Early ontogeny and distribution of the orthocerid cephalopod Calorthoceras Chen, 1981 - taxonomic and palaeoecological implications

Calorthoceras Chen, 1981 (?Middle Ordovician, Llandovery, to Middle Devonian) is an orthocerid with a straight, annulated shell and a reticulate sculpture.Three slightly curved, endogastric apical parts of shells of C. butovitzense Chen, 1981 from the Silurian of Bohemia are described.The most apical part of the shell is blunt and smooth, bordered by a relief fold.The adjacent conical part of the shell shows an accretion structure and is straight or curved.The following part of the shell is straight and slowly expanding.A change in sculpture and shell shape indicating the embryonic conch occurs at shell heights of 1.3-1.6 mm and shell lengths of 4.4-4.7 mm.The first annulus appears at a shell height of 1.6-2.5 mm and a shell length of 5.5-13.5 mm.The dimensions of the embryonic shell indicate a maximum egg size of about 10 mm.The morphology of the juvenile shell agrees with a pelagic habit.Calorthoceras is reclassified into the family Dawsonoceratidae Flower, 1962.The morphology of the early ontogenetic stages of the shell demonstrates the affinities of Calorthoceras to the Early Devonian Cyrtoceras pugio Barrande, 1866 and Suloceras pulchrum (Barrande, 1968).The shell of these cephalopods shows morphological convergence with Spyroceras Hyatt, 1884 in which the juvenile stages were probably demersal.The distribution of the Silurian Calorthoceras is discussed, particularly in the Prague Basin where it represents one of the immigrants from the Baltic-Avalonian area.Merocycloceras declive Ristedt, 1968 and Dawsonoceras multiliratum Foerste, 1928 are described for the first time from the Silurian of the Prague Basin.