The different groups of Lissamphibia – Part 1
As exposed in
the previous article, lissamphibians, or modern amphibians, is a monophyletic
clade including four groups: Albanerpetontidae, Apoda, Salientita, and Urodela.
Let have a focus on the two least known: the mysterious apods and the cute
albanerpetontids!
I)
Albanerpetontidae
Figure 1: Representation of an albanerpetontid (credits: nix).
Probably the
least known clade among lissamphibians, mostly because it is extinct (and
therefore not truly “modern”), but also because its position within
Lissamphibia or as a sister-group is still open to debate (Matsumoto and
Evans, 2018). Albanerpetontids constitute a small group of
amphibians, salamander-like forms, that occurred from the Middle Jurassic to
the Late Pliocene (Gardner et al.,
2003; Venczel and Gardner, 2005; Delfino and Sala, 2007).
Albanerpetontid
are found either in vertebrate microfossil assemblage, in fissure fills, or in
floodplain deposits. The fossil record is mostly located in Laurasia, the
supercontinent grouping North America, Europe, an Asia, although one occurrence
is from the Early Cretaceous of Morocco. Up to date, 5 genera and 13 species
have been described, and more material has been attributed to undetermined genera
or species. However, some of these descriptions may require a revision, since
the phylogeny of this group retains some issues.
Figure 2: Cladogram of Albanerpetontidae, with their time and
geographical range.
One of the
major problems with albanerpetontid lies in their material mostly biased toward
fragmented and isolated elements. However, despite this issue, Albanerpetontids
are characterized by a small set of synapomorphies easily recognizable:
1.
the
fused and dorsally ornamented frontal;
2.
the
“mortise and tenon” interdentary joint, where right and jaw dentary
symphysis bear an interdigitating joint;
3.
the
amniote-like atlas-axis complex involving a tripartite facet;
4.
the
chisel-shaped, non-pedicellate, tricuspid teeth.
Very few
complete specimens are available (McGowan and
Evans, 1995), which befog their identification and study for
systematics, especially in the Mesozoic period. Indeed, as it can be hard to recognize,
the post-cranial skeleton bears very few characteristics and it is not included
in systematics analyses. Furthermore, fused frontals are not only a
synapomorphy of the group, but they can also be used as diagnostic features to
segregate the different species.
Figure 3: Frontals from
different genera of albanerpetontids (from Matsumoto and Evans, 2018)
Their
morphology would suggest their fossorial ecology (Estes and
Hoffstetter, 1976). They supposedly lived in humid soil near freshwater
ponds, where the skull would be used as a ram or a shovel, feeding on
arthropods with tough chitinous shell (Wiechmann, 2000), thanks to the shearing bite allowed by their
dentition (Gardner, 2001).
However,
albanerpetontids constitute one major group of microvertebrates from the Late
Jurassic of Portugal. Indeed, they represent one of the most abundant taxa in
Guimarota mine, with thousands of remains recovered, but hundreds have also
been found in the Lourinhã Formation (Wiechmann, 2003;
Guillaume et al., 2019), and they are now under description.
II)
Apoda
Figure 4: Representation of
an apod (credits: Manimalworld).
Another
poorly known amphibian clade outside herpetology. They are a small group of
vermiform animals that occurred in the Early Jurassic. One of their difficulties
lies in the scarcity of their fossil record, especially for the Mesozoic period
(Jenkins and Walsh, 1993; Evans and Sigogneau‐Russell,
2001; Jenkins et al., 2007). The known species fit poorly within the modern
clades, which require the use of molecular data to understand their
evolutionary pattern (Roelants et al.,
2007; Zhang and Wake, 2009; Wilkinson et al., 2011; Schoch, 2014).
Figure 5: Cladogram of Apoda,
with the occurrence of some noticeable fossil species.
Due to their
highly specialize morphology, apods share a unique set of synapomorphies:
1.
the
trunk greatly elongated, exhibiting an increase of presacral vertebrae, ranging
from 95 to 285;
2.
as
a consequence, and as other vertebrates with similar adaptation, lungs are
asymmetric, the right one being the larger;
3.
the
skin is segmented with primary and secondary rings, the former
correlating with myosepta;
4.
male
individuals display a phallodeum, an unpaired, intromittent organ formed
by a projected portion of the cloaca;
5.
the
absence of limbs and girdles in all extant and extinct species, with the
unique exception of Eocaecilia, the most basal taxon;
6.
the
short tail skeleton, which can be even lost in some species;
7.
the
presence of a massive skull, weakly fenestrate or entirely closed, with
solid fusion of the different bones;
8.
as
a consequence, eyes are largely reduced, or can even be covered by
dermal bones in some species;
9.
the
presence of a tentacular organ between the eye and the nose, used a chemosensory.
The extant
groups have a tropical distribution, and molecular analyses suggest their
dispersal road may be linked to the breaking of Gondwana during the Cretaceous (Zhang and Wake,
2009). However, the early radiation should have occurred
before the breaking of Pangea (Roelants et al.,
2007; Wilkinson et al., 2011). As reflected by their skull morphology, they are
fossorial and nocturnal animals. They live mostly in the topsoil layers and occupy
a full range of environment: decaying plant material, humus, wet mud of
riverbanks, or fully aquatic habitats. They usually feed on earthworms and arthropods
(beetles and termites), but the biggest species can also prey on lizards, snakes,
and even small birds!
Figure 6: Geographic
distribution of extant apods.
Nowadays, no
apods are living in Portugal and unfortunately, none have been found in the
fossil record neither. However, if we are not focusing on them in Lourinhã, it
does not mean they should be forgotten! We still have a lot to learn about
them, especially since recent studies may suggest a different path in their
evolution (Pardo et al.,
2017), and we will go back on this when we talk on the
origin of Lissamphibia.
Next time, we
will see the Batrachia, the clade that groups both Salientia and Urodela, the best-known
amphibians. Until then, stay home and be safe!
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