Mongolarachne
Name:
Mongolarachne
(Mongolia spider).
Phonetic: Mon-go-lah-rak-ne.
Named By: P. A. Seldon, C. K. Shih
& D. Ren - 2013.
Synonyms: Nephila jurassica.
Classification: Arthropoda, Arachnida, Araneae,
Araneomorphae, Mongolarachnidae.
Species: M. jurassica
(type).
Diet: Carnivore.
Size: Female total body length estimated at 24.6
millimeters long, with forwards length 56.5 millimetres long.
Male total body length 16.54 millimetres long.
Known locations: Inner Mongolia - Jiulongshan
Formation.
Time period: Mid Jurassic.
Fossil representation: Partial female and male
specimens.
Mongolarachne
was originally described in 2011 as Nephila jurassica,
a
prehistoric species of the Nephila genus of orb
weaving spiders that we
know today. Nephila jurassica made headlines at
the time because it
was the largest known spider in the fossil record, though comparable
in size to modern golden orb weaver spiders that are alive today. It
was also credited as being the oldest known species of the Nephila
genus, extended the temporal range of the genus back one hundred and
thirty million years.
Then
two years later a new study by Kuntner et al cast
serious doubt upon
the fossil spider’s inclusion into the Nephila
genus. The main
argument against the fossil inclusion into the Nephila
genus was that
it appeared to be a cribellate spider while known members of the
Nephila genus are ecribellate. The difference is
that ecribellate
spiders spin a sticky silk out of their spinnerets to snare prey like
insects. Cribellate spiders have a Cribellum organ different to other
spinnerets in that instead of weaving sticky silk, it produces a
woolly silk which has a super fine series of fibres that are very
effective at trapping insects.
The
suggestion that the spider fossils in question were of a cribellate
spider lead to new analysis of the specimens with study of a male
individual. When the pedipalps (appendages between the fore legs
and fangs) of the male specimen were studied they were found to
actually have a very different construction to known males of the
Nephila genus, and this combined with the
identification of the
fossils being of a cribellate spider led to only one possible
conclusion: the fossils did not represent a species of Nephila.
This led to the creation of the Mongolarachne
genus, and since
normal practice when establishing a genus from a species is to retain
the old species name, Nephila jurassica became Mongolarachne
jurassica.
At
the time of its description (and time of writing) Mongolarachne
jurassica represents the largest fossil spider known to us,
and
although now known to not be related females of Mongolarachne
jurassica are known to comparatively equal to the females of
the
Nephila genus in size. Like in most spider
species, the males of
Mongolarachne jurassica are much smaller than the
females, and just
like with modern spiders, the males may have occasionally been eaten
by females when they approached to court them.
When
described as Nephila jurassica, Mongolarachne
was often
reconstructed as sitting within a broad spider web like other orb
weaving spiders, though as a cribellate spider such a web
construction would be highly unlikely do to the type of silk spun by
the spider. Some cribellate spiders don’t spin silk for prey
capture, instead preferring to chase prey down, while others spin
tangle webs to ensnare passing insects, while other still use he silk
to reinforce a burrow so that they can leap out at passing insects that
pass a sensory silk line (usually monitored by the spider keeping a
foot on the line so it can feel vibrations along it).
Some
cribellate spiders however have a very elaborate method of prey
capture, spinning a net like web and casting it over their prey.
This might be a plausible method of hunting for Mongolarachne
since it
is morphological similar to these ‘net casting’ spiders. Woolly
silk stretches well and when allowed to shrink back to size will trap
and snare almost anything that is within it. Some spiders will
stretch this net over their prey and allow it to collapse while others
will stretch it out between their legs, scoop up their prey into the
net and then bring their legs in to trap the prey in the net.
It
is impossible to say if this was the hunting method for Mongolarachne
as it is impossible for us to see one hunting in the wild, but as a
cribellate spider, we can at least have a better idea about it. In
addition, most cribellate spiders seem to be either nocturnal, or
preferring to reside in dark areas such as caves and cervices in trees
or amongst rocks. This would mean that Mongolarachne
were probably
hidden away from potential predators such as small dinosaurs during the
day.
Further reading
- A golden orb-weaver spider (Araneae: Nephilidae: Nephila)
from the Middle Jurassic of China. Biology Letters 7 (5):
775–8 - P. A. Seldon, C. K. Shih & D. Ren
- 2011.
- A molecular phylogeny of nephilid spiders: Evolutionary history
of a model lineage". Molecular Phylogenetics and Evolution 69
(3): 961–979 - M. Kuntner, M. A. Arnedo, P.
Trontelj, T. Lokovše & I.Agnarsson - 2013.
- A giant spider from the Jurassic of China reveals greater diversity
of the orbicularian stem group. Naturwissenschaften 100 (12):
1171–1181 - P. A. Seldon, C. K. Shih & D.
Ren - 2013.
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