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Conodont-Vertebrate Phylogenetic Relationships Revisited
An evidence-based reassessment of the phylogenetic relationships of
conodonts by Turner & Burrow has shown that conodonts are not “stem”
gnathostomes, vertebrates or craniates. According to Turner & Burrow
there is a significant group of workers studying conodonts that have
proposed or accepted a designation of craniate for the conodont animal,
and this interpretation is increasingly becoming established as accepted
“fact”. The conclusion of Turner & Burrow, against this trend is that
based on a revised cladistics analysis of traditional morphological
features of both discrete conodont elements and apparatuses,
histological investigation, and a revised cladistic analysis that
modified that used in the keystone publication that has been promoted as
proof of the hypothesis that
conodonts are vertebrates. The results of the study by Turner &
Burrow suggests that conodonts were possibly not even chordates, but
Turner & Burrow demonstration of this is beyond the scope of this paper.
In summary there is low cephalisation in conodonts; the presence of
simple V-shaped trunk musculature and unique large-crystal albid in the
elements; lack of dermal skeleton, including characteristic hard tissues
of bone, dentine and enamel; lack of odontodes with bone attachment and
a unique pulp system; lack of paraxial elements that are segmentally
arranged and dermal elements in median fins, all of which, according to
Turner & Burrow do not support a vertebrate or a craniate relationships
for conodonts.
Conodonts are small animals that are exclusively marine that date from
the Palaeozoic to
the Early Mesozoic
eras. Ever since their phosphatic parts, calcium phosphate or apatetic
conodont “elements” were first described in 1856 by Pander their
zoological relationships have been vigorously debated. An example is the
listing of almost 50 publications (Müller, 1981) for the period
1856-1975 that suggested a variety of affinities with 10 different
taxonomic entities, including such diverse groups as vertebrates,
annelids and plants. Chinese terminology has 11 different words
equivalent to higher taxa for conodonts (Wang Cheng-Yuan, pers. comm. to
ST, 1984). Most relationships were easily dismissed (Sweet, 1988:
170-184), including strong evidence against any affinity with myxinoids
(hagfishes), even questioning a relationship with chordates.
Sweet (1988: 172, 173) discussed the claim (Tillier & Cuif, 1986)
though provided no refutation to
their claim that conodonts may be related to aplacophoran molluscs (but
see Briggs et al., 1987),
where they noted similarities between the 2, as they discovered calcium
phosphate in the teeth and mandibles of one aplacophoran taxon. Calcium
is also found in several “invertebrate” taxa such as in phyllocarids,
the stylets of nemerteans, shells of some brachiopods, and is likely to
also be found in the radula teeth of some chitons (Watabe, 1990),
therefore the conclusions of Tillier & Cuif (1986) should, according to
Turner & Burrow, be considered as being based on comparisons that are
rather simplistic. The analysis by Sweet (1900) of possible affinities
of the conodont organism led to him concluding that they could probably
best be assigned to a separate phylum, and was the result of yet another
evolutionary experiment that eventually went extinct.
In this paper Turner & Burrow continue the debate as they do not
accept the increasingly prevailing paradigm that conodonts are
vertebrates.
In this paper Turner & Burrow concentrate on only the euconodonts (=
conodonts s.s.) or “complex”
conodonts, as did (e.g. Donoghue & Aldridge, 2001, Donoghue et
al., 2000, 2008), as well as
others, who claim the conodonts are vertebrates, restricting their
hypothesis of conodont interrelationships to euconodonts.
Conclusions
This paper was written as a further refutation of the hypothesis that
conodonts are “stem
gnathostomes” or vertebrates. According to Turner & Burrow conodonts
are not recognised as a group of early vertebrates that had experimented
with biomineralisation of their skeletons. Turner & Burrow suggest that
conodonts might represent a
cephalochordate grade of evolution at most, similar to the amphioxus
Branchiostoma Contra,
(e.g. Donoghue et al., Kuhn &
Barnes, 2005; Janvier, 2008), the plesion Conodonta (Eichenberg, 1930),
is in no way a sister group or a member, stem or otherwise of the phylum
Craniata (Linnaeus, 1758), nor the superclass Gnathostomata (Cope,
1889). Turner & Burrow also say that the placing of higher taxa
Conodontaphorida or Conodonta in the Chordata (Bateson, 1886) or
Craniata (e.g. Farrell, 2004; Nelson, 2006; King et
al., 2009) is not accepted
for the reasons outlined above. Based on the evidence that is provided
here, Turner & Burrow support the hypothesis that:
·
The phylogenetic status of conodonts
s.l. that includes proto-,
para- and euconodonts has not been resolved; at the present the 3 groups
are informal, with proto-conodonts probably not being monophyletic. It
is considered (Müller & Hinz-Schallreuter, 1998) that the 3 groups are
related. The idea that protoconodonts that belong to the evolutionary
lineage of
Phakelodus is probably
the stem group of chaetognaths is favoured by Szaniawski (2002). An
evolutionary lineage that has been well documented from a paraconodont
species to a euconodont species has been described (Szaniawski &
Bengston, 1993). According to Turner & Burrow authors who relate
euconodonts to craniates consider the 3 groups have different
phylogenetic relationships;
·
Conodont elements are not odontodes and do not have vertebrate hard
tissues, which includes globular cartilage, bone, lamellin, dentine,
enameloid or enamel, and lack vertebrate morphogenesis; Turner & Burrow
say that conodont elements that are broken could be repaired during the
life of the animal and, therefore, the elements had to be infolded in
tissue, at least at times, unlike odontodes, scales, teeth etc.
·
Conodont elements are not homologous with vertebrate teeth and do not
represent the first vertebrate experiment in skeletonisation or feeding
apparatus that are mineralised;
·
Conodont animals (euconodont) animals are not craniates, vertebrates nor
“stem gnathostomes”.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |