6A,B,7). It underlies the otic capsule and forms the floor of the anterior section of the fissura metotica (called the recessus scalae tympani, which houses the perilymphatic sac). The subcapsular process covers from the lateral side the anterior half of the perilymphatic foramen and dorsally reaches the ventral level of the fenestra ovalis. The posterior margin of the subcapsular process lies at the level of the stapes. The ramus hyomandibularis of the facialis nerve runs along the dorsal margin of the subcapsular process and then curves ventrally and runs along its posterior margin (Figs. 4A, 7). From the above-described morphology and position of the subcapsular process, it can be concluded that the structure described and figured by DeBeer and Barrington (1934), partially on the basis of the results of Shiino (1914), does not exist either in C. porosus (Shiino, 1914) or in A. mississippiensis (Klembara, 1991). Further, the subcapsular process and the metotic cartilage have different topologies relative to the structures of the lateral wall of the neural endocranium and the ramus hyomandibularis of the nervus facialis.
However, the metotic cartilage has, in many respects, a similar position to the lateral wall of the endocranium as the juxtaotic lamina. These similarities are: 1. Both structures cover the posterior portion of the otoccipital region of the neural endocranium from the outside, and anteriorly reach the level of the stapes. 2. Dorsally, they fuse with the external wall of the lateral semicircular canal. 3. Ventrally, they are joined with the basal plate and pila occipitalis in the region of the exit of the glossopharyngeal and vagus nerves from the cranial cavity. However, the place of their junction with the basal plate and pila occipitalis is not identical (see below). 4. They join the quadrate in later ontogenetic stages.
However, despite these topological similarities of the metotic cartilage and the juxtaotic lamina, there are several fundamental differences between the two structures. These differences are: 1. The cartilago metotica is a cartilaginous structure. The juxtaotic lamina is membranous bone from the beginning of the ossification of its original two components (anterior and posterior juxtaotic lamina). In the Alligator mississippiensis skull, a similar mode of ossification is also seen in the lamina palatoquadrati anterior and the lamina prootici anterior (Klembara, 2004). The bony trabeculae of all these structures start to form at and close to the perichondrium of the given endocranial cartilaginous structure. In early phases of the growth of the bony trabeculae the cartilage from which they extend is not resorbed and the bones spread within the tissue layers. All these structures in A. mississippiensis, the lamina palatoquadrati anterior, lamina prootici anterior (Klembara, 2004), and the anterior and posterior juxtaotic laminae described here, start to develop relatively late in ontogeny. 2. Although Engelbrecht (1958) observed the ontogenetic origin of the metotic cartilage from anterior and medial components that fuse together later in ontogeny, the metotic cartilage originates mostly from one independent mesenchymal component. The juxtaotic lamina arises from two, anterior (anterior juxtaotic lamina) and posterior (posterior juxtaotic lamina), components that with growth fuse together. Moreover, the anterior and posterior juxtaotic laminae are joined with the external surface of the basal plate and pila occipitalis, respectively, from the beginning of their development. 3. The juxtaotic lamina bears the external jugular foramen (or foramen vagi) in its ventral portion and this foramen arises as a consequence of the fusion of the anterior and posterior juxtaotic laminae. Although there is some variation in the developmental origins of the foramina for the glossopharyngeal and vagus nerves in birds (e.g., Sonies, 1907;Crompton, 1953, and above), the ventral margin of the metotic cartilage fuses with the basal plate in such a manner that the foramen vagi of later ontogenetic stages is the remnant of the metotic fissure (May, 1962). Hence, the metotic cartilage does not contain the foramen for the vagus nerve, i.e., there is no external jugular foramen in birds. 4. The ramus hyomandibularis of the facialis nerve runs anteriorly or anteromedially to the metotic cartilage. This nerve runs laterally to the juxtaotic lamina and posteriorly to its anterior margin (Figs. 4A, 7). 5. The metotic cartilage encloses the cavum metoticum (inferior tympanic recess in hatchlings) and has no topological and functional relation to the Eustachian tube. The anterior juxtaotic lamina forms the floor of the posterior section of the Eustachian tube. Although the juxtaotic lamina is involved in the formation of the ventral support of the Eustachian tube, it has to be emphasized, however, that other Recent reptiles having the Eustachian tube have no juxtaotic lamina.
The above-described conditions show that the topologically similar structures (the metotic cartilage on one side and the juxtaotic lamina on the other side) have different developmental origins in two crown taxa (crocodiles and birds) of the archosauromorph clade, and also different relations to the Eustachian tube and ramus hyomandibularis of the facialis nerve. Besides this, there is no external jugular foramen in birds. These differences probably reflect the different phylogenetic history of the corresponding neurocranial regions of both compared taxa. The phylogenetic origin of both compared structures, however, remains unknown.