Postparietal and prehatching ontogeny of the supraoccipital in Alligator mississippiensis (Archosauria, Crocodylia)

The first record of the postparietal bone of Alligator mississippiensis, documented by transverse histological sections, is presented. It is the first evidence of the presence of this bone within Recent reptiles. The postparietal is present in a specimen with a head length of 32.3 mm. The bone is a small dermal plate lying ventrally and posteriorly to the posterior margin of the parietal and dorsally to the trabecular bone, forming a dorsal surface of the supraoccipital portion of the neural endocranium. The trabecular bone develops perichondrally from the dorsal surface of the tectal cartilaginous bridge spanning between the dorsal portions of the otic capsules and occipital pilae. The bridge probably represents the fused tectum synoticum posterior plus tectum posterius. Later in ontogeny, the bridge ossifies endochondrally. The endochondrally ossifying bridge together with its perichondrally ossifying trabecular bone form the future supraoccipital. The trabecular bone is the integral part of the cranial endoskeleton and ontogenetically distinct from the dermal postparietal bone. J. Morphol. 249:147–153, 2001. © 2001 Wiley‐Liss, Inc.

Although there is no general agreement on what bone in osteolepiform ancestors of tetrapods (extrascapular medial or the bone[s] lying immediately to it) is homologous with the postparietal in tetrapods (reviewed by Jarvik, 1996), there is general agreement that the postparietal bones (sometimes fused into an unpaired bone) of early tetrapods lie at the posterior margin of the dermal skull roof and may descend as flanges onto the posterior surface of the neurocranium (Romer and Parsons, 1977). In the evolutionary line leading to mammals, the postparietals gradually obtain the position at the posterodorsal margin of the occiput. The postparietal bone is synonymously also termed the dermal supraoccipital (or dermo-supraoccipital) or interparietal (Romer, 1976;Romer and Parsons, 1977).
Within Recent tetrapods, bones that are homologized with the postparietal bones of the extinct tetrapods develop ontogenically dorsally to the posteriormost cranial tecta, tectum synoticum, and tectum posterius, and have been occasionally recorded as independent bones in various ontogenetic stages in anurans and mammals. During ontogeny, these independent bones may fuse with the frontoparietals in anurans (Roček, 1981;Smirnov, 1992); in mammals they fuse with the dorsal margin of the enchondrally ossifying supraoccipital cartilage to form together the occipital bone (Romer and Parsons, 1977;Jarvik, 1996). However, they may also persist as independent bones in adult anurans (Smirnov, 1977) as well as in mammals (Wegner, 1960).
Since the earliest investigations of the skull anatomy of vertebrates in the nineteenth century, only two articles record the possible presence of the postparietal bone in Recent reptiles. Both records were made on crocodylians: in Alligator mississippiensis (described as dermo-supraoccipital by Mook, 1921) and in two species of Crocodylus: C. palustris and C. porosus (described as interparietal by Deraniyagala, 1939). Mook's dermo-supraoccipital was identified in dried skulls and Deraniyagala's interparietal in dried skulls and cleared embryos.
The postparietal was not recorded in the two cleared embryos of Crocodylus niloticus studied by Iordansky (1973). Rieppel (1993) studied the process of ossification of the supraoccipital of Alligator mississippiensis on cleared and stained embryos, but did not find a separate dermal postparietal ossification in his material. All hatchling and embryonic specimens of A. mississippiensis studied by Brochu (1999) lacked the postparietal bone.
The aim of this article is to describe prehatching ontogeny of the supraoccipital of Alligator mississippiensis and the first discovery of a completely inde- Figure 1 pendent postparietal dermal ossification recorded on transverse histological sections of an embryo of A. mississippiensis. A cartilaginous bridge, spanned between the posteriormost dorsal portions of the otic capsules and the occipital pilae, which ossifies as the supraoccipital, is variously called the tectum synoticum (Rieppel, 1993) or tectum posterius (Shiino, 1914). It probably represents the tectum synoticum posterior plus tectum posterius (tspϩtp) (de Beer, 1937;Klembara, 1991). Rieppel (1993) calls the bone that develops from the dorsal surface of the tspϩtp during the early ontogeny of Alligator mississippiensis the trabecular bone (of dermal appearance). With growth, this bone fills the space between the endochondral supraoccipital ossification (ossifying in the tspϩtp) and the posterior margins of the parietals. For this portion of the future supraoccipital, which Rieppel (1993) calls the trabecular bone (of dermal appearance), I also use the term trabecular bone. It designates the trabecular architecture of the bone developing perichondrally from the dorsal surface of the tspϩtp. It is designated the tb.tspϩtp here.

MATERIALS AND METHODS
For this study, the following transversely sectioned ontogenetic stages of Alligator mississippiensis arranged according to the head lengths were used (FS -stages after Fergusson [1985]

RESULTS
The following description of the ontogeny of the supraoccipital of Alligator mississippiensis starts at Stage 7, in which the perichondrally ossifying tb.tspϩtp first appears. It is followed by a description of the progressive growth of the tb.tspϩtp up to Stage 11A, at which stage the boundary between the endochondral ossification of the tspϩtp is almost indistinguishable from the tb.tspϩtp. The presence of the dermal postparietal bone in the Stage 9B is related to the ontogenetic status of the tb.tspϩtp. The latter structure conveys the impression of being a dermal ossification and it has been considered by some previous authors (see Discussion) to be a separate postparietal dermal ossification.

Stage 7A
In Stage 7A the anterior margin of the tspϩtp extends into a pair of plate-like processes (plp.tspϩtp, Fig. 1A) that are not connected by their lateral margins with the otic capsules and that are not connected together in the mesial plane. The dorsal surface of the posterior portion of the tspϩtp, in the depression bilaterally flanked by the cartilaginous elevations of the posteriormost portions of the otoccipital region of the endocranium, starts to ossify perichondrally. The perichondral cells form a distinct lining from which several clusters of cells spread slightly dorsally. They represent the initial phase of the formation of the future bony trabeculae (tb.tspϩtp, Fig. 1B).

Stage 8A
In Stage 8A, as in Stage 7A, the anterior margin of the tspϩtp bears two plp.tspϩtp. Immediately posteriorly, the tspϩtp is not covered with the trabecular bone. The bony lining and very short trabeculae cover the posteriormost section of the anterior third and the whole middle third of the length of the tspϩtp (tb.tspϩtp, Fig. 1C). It may be noted that in its posterior section the layer of the tb.tspϩtp is dorsoventrally thicker than the underlying cartilaginous tspϩtp (Fig. 1D). Some of the bony trabeculae spread slightly dorsally from the bony lining of the tspϩtp; however, most of them are situated more dorsally in the tissue and with no connection with the bony lining. The posterior section of the posterior third of the tspϩtp is not covered by trabecular bone. The cartilage of the tspϩtp shows no trace of resorption.

Stage 9B
In Stage 9B both of the anterior plate-like processes protruding from the anterior margin of the cartilaginous bridge (plp.tspϩtp) are still present, as in the two preceding stages. The tspϩtp is partially resorbed in this stage, most intensively in the middle section of its length, below the tb.tspϩtp ( Fig.  2A-C). The tb.tspϩtp is well developed and it is thicker than the partially resorbed underlying tspϩtp. There is no trace of separation between the tb.tspϩtp and the endochondrally ossifying tspϩtp. The posteriormost section of the tspϩtp remains cartilaginous and no tb.tspϩtp is formed here (Fig.  2D).
The bone interpreted here as the postparietal is present in Stage 9B on the left side of the head only ( Fig. 2A-C). It is a small dermal plate lying immediately dorsally to the tb.tspϩtp and posteroventrally to the posterior margin of the parietal. No part of the postparietal is fused to the tb.tspϩtp or to the parietal.

Stages 10A and 11A
In Stages 10A and 11A, the tspϩtp is substantially resorbed in some portions, but it is mostly ossified through its length. The trabecular architecture of this endochondral ossification is basically identical to that of the tb.tspϩtp. In both stages no independent postparietal has been recorded. It would be impossible to recognize, once it had fused with the tb.tspϩtp or with the parietal during ontogeny.

DISCUSSION
The postparietal in Stage 9B is a very small bone relative to the size of the parietal or the supraoccipital portion of the endocranium. It is present only on one side of the head. It may be that the right-left asymmetry in the development of this bone is only intraspecific variation. The position of the postparietal on one side clearly indicates that this bone is not a median, unpaired element in Alligator mississippiensis, at least in the stage of its early ontogeny. The postparietal appears late in ontogeny; all other exocranial bones are already well developed.
The postparietal of Alligator mississippiensis, as identified in Stage 9B, casts doubts on the correctness of the identification of the dermo-supraoccipital of Mook (1921) and of the interparietal of Deraniyagala (1939) as an independent dermal bone of the posterior skull table. In addition, the identification of the postparietal in Stage 9B and its topology clarify relationship of the postparietal to the individual ontogenetic components of the supraoccipital.

Dermo-supraoccipital of Alligator mississippiensis of Mook (1921)
Mook (1921) described a postparietal bone in two juvenile skulls of Alligator mississippiensis: 1) in the first skull (skull length, 35.3 mm), the postparietal lies posterior to the parietal and anterior to the supraoccipital and the postparietal is united with both bones by means of sutures; 2) in the second skull (skull length, 39.5 mm), the postparietalparietal suture is obscure and the postparietal appears to be partially fused with the parietal.
The head length of the specimen examined here is slightly shorter than the skull length of the first skull of Mook (1921), in which the postparietal has been identified as a separate bone. However, because in specimens Stage 10A and S 11A no postparietal is present, the identification of the dermo-supraoccipital in larger specimens of A. mississippiensis by Mook (1921) seems doubtful. Further, in his specimens the dermo-supraoccipital is a relatively large bone, much larger than the postparietal as preserved in Stage 9B. The outline of the dermo-supraoccipital of Mook (1921) corresponds well with the outline of the portion of the supraoccipital ontogenetically formed by its trabecular bone component (tb.tspϩtp). Deraniyagala (1939) did not describe the process of fusion of the postparietal with the parietal or supraoccipital during ontogeny of Crocodylus palustris and C. porosus.

Interparietal of Crocodylus of Deraniyagala (1939)
The interparietal of both species of Crocodylus of Deraniyagala (1939) has the same position and outline as the dermo-supraoccipital in Alligator mississippiensis of Mook (1921). The postparietal was not recorded in two embryos (skull lengths, 29.4 and 35.6 mm) of C. niloticus by Iordansky (1973) and it is absent in this genus in adult specimens (Iordansky, 1973;personal observation). Rieppel (1993) expressed doubts on the correctness of the identification of the dermo-supraoccipital of Deraniyagala (1939) and concluded that his illustrations suggest that his dermo-supraoccipital is nothing but the supraoccipital ossifying in the synotic tectum. However, Rieppel (1993), studying the process of the ossification of the otoccipital neurocranial structures of A. mississippiensis, did not exclude the possibility of the identification of this element as a dermosupraoccipital. However, the skull length of the embryo of C. palustris with the interparietal (Deraniyagala, 1939: fig. 126) corresponds to about Stage 5 A. mississippiensis, and the tb.tspϩtp is not formed in embryos smaller than Stage 7A, while the cartilaginous bridge starts to ossify endochondrally even later. In this instance, knowledge of the sequence of ossification of the neurocranial structures in the skull of Crocodylus is needed. Nevertheless, as in the case of the dermo-supraoccipital of Mook (1921), the outline of the interparietals of Deraniyagala's (1939) Crocodylus specimens corresponds very well to the outline of the trabecular bone portion of the supraoccipital (tb.tspϩtp) in A. mississippiensis.

Ossification of the Supraoccipital in
Alligator mississippiensis Rieppel (1993) found that in his 53-day-old specimen of Alligator mississippiensis the trabecular bone (of dermal appearance) covered the entire endochondral supraoccipital ossification and filled the space between the endochondral supraoccipital ossification and the posterior margins of the parietals. He found no separation between the trabecular bone (of dermal appearance) and the underlying endochondral bone in the area of the supraoccipital. Rieppel (1993) concluded that additional histological investigations are required to clarify the relation between the endochondral and dermal ossifications in the area of the supraoccipital in A. mississippiensis.
The process of ossification of the supraoccipital described here differs slightly from that observed and described by Rieppel (1993) in cleared and stained embryos. The tb.tspϩtp does not start to develop on the anterior margin of the tspϩtp, as observed by Rieppel (1993), but rather about in the middle section of the tspϩtp length. Later, it spreads anteriorwards and posteriorwards. In Stage 8A (FS 25), the tb.tspϩtp is still absent dorsal to the anterior portion of the tspϩtp. Rieppel (1993) stated that in his 53-day-old specimen (FS 25) the parietals meet in the loosely defined suture in the dorsal midline of the skull, and the trabecular bone (of dermal appearance) covers the entire supraoccipital endochondral ossification (unfortunately, he did not report the cranial length of this specimen). In Stage 9B (FS 27) described here, the parietals are already fused in their posterior sections; hence, this specimen is of larger size than that of 53 days of Rieppel (1993). However, in Stage 9B the posterior section of the tspϩtp is not covered by the tb.tspϩtp (Fig. 2D). The postparietal as described here is a small, completely independent dermal plate lying dorsal to the tb.tspϩtp. Therefore, the tb.tspϩtp of the future supraoccipital on one side and the dermal postparietal bone on the other are two different structures. The tb.tspϩtp is a bone that develops from the perichondral lining of the tspϩtp, although it does possess a dermal appearance as stated by Rieppel (1993).
The postparietal as recorded in specimen Stage 9B of Alligator mississippiensis is the only record of this bone within Recent reptiles. Within other groups of Recent tetrapods, the dermal bone that develops dorsal to the tectum synoticum and tectum posterius and almost completely or completely behind the (fronto)parietal(s) has been recorded in several Recent anurans and mammals, both in embryos and in adults.
In Recent mammals, the dermal bone that develops dorsal to the tectum synoticum plus tectum posterius is called the interparietal (reviewed by Wegner, 1960) and it generally develops as a paired element. Later in ontogenesis, both elements fuse into one median interparietal (Kadam, 1973;Köhncke, 1985;Timm, 1987). This may occasionally persist as an independent bone in adults, but normally it fuses with the dorsal portion of the enchondrally ossifying supraoccipital, and is thus included in a single occipital bone. However, in ruminants and some rodents the interparietal fuses with the parietal (Stadtmü ller, 1936). According to Jarvik (1996), the interparietal of Homo sapiens is homologous with the medial extrascapular of the osteolepiform fish Eusthenopteron foordi.
It is highly probable that all these bones, paired or unpaired, having the same topology as the postparietal in Stage 9B of Alligator mississippiensis, are homologous structures.

ACKNOWLEDGMENTS
For the embryos of Alligator mississippiensis, I am grateful to Drs. D.L. Auth and W. Auffenburg from the Florida State Museum. I thank Dr. A.R.