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Postparietal and Prehatching Ontogeny of the Supraoccipital in Alligator Mississippiensis (Archosauria, Crocodylia)

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Postparietal and Prehatching Ontogeny of the Supraoccipital in Alligator Mississippiensis (Archosauria, Crocodylia) JOURNAL OF MORPHOLOGY 249:147–153 (2001) Postparietal and Prehatching Ontogeny of the Supraoccipital in Alligator mississippiensis (Archosauria, Crocodylia) Jozef Klembara* Comenius University, Faculty of Natural Sciences, Department of Ecology, Bratislava, Slovak Republic ABSTRACT The first record of the postparietal bone of pilae. The bridge probably represents the fused tectum Alligator mississippiensis, documented by transverse his- synoticum posterior plus tectum posterius. Later in ontog- tological sections, is presented. It is the first evidence of eny, the bridge ossifies endochondrally. The endochon- the presence of this bone within Recent reptiles. The post- drally ossifying bridge together with its perichondrally parietal is present in a specimen with a head length of ossifying trabecular bone form the future supraoccipital. 32.3 mm. The bone is a small dermal plate lying ventrally The trabecular bone is the integral part of the cranial and posteriorly to the posterior margin of the parietal and endoskeleton and ontogenetically distinct from the dermal dorsally to the trabecular bone, forming a dorsal surface of postparietal bone. J. Morphol. 249:147–153, 2001. the supraoccipital portion of the neural endocranium. The © 2001 Wiley-Liss, Inc. trabecular bone develops perichondrally from the dorsal surface of the tectal cartilaginous bridge spanning be- KEY WORDS: Crocodylia; Alligator mississippiensis; on- tween the dorsal portions of the otic capsules and occipital togeny; postparietal; supraoccipital; homology Although there is no general agreement on what Since the earliest investigations of the skull anat- bone in osteolepiform ancestors of tetrapods (extras- omy of vertebrates in the nineteenth century, only capular medial or the bone[s] lying immediately to two articles record the possible presence of the post- it) is homologous with the postparietal in tetrapods parietal bone in Recent reptiles. Both records were (reviewed by Jarvik, 1996), there is general agree- made on crocodylians: in Alligator mississippiensis ment that the postparietal bones (sometimes fused (described as dermo-supraoccipital by Mook, 1921) into an unpaired bone) of early tetrapods lie at the and in two species of Crocodylus: C. palustris and C. posterior margin of the dermal skull roof and may porosus (described as interparietal by Deraniyagala, descend as flanges onto the posterior surface of the 1939). Mook’s dermo-supraoccipital was identified in neurocranium (Romer and Parsons, 1977). In the dried skulls and Deraniyagala’s interparietal in evolutionary line leading to mammals, the postpari- dried skulls and cleared embryos. etals gradually obtain the position at the posterodor- The postparietal was not recorded in the two sal margin of the occiput. The postparietal bone is cleared embryos of Crocodylus niloticus studied by synonymously also termed the dermal supraoccipi- Iordansky (1973). Rieppel (1993) studied the process tal (or dermo-supraoccipital) or interparietal (Ro- of ossification of the supraoccipital of Alligator mis- mer, 1976; Romer and Parsons, 1977). sissippiensis on cleared and stained embryos, but Within Recent tetrapods, bones that are homolo- did not find a separate dermal postparietal ossifica- gized with the postparietal bones of the extinct tet- tion in his material. All hatchling and embryonic rapods develop ontogenically dorsally to the posteri- specimens of A. mississippiensis studied by Brochu ormost cranial tecta, tectum synoticum, and tectum (1999) lacked the postparietal bone. posterius, and have been occasionally recorded as The aim of this article is to describe prehatching independent bones in various ontogenetic stages in ontogeny of the supraoccipital of Alligator mississip- anurans and mammals. During ontogeny, these in- piensis and the first discovery of a completely inde- dependent bones may fuse with the frontoparietals in anurans (Rocˇek, 1981; Smirnov, 1992); in mam- mals they fuse with the dorsal margin of the enchon- Contract grant sponsor: the Scientific Grant Agency of the Ministry drally ossifying supraoccipital cartilage to form to- of Education of Slovak Republic and Slovak Academy of Sciences; gether the occipital bone (Romer and Parsons, 1977; Contract grant number: 1/6162/99. Jarvik, 1996). However, they may also persist as *Correspondence to: Jozef Klembara, Comenius University, Faculty independent bones in adult anurans (Smirnov, of Natural Sciences, Department of Ecology, Mlynska´ dolina, 842 15 1977) as well as in mammals (Wegner, 1960). Bratislava, Slovak Republic. E-mail: [email protected] © 2001 WILEY-LISS, INC. Figure 1 POSTPARIETAL OF ALLIGATOR MISSISSIPPIENSIS 149 pendent postparietal dermal ossification recorded on Stage 7, in which the perichondrally ossifying transverse histological sections of an embryo of A. tb.tspϩtp first appears. It is followed by a descrip- mississippiensis. A cartilaginous bridge, spanned tion of the progressive growth of the tb.tspϩtp up to between the posteriormost dorsal portions of the otic Stage 11A, at which stage the boundary between the capsules and the occipital pilae, which ossifies as the endochondral ossification of the tspϩtp is almost supraoccipital, is variously called the tectum synoti- indistinguishable from the tb.tspϩtp. The presence cum (Rieppel, 1993) or tectum posterius (Shiino, of the dermal postparietal bone in the Stage 9B is 1914). It probably represents the tectum synoticum related to the ontogenetic status of the tb.tspϩtp. posterior plus tectum posterius (tspϩtp) (de Beer, The latter structure conveys the impression of being 1937; Klembara, 1991). a dermal ossification and it has been considered by Rieppel (1993) calls the bone that develops from some previous authors (see Discussion) to be a sep- the dorsal surface of the tspϩtp during the early arate postparietal dermal ossification. ontogeny of Alligator mississippiensis the trabecular bone (of dermal appearance). With growth, this bone Stage 7A fills the space between the endochondral supraoccip- ital ossification (ossifying in the tspϩtp) and the In Stage 7A the anterior margin of the tspϩtp ex- posterior margins of the parietals. For this portion of tends into a pair of plate-like processes (plp.tspϩtp, the future supraoccipital, which Rieppel (1993) calls Fig. 1A) that are not connected by their lateral mar- the trabecular bone (of dermal appearance), I also gins with the otic capsules and that are not con- use the term trabecular bone. It designates the tra- nected together in the mesial plane. The dorsal sur- becular architecture of the bone developing peri- face of the posterior portion of the tspϩtp, in the chondrally from the dorsal surface of the tspϩtp. It depression bilaterally flanked by the cartilaginous is designated the tb.tspϩtp here. elevations of the posteriormost portions of the otoc- cipital region of the endocranium, starts to ossify MATERIALS AND METHODS perichondrally. The perichondral cells form a dis- tinct lining from which several clusters of cells For this study, the following transversely sec- spread slightly dorsally. They represent the initial tioned ontogenetic stages of Alligator mississippien- phase of the formation of the future bony trabeculae sis arranged according to the head lengths were (tb.tspϩtp, Fig. 1B). used (FS — stages after Fergusson [1985]): Stage 5 (FS 21) — 17.9 mm; Stage 6B (FS 23) — 22.3 mm; Stage 8A Stage 7A (FS 24) — 23.8 mm; Stage 8A (FS 25) — 25.4 mm; Stage 9B (FS 27) — 32.3 mm; Stage 10A In Stage 8A, as in Stage 7A, the anterior margin of (FS 27) — 33.1 mm; Stage 11A (hatchling) — 36.5 the tspϩtp bears two plp.tspϩtp. Immediately pos- mm. The thickness of histological sections of Stages teriorly, the tspϩtp is not covered with the trabec- 5, 6B, 7A, 8A, and 9B is 12 ␮m and of Stages 10A ular bone. The bony lining and very short trabeculae and 11A is 15 ␮m. The histological sections were cover the posteriormost section of the anterior third stained with Alcian blue, erythrosin, and Mayers’s and the whole middle third of the length of the hematoxylin. The specimens are from Florida, Dade tspϩtp (tb.tspϩtp, Fig. 1C). It may be noted that in County, Everglades National Park (collector: Dr. its posterior section the layer of the tb.tspϩtp is J.A. Kushlan), USA. Histological sections were ex- dorsoventrally thicker than the underlying cartilag- amined and photographed using a WILD M8 ste- inous tspϩtp (Fig. 1D). Some of the bony trabeculae reomicroscope. spread slightly dorsally from the bony lining of the tspϩtp; however, most of them are situated more RESULTS dorsally in the tissue and with no connection with the bony lining. The posterior section of the poste- The following description of the ontogeny of the rior third of the tspϩtp is not covered by trabecular supraoccipital of Alligator mississippiensis starts at bone. The cartilage of the tspϩtp shows no trace of resorption. Fig. 1. Alligator mississippiensis. Transverse sections through Stage 9B the supraoccipital portion of the otoccipital region of the neuro- In Stage 9B both of the anterior plate-like pro- cranium of Stage 7A (A,B) and Stage 8A (C,D) in anterior to posterior sequence, respectively, showing the relative positions of cesses protruding from the anterior margin of the the structures of the future supraoccipital. br, brain; ot.c, otic cartilaginous bridge (plp.tspϩtp) are still present, as capsule; otoc, endochondrally ossifying posterolateral portion of in the two preceding stages. The tspϩtp is partially the otoccipital region of the neural endocranium;
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