Diplocercides kayseri from the Upper Devonian. The species 

 of this group share several distinctive features and are easily 

 identified. E.\cept for apparent reduction in ossification and 

 fragmentation of some bones, Latimena is typical of this 

 group, which differs in many ways from the other crossop- 

 terygians. These differences, such as the rostral organ — a 

 sac-like structure which has three openings to either side on 

 the snout, the reduction of the bones of thejaw margins, and 

 the cranial roof and cheek pattern, suggest specialization. 

 The peculiar ossifications of the endocranium suggest an in- 

 dependent history of fragmentation of the solid structure of 

 the ancestral form. The peculiar hyomandibula is also note- 

 worthy. Only in the single-pieced ceratohyal is there a 

 greater resemblance to the amphibian than that shown by 

 other crossopterygians. 



Dipnoan 



This last group of the choanates contains the lungfishes. 

 Their fossil history began in the Lower Devonian with 

 Dipniirhynchus but it was not until the Middle Devonian 

 Difilerus platycephalus that much detail was preserved. 



Dipnorhynchus is represented by two good cranial roofs, 

 showing the supraorbital sensory canal completely separate 

 from the temporal canal; there is also a parietal foramen 

 (Figure 4-35). In Dipterus (Figure 4-36) these canals joined 

 and there was a distinct anterior pit line and canal; the parie- 

 tal foramen was lacking. In both genera the snout was cov- 

 ered by an unbroken cosmine layer, which concealed the 

 underlying plates, if present. The external nostril lay on the 

 margin of thejaw and there was no evidence of premaxilla 

 or maxilla. The cheek was much shortened but it had three 

 canal units in about the position of those of the porolepi- 

 form; this suggests homology. The last of these units partly 

 overlay the posterior end of the mandible and connected 

 directly with the mandibular canal series. The quadrato- 

 jugal was present and bore a pit line. 



There was a temporal series of bones comparable to that 

 of the rhipidistian, but here the similarity ends. There were 

 many individual roof plates, which appear to be quite vari- 

 able in shape, interrelationships, and even occurrence. In 

 the history of this group, there appears to have been a com- 

 petition for space among the individual plates, with the liv- 

 ing species retaining only a small number. 



The endocranium, known from the Devonian genus 

 Chirudipterus, was well ossified and formed a single unit to 

 which the palatoquadrate was fused (autostylic type of jaw 

 suspension) (Figure 4-37). Covering the ventral aspect of the 

 cranium was a large parasphenoid with a hypophyseal fora- 

 men near its anterior end. The notochord was unconstricted. 

 The visceral skeleton is unknown for the early forms. 



Of the living forms, Neoceratodus (Figure 4-38) appears to 

 be least modified (more like the fossil forms in appearance). 

 The cranial roof is much altered in this group, and the 

 various plates can only arbitrarily be assigned names. 



.Among living fishes the lungfishes are peculiar in having 

 midline plates in the cranial roof The endocranium is car- 

 tilaginous, except for the exoccipitals, which in Protopterus 

 (Figure 6-59) meet above the foramen magnum. The vis- 

 ceral skeleton oi Neoceralndus is especially of interest since it 

 appears to be intermediate between the crossopterygian and 

 amphibian types. 



The hyomandibula is small and cartilaginous in Neocera- 

 todus; it is lacking in Protopterus and Lepidosiren. A second part 

 of the hyoid arch lies below the head vein and the hyoman- 

 dibular nerve; it is fused with the otic capsule below and 

 behind the posterior entrance of the cranioquadrate fissure. 

 This cartilaginous process connects by a ligament with the 

 cartilaginous process connects by a ligament with the 

 palatoquadrate and with the suspensory ligament of the 

 ceratohyal. 



The ceratohyal is a single ossification with cartilaginous 

 ends; it articulates with the hypohyal in front. The hypohyals 

 are joined across the midline by connective tissue and 

 articulate in front with a short cartilaginous basihyal. In 

 the position of a urohyal there is a cartilaginous rod. There 

 are five branchial arches. The first four consist of epibran- 

 chials, which are only nodules, and cartilaginous ceratobran- 

 chials. The fifth arch has only the ceratobranchials; it has no 

 hypobranchials, basibranchials. or copula. 



supraorbital canal 



parietal foramen 



spiracular 



Figure 4-35. Cranial roof of Dipnorhynchus sussmtichi (After Westell, 

 1949) 



CHOANATE FISHES • 99 



