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Evolutionary
Biology
31, 105-154 (2000). Conclusions.
There are several anatomical traits shared by the most primitive Early
Cambrian chordates Yunnanozoon, the Middle Cambrian probable
westergaardodinid Odontogriphus, and the Vendian problematicum Dickinsonia,
and their body plans resemble that of the nemerteans. The rhynchocoel of the
nemerteans and (at least) the most anterior dorsal chamber of Dickinsonia may
appear thus homologous, which is consistent with the idea of its homology with
the notochord and myocoel of the chordates. The phosphatic elements of the
westergaardodinid ‘paraconodont’ apparatuses seem to represent a
stage in the development of mineral skeleton transitional between nemertean
stylets and conodont elements. Odontogriphus had an oral grasping
apparatus composed of such mineralized, probably phosphatic elements. Some
kind of oral apparatus of unknown internal structure also occurred in Yunnanozoon,
which probably also bore complex sensory head organs. Dickinsonia, Odontogriphus,
Yunnanozoon, and Pikaia may thus represent a developmental
series, from muscular dorsal chambers to the organization of myomeres typical
for all later chordates (Fig. 13). In this respect even cephalochordates and
tunicates are anatomically highly derived. Amphioxus and the tunicates
share the presence of the filtratory basket surrounded by the atrium, which
indicates that they are closely related and evolutionarily late. The earliest,
more or less reliable, fossil evidence of this structure is in the
Carboniferous Typhloesus. It shares with Recent salps a very
characteristic concentration of the alimentary tract in a globular body (although
bilobed in Typhloesus). Notably, the latest of the anaspid agnathans
with weakly mineralized body covers, the Late Devonian Legendrolepis,
shows a very elaborate gill apparatus (Arsenault and Janvier, 1991),
suggestive of filtratory adaptations. The time order in appearance of these
anatomies is consistent with the idea that salps and Amphioxus derived
from chordates related to the anaspids. This implies a more complex anatomy of
the common ancestor of the tunicate and cephalochordate evolutionary branches
than usually assumed, but is consistent with neontological data by Lacalli
(1996) and Williams and Holland (1996). The mode of secretion of the conodont
crown tissue, with its early mineralization, and distribution of shapes of the
secretory cells suggest that localized intensity of secretion of calcium
phosphate, instead of cell migration, was the main factor controlling the
morphology of elements. Calcium phosphate secretion was extremely high at
denticle tips which requires (as phosphate and calcium ion transport took
place above the surface of the element) a relatively thick cover of soft
tissue to fulfill supply needs. In typical conodonts, this tissue was probably
protected on the outside with horny caps. The primitive panderodontid
conodonts had a grasping apparatus composed of at least seven element pairs
morphologically similar to that of the Cambrian and Recent chaetognaths. Any
homology of their crown tissue or the organic cap, with the grasping spines of
the chaetognaths (‘protoconodonts’) is unlikely. The typical
organization of the conodont apparatus originated in the early Ordovician
within the clade of coniform Protopanderodontida. Their apparatuses were
composed of a pair of incisor-like elements in front, a set of four pairs of
relatively gracile elements connected into a single unit by one posteriorly
located symmetrical element, and two pairs of robust elements hidden within
the throat. This apparatus architecture did not undergo any basic
modifications in the ozarkodinid and prioniodontid clades. In the most
advanced prioniodontids (e.g., Promissum) the apparatus was probably
evertible, with the throat being sinuously folded at the resting position.
Elements of some locations were doubled in these conodonts. A homology between
the organic caps of elements of advanced conodonts and horny jaws of Recent
hagfish is likely, which would make the Myxinoidea close relatives of the
conodonts or even members of their class. The homology between the crown and
basal filling tissues of the conodonts and the enamel and dentine of
vertebrates, respectively, implies that the developmental mechanisms which
arose at the origin of the conodonts were subsequently used by the agnathans
to build up their protective dermal scales. |