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Australia: The Land Where Time Began |
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Jawed Vertebrates - Marginal Dentition and Multiple Dermal
Jawbones as Ancestral Condition
In extant fishes and land vertebrates the dentitions vary in patterns as
well as type of tooth replacement. The common ancestral condition has
been argued to have been likely to have resembled the nonmarginal,
radially-arranged tooth files of arthrodires, which were an early group
of armoured fishes. In this study microtomography was used to describe
the fossil dentitions of
acanthothoracids,
the most basal, phylogenetically, group of jawed vertebrates that had
teeth of the genera
Radotina,
Kosoraspis, and
Tlamaspis (that were
found in Early
Devonian deposits in the Czech Republic). The dentitions of these
genera differ fundamentally from those of arthrodires; they are
marginal, carried by cheek bones or a series of short dermal bones along
the edges of the jaw, and teeth are added lingually, as is the case in
many chondrichthyans
(cartilaginous fish) and osteichthyans (bony fish and tetrapods).
Vaškaninová et al. proposed
these characteristics as ancestral for all jawed vertebrates.
One of the key events in evolutionary history from jawless to jawed
vertebrates was the origin of teeth. There are whorl-like tooth files
arranged transversely with lingual addition in stem osteichthyans
(Botella et al., 2007; Chen
et al., 2016) and most
chondrichthyans (Smith & Coates, 2001).Teeth are added in diverging
files from the progenitor tooth or region in arthrodires, a clade of
jawed stem gnathostomes,
and the number of files is highly variable, and addition can be labial
as well as lingual or longitudinal (Smith, 2003;
Řrvig, 1980; Sith & Johanson, 2003;
Smith & Johanson, 2003; Rücklin et al.,
2012; Hu et
al., 2019). A proposed
evolutionary succession (Zhu et
al., 2016) in which a
bridge is formed between an arthrodire-like condition (which is assumed
to be primitive for jawed vertebrates) and the osteichthyan condition
(assumed to be primitive for the crown gnathostomes), as the result of a
recent discovery of osteichthyan-like marginal dermal jawbones combined
with an arthrodire-like
body plan in
Entelognathus and
Qilinyu (Zhu
et al., 2013; Zhu
et al., 2016). The
dentitions of
Entelognathus and
Qilinyu are, however,
poorly understood.
In this study dentitions have been presented of the acanthothoracid stem
gnathostomes
Radotina,
Kosoraspis, and
Tlamaspis from the Early
Devonian of the Czech Republic, and the only described acanthothoracid
dentition, specimen CPW.9 from the Canadian Arctic (mistakenly assigned
to
Romundina by some
authors) (Smith & Johanson, 2003; Goujet & Young, 2004) has been
re-examined. The only jawed vertebrates that have an anteriorly
projecting precerebral trabecula region (Dupret
et al., 2014) are the
Acanthothoracids, antiarchs and
Brindabellaspis (Young,
1980). This distinctive facial geometry resembles those of jawless stem
gnathostomes especially galeaspids (Kuratani & Ahlberg, 2016). Antiarchs,
Brindabellaspis, and
acanthothoracids as basal to other jawed vertebrates (Zhu
et al., 2016; Dupret
et al., 2014; Qiao
et al., 2016), a
position they retain when the (not analysed previously) genera that are
discussed herein are included in the analysis. The jaws and dentition of
Brindabellaspis are not
known, whereas antiarchs don’t have teeth (Young, 1984). The teeth of
acanthothoracids have, therefore, the unique potential to illuminate the
origin of vertebrate dentition. In the dentition of CPW.9, the tooth
addition, which comprises a pair of supragnathal plates on a snout
(Goujet & Young, 2004), has been described as concentric (Smith &
Johanson, 2003; Smith, Clark, Goujet & Johanson, 2017). As a result of
this, a dermal tessera that is detached that has concentrically arranges
odontodes from the same formation was misinterpreted as an
acanthothoracid supragnathal (Smith, Clark, Goujet & Johanson, 2017;
Rücklin & Donoghue, 2015; Burrow, Hu & Young, 2016). By reanalysing
micro-computed tomography (micro-CT) scans of CPW.9 the supragnathals
suture labially with the dermal premedian plate. Tooth Addition is
radial, in fact, from a labial founder region, and there is no addition
of a labial tooth. The oldest small teeth in the founder region become
overgrown by larger dermal odontodes instead, as was observed in the
stem osteichthyans
Andreolepis (Chen
et al., 2016).
In this study propagation phase contrast synchrotron microtomography was
used, and the previously unknown dentition of
Radotina,
Kosoraspis, and
Tlamaspis was discovered
(Vaškaninová & Ahlberg, 2017). All differ substantially from that of
CPW.9. The dentition of
Radotina was comprised of
4 rows of teeth that were carried on the ventral face of a large dermal
cheek bone that was sutured to the external surface of the
palatoquadrate. A ventral trough is formed by this ventral face, and the
teeth are carried on the labial flank and the floor of the trough that
is marked by a row of vascular grooves. The dentition is, therefore,
unambiguously marginal, and is located inside the margin of the jaw but
external to the palatoquadrate, and a distinct labial edge of exposed
bone from the external surface of the cheek. Addition of tooth rows is
from the lingual side, and there is a pronounced mineralisation gradient
from the oldest to the youngest teeth (S4). The shape of the teeth is
elongated bladelike stellate that matches that of dermal ornament
odontodes on the tesserae of the cheek. Unlike those odontodes, however,
a distinct basal attachment tissue fuses the teeth into rows. The arms
of the stellate teeth, which consist of pallial mesodentine, is attached
to a central cone of circumpulpal semidentine (Řrvig, 1997; Řrvig,
1875). The large spaces of odontoblast clusters that are enclosed in the
arms anastomose with the small, sparser cell spaces in the cone, across
the boundary between the pallial and circumpulpal dentine. Each arm is
equipped with a blade that is enameloid coated. This is shown to be a
functional cutting dentition by well-developed use facets on the teeth.
A broken tooth in the lingualmost row has been repaired with a new crown
inserted into the break, and there is no evidence of resorption or
replacement. There are multiple short jawbones that bear teeth in
Kosoraspis (Gross, 1959)
and
Tlamaspis (Vaškaninová &
Ahlberg, 2017). In
Kosoraspis their length
and shape vary according to their position, though they invariably bear
a prominent facial Lamina. The facial lamina carries tooth like
odontodes that grade into recurved, pointed teeth on the oral lamina,
forming whorl-like files that are obliquely transverse, resembling the
first generation tooth-like odontodes and non-shedding teeth in
Andreolepis (Chen
et al., 2016). Their
shape differs from the stellate odontodes on the cheek. The odontodes
and teeth both consist of parallel semidentine and the circumpulpal
dentine that infills the pulp cavity in mature teeth is an atubular
dentinous tissue with fewer cell spaces that are enclosed. The facial
lamina of
Tlamaspis, by contrast,
is mostly unornamented labially, with the exception of 2 or 3 rows of
tubercles that abruptly transition into a single row of conical teeth,
which are bordered lingually by a toothless flange. Though the teeth are
not shed or replaced this is reminiscent of the marginal jawbones of
crown osteichthyans. In this case the histology of the tooth is not
known.
Acanthothoracids are separated from the gnathostome crown group by 4
internodes in the phylogenetic analysis that was undertaken in this
study. There are, nevertheless, distinct similarities between dentitions
acanthothoracids and osteichthyans. There are, notably, short dermal
jawbones, which are to those of
Kosoraspis
Tlamaspis, are present in
the stem osteichthyan
Lophosteus, which has
acanthothoracid-like dermal ornament (Jerve
et al., 2016). In
Kosoraspis the cheek
pattern also approaches closely that of primitive crown osteichthyans
(Zhu, Yu & Janvier, 1999). Vaškaninová et
al. suggested that the
possibility that the gnathostome crown is rooted close to
acanthothoracids, with arthrodires as well as certain other so-called
placoderm groups forming a divergent clade rather than a paraphyletic
segment of the gnathostome stem, should be seriously considered.
The acanthothoracid dentitions that are newly described exhibit a
combination of shared characteristics and remarkable diversity. There
are functional and architectural aspects to the diversity of dentitions.
It appears that the dentition of CPW.9 is functionally adapted to
crushing, that of
Radotina for cutting, and
those of
Kosoraspis and
Tlamaspis adapted for
different styles of grasping and piercing. The most striking distinction
is, architecturally, between
Radotina in which the
teeth resemble ornament odontodes and are attached to the cheek plate –
and the 3 other taxa, all with separate dermal jawbones and have teeth
that do not resemble their dermal ornament. The teeth in all the
dentitions are non-shedding and no evidence has been found of resorption.
All are carried by dermal bones that have either ornamented facial
laminae or (as in the case of CPW.9) suture to facial bones; none show
labial tooth addition. Among the least crownwards jawed vertebrates
their phylogenetic position suggests that both marginal dermal jawbones,
are ancestral for all gnathostome dentitions.
Vaškaninová, V., et al. (2020). "Marginal dentition and multiple dermal
jawbones as the ancestral condition of jawed vertebrates." Science
369(6500): 211-216.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |