The Pterygoid Series in Hippocampus zosterae and Syngnathus scovelli (Pisces: Syngnathidae)

Abstract

The presence or absence of the endochondral metapterygoid has long been disputed in syngnathoid fishes, owing to the fusion and expansion of cranial bones in the atypical morphology of the syngnathoid cranium. The presence of precursor cartilage and subsequent ossification of elements of the syngnathoid skull have also been questioned. Osteological examination of embryonic through juvenile specimens of the dwarf seahorse, Hippocampus zosterae (4.5-9.0 mm notochord length (NL)); 9.0-18.0 mm TL), and the Gulf pipefish, Syngnathus scovelli (5.0-9.0 mm NL; 11.0-52.0 mm TL), show clear evidence of the presence of the endochondral metapterygoid and dermal ectopterygoid in early embryonic stages. The endopterygoid appears shortly after parturition in both species. The presence of precursor cartilage for the metapterygoid was also noted. The sup- posed absence of the metapterygoid reported by many workers is because of the elongation and fusion of this bone with other cranial elements by the early juvenile stage. Earlier investigations of the pterygoid and circumorbital bones are reviewed in light of this new evidence. The presence or absence of the endochondral metapterygoid has long been disputed in syngnathoid fishes, owing to the fusion and expansion of cranial bones in the atypical morphology of the syngnathoid cranium. The presence of precursor cartilage and subsequent ossification of elements of the syngnathoid skull have also been questioned. Osteological examination of embryonic through juvenile specimens of the dwarf seahorse, Hippocampus zosterae (4.5-9.0 mm notochord length (NL)); 9.0-18.0 mm TL), and the Gulf pipefish, Syngnathus scovelli (5.0-9.0 mm NL; 11.0-52.0 mm TL), show clear evidence of the presence of the endochondral metapterygoid and dermal ectopterygoid in early embryonic stages. The endopterygoid appears shortly after parturition in both species. The presence of precursor cartilage for the metapterygoid was also noted. The sup- posed absence of the metapterygoid reported by many workers is because of the elongation and fusion of this bone with other cranial elements by the early juvenile stage. Earlier investigations of the pterygoid and circumorbital bones are reviewed in light of this new evidence. THE presence and homology of the endo- chondral metapterygoid bone in syngna- thoid fishes has been a subject of dispute since the late 1800s. The presumed absence of this structure has even been proposed as a distinc- tive feature of the family (ungersen, 1910; Berg, 1940). Misidentification of the metapterygoid and lack of consensus as to its existence stems in part from the atypical morphology of the syngnathoid skull. The elongation of the sym- plectic and its great separation from the hyo- mandibular make it difficult to ascertain the ho- mologies of the pterygoid series. Additionally, in adult syngnathoids extensive laminae are sent out from several bones, encasing the walls of the tubular snout, a condition which prompted Swinnerton (1902) to suggest that only a study of ontogeny could determine the true distri- bution of the original cartilage. THE presence and homology of the endo- chondral metapterygoid bone in syngna- thoid fishes has been a subject of dispute since the late 1800s. The presumed absence of this structure has even been proposed as a distinc- tive feature of the family (ungersen, 1910; Berg, 1940). Misidentification of the metapterygoid and lack of consensus as to its existence stems in part from the atypical morphology of the syngnathoid skull. The elongation of the sym- plectic and its great separation from the hyo- mandibular make it difficult to ascertain the ho- mologies of the pterygoid series. Additionally, in adult syngnathoids extensive laminae are sent out from several bones, encasing the walls of the tubular snout, a condition which prompted Swinnerton (1902) to suggest that only a study of ontogeny could determine the true distri- bution of the original cartilage. In this paper, I present evidence for the ex- istence of the metapterygoid in syngnathoids based on developmental osteology of the dwarf seahorse, Hippocampus zosterae, and the Gulf pipefish, Syngnathus scovelli. Ontogenetic devel- opment of the chondrocranium in both species is described for embryonic through post-par- turition juvenile stages. The findings of pre- vious researchers on osteology of the syngna- thoid skull are compared and discussed.