A new calcichordate from the Ordovician of Bohemia and its anatomy, adaptations and relationships

Abstract

The paper describes a new member of a group of Lower Palaeozoic marine fossils which partly bridge the gap between echinoderms and chordates. Evidence suggests that this group included the ancestors of the vertebrates. Its members are traditionally regarded as primitive echinoderms, but are better seen as primitive chordates with echinoderm affinities. They form a basal subphylum of chordates-the Calcichordata Jefferies 1967. The Calcichordata, in accordance with an early suggestion by GislCn, are probably ancestral to all living chordates.The new calcichydate is named Reticu/ocarposvhanui gen et sp. nov. I t comes from the Lower Ordovician Sirka Formation (Llanvirn) of Sdrka near Prague, Czechoslovakia and is placed in the family Amygdalothecidae Ubaghs 1970. I t is important because of its position in the Calcichordata. This group is divided into two very different orders-the Cornuta and the Mitrata. The Cornuta are the more primitive order and gave rise to the Mitrata, which had the structure of giant, calcite-plated tunicate tadpoles. Many features show that the new species is a very advanced cornute, closely related to the stock that gave rise to the mitrates. For this reason it is important in the general history of the chordates, since some primitive mitrate was probably the latest common ancestor of the living chordate subphyla i.e. of tunicates, of amphioxus and its allies and of the vertebrates.Being a rnitrate-like cornute, the new species allows the cornutes and mitrates to be compared more confidently than before. Four results are especially important. Firstly it is likely that the stem (=tail) of mitrates is equivalent only to the anterior part of the stem of cornutes. This is significant, because traditional views as to which was the upper surface in rnitrates have been based on stem homologies now seen as false. Secondly Reticulocarpos hanusi is adapted to stay up on very soft mud, using only the strength of the mud for support. The mitrates, on the other hand, supported themselves on soft mud by a much more reliable method resembling buoyancy. Thirdly, the new form had paired transpharyngeal eyes which are otherwise known only in mitrates, and which are the earliest type of paired eyes in chordates. Fourthly, it becomes possible to homologize the thecal plates of cornutes with those of mitrates.Reticulocarpos hanusi represents an important phase in chordate evolution dominated by the necessity of staying up on mud by a very precarious method. During this phase many pre-adaptations for swimming were acquired. Primitive mitrates, descended from a very similar form, were probably the first chordates that could swim.V prici je popdn Reticulocarpus hanusi n. gen. n. sp., novL druh primitivnich chordhi z podkmene Calcichordata Jefferies, 1967. Calcichordata jevi blizkt vztahy k ostnokoicum a takt se jim do znaEnC miry podoba ji. Zahrnujeme do nich dva Tidy, Mitrata a Cornuta, ktert byly diive, spolu s dalgimi dv'ema Mdy, Soluta a Cincta fazeny do th'dy Carpoidea Jaekel, 1900. 6 9 7 0 R. P. S. JEFFERIES...

The pre‐radial history of echinoderms

“…Jaekelocarpus has distinct gaps immediately to left and right of the mouth that may represent atrial openings, but such openings are less convincingly displayed in other mitrates. Jefferies and Prokop (1972) and Cripps (1989) believed that certain sutures in the head of mitrates might be capable of moving apart, but the evidence for this is far from convincing and has been forcibly challenged by Ruta (1999a). So, if mitrates had internal gills, but lacked convincing sutural openings, as Ruta (1999a) and Philip (1979) contend, how did they function?…”

Sessile lifestyle and origin of chordates

“…Jefferies and co-workers (Jefferies 1967(Jefferies , 1968(Jefferies , 1973(Jefferies , 1975(Jefferies , 1981(Jefferies , 1984(Jefferies , 1986Jefferies & Prokop 1972;Jefferies & Lewis 1978;Jefferies et al 1987;Craske & Jefferies 1989), with the backing of Cripps (1989aCripps ( , b, 1990Cripps ( , 1991, advanced the calcichordate hypothesis, a closely argued phylogenetic system propounding the theory that the fossil, extinct stylophoran groups (Cornuta, Mitrata, and Soluta) are true chordates, ancestral to all other chordate groups. In support of this scheme, Jefferies (1967) removed the cornutes and mitrates from their former position among the echinoderms and placed them in the new subphylum Calcichordata of the phylum Chordata.…”

Early Paleozoic Echinoderms

“…Jefferies, however, has advocated the rather revolutionary theory that they are better seen as primitive chordates with echinoderm affinities (Jefferies 1967, I 968a,b, 1969, Jefferies & Prokop 1972.1 Consequently he has called them Cakichordata, placed them in I Possible relationships of carpoid echinoderms with chordates have been suggested previously by Matsumoto (1929) and Gislim (1930). The latter, however, has expressly written that it was not his intention to assume certain primitive Paleozoic echinoderms to be the actual ancestors of the higher Deuterostomia.…”

“…No mitrate has heen discovered in deposits older than Lower Ordovician. The recent discovery of a Lower Ordovician comute with distinct mitrate features suggests possible derivation of the mitrates from a comute ancestor (Jefferies & Prokop 1972). …”

“…Jefferies, however, in order to remain consistant with his interpretation of the theca and the orientation of mitrates that he advocates, is forced to give the mitrate aulacophore an orientation opposite to that of the comute appendage, and accordingly to consider the massive ossicles of the medial and distal regions to be "ventral" in cornutes and "dorsal" in mitrates. Jefferies & Prokop (1972) try to solve this difficulty by supposing that in giving rise to the mitrates, their presumed comute ancestors have lost the medial and distal regions of their aulacophore, and evolved entirely new, yet similar, structures from the remaining proxi m al region. It will probably be agreed that the explanation is difficult to believe, when it would be so much more satisfactory to take things as they are and reorient mitrates without regard to any theory or preconceived interpretation.…”