The fossil record is an incredible thing. It gives us an invaluable glimpse into the ancient
prehistory of our planet and the life that once inhabited it, and without it we’d have very little
clue about the long and complex evolutionary history of all the wonderful organisms around
us today, or indeed our own origins. But sometimes, the fossil record turns up
something so incredibly weird and confusing that it just
sends palaeontologists absolutely mad. This is indeed the case with an organism called
Ainiktozoon loganense, a creature so bizarre that for many years since its initial description in
1937, palaeontologists were looking at this animal upside down, with it not being until 1997 that
this mistake was finally realised. Remarkably, not a great deal of research actually seems to
have been done on this organism, despite the appeal of solving such a strange fossil mystery,
and Ainiktozoon remains a pretty obscure prehistoric organism.
The first fossils of this creature were uncovered possibly as early as the mid 1890s,
with most of the original specimens used as the basis for its description
being found during a geological expedition around 1904. 32 fossils were originally recovered, and
the site of the discovery was in the Silurian-aged Ludlow Group that outcrops in Scotland.
After they were initially found, the specimens changed hands a few times.
Eventually though, in 1937, the fossils were studied and described by crustacean expert
D. J. Scourfield, who recognised that they likely all belonged to the same species and subsequently
attempted to interpret their anatomy. The main feature that was obvious in
all the specimens he studied was a so-called ‘capsule’, ovoid in shape,
that on one end connects to a broad zone of parallel bars he named the ‘palisade’.
On the upper surface of this palisade he noted some fine, branched ‘cirri’, and then also
attached to the capsule were two segmented rods, which he named the primary and secondary rods.
The Primary segmented rod was the lower one in his interpretation and possessed a series of 8 to 9
sharply pointed bifurcated spines, while the Secondary rod is almost constant in
width and meets with the primary at the end. The two rods are then also connected
along their length by structures that continue past the secondary rod and end in lobes.
9 or 10 of these lobes were said to be present and in this original interpretation they were
not thought to be paired, instead running in a single row down the middle of the rods.
Other structures that were found preserved, one of which projected from the capsule,
were named ‘fascicles’ by Scourfield. The one connected to the capsule was illustrated as
projecting downwards, tapering towards the end and also curving slightly forwards. At the end
of this fascicle was then what he thought looked like a hook that was probably paired. At the upper
front margin of one of the fossil specimens was also a patch of subtriangular, regularly arranged
little bumps that he called the ‘shagreen patch’, next to an even smaller, C-shaped
patch he named the 'auriculate organ’. Various bands were also noted as crossing the
fossil in various places, and some indistinct regions of anatomy were also briefly noted.
All in all, Scourfield was quite perplexed by what this incredibly unusual morphology
meant in terms of the animal’s relationships to other known groupings. But he felt fairly
confident that it was not a crustacean at least, and instead classified it as some kind of early,
basal chordate, the phylum to which we also belong. He therefore gave it the
name Ainiktozoon, based on the Greek for ‘riddling’ or ‘enigmatical’ animal.
It was then not until 1985 that another paper re-examining this organism was published by
Australian palaeontologist Alexander Ritchie - though it should be noted that a couple of
other researchers had made their own brief interpretations of the animal before then,
but did not add much of significance. Ritchie ultimately agreed with Scourfield’s
interpretation that Ainiktozoon was a sort of early chordate, but based on newly available
specimens that had been discovered in the years since the initial publication,
some more anatomical details were revealed. These include a mystery ‘tube’ organ inside the
body cavity, the function of which was not understood, as well as a ‘fringe’ structure
apparently attached to the tube. This ‘fringe’ comprises curved rods that display many small,
hair-like filaments along them, and actually sits in the same place as the hook structure,
but both hook and fringe never occurred in the same specimen.
The fringe was proposed to have some kind of reproductive function and thought to
be placed internally, while the hook was an external structure. The hook then also
seems to have supported a very delicate structure called the reticulate area,
the function of which was again described as uncertain.
The most significant of the new anatomical discoveries in this paper, though, was the
reinterpretation of the previously mentioned ‘shagreen patch’ and 'auriculate organ’. In
one of the newly discovered specimens these structures were preserved much more clearly,
and very obviously show that these are the remains of a compound eye made up of many tiny lenses.
The smaller, 'auriculate organ’ was the retinal body, with a clear gap separating it from the
outer lenses, represented by the ‘shagreen patch’. Interestingly, the eye was not interpreted
as being paired, with it instead looking like Ainiktozoon possessed a single, large eye in the
middle of the front-facing side of the body. Now, you may be thinking that the identification
of a compound eye would be some pretty damning evidence against this organism being a chordate,
and in favour of an arthropod affinity, considering that arthropods are well-known
for possessing this type of eye morphology. However, Ritchie defended the chordate
interpretation by explaining how a lot of Ainiktozoon’s other anatomy
does not conform with that of arthropods. This creature apparently lacked any antennae,
jointed appendages or mouthparts, was only segmented in the tail and not in other
locations across the body, and had skeletal material that was interpreted as being more
like certain unmineralised structures seen in tunicates - a type of chordate - than what is
seen in arthropods. Other justifications were also given, including arguing that the compound eye
was an example of convergent evolution, leading Ritchie to agree with Scourfield that Ainiktozoon
was a very bizarre chordate. He considered it to be one that led an active, free-swimming lifestyle
and an inhabitant of deep, quiet waters where it survived by filter-feeding.
So, while some more details of its anatomy had been discovered, this paper did not change a
great deal about our understanding of this organism from what was already
described in the original paper naming it. But then, just over a decade later in 1997,
a different team of palaeontologists quite literally turned everything upside down.
Looking at the illustrations and interpretations made by Scourfield and then expanded upon by
Ritchie, these palaeontologists realised that if the side considered to be the
upper surface was instead the lower one, the fossils actually display a lot of anatomies
that do in fact conform with arthropods. Examining a few more newly-found fossils too,
this research interprets Ainiktozoon as possessing a tall carapace at the front which covered a
segmented body, from which 8 branched limbs with paddles on the end project - these being the
lobed structures that Scourfield originally identified as projecting from the rods.
The capsule with the filamentous cirri on top, now positioned upside down,
is considered to be a very muscular foregut and the cirri thought to be gills, and the so-called
‘fascicles’ - the odd bands inside the carapace and also connected to the capsule - are said to
be traces of phosphatised muscles which would have dilated the stomach and moved the carapace.
One of the newly discovered specimens at the time of this research’s publication
also revealed something else that was very interesting.
Near the front of the body, under the carapace, were what looked like a set of raptorial limbs
all lying one on top of the other. These limbs, said to look like ‘jack-knives’ by the authors,
had large segments that were equipped with tough spines, and were presumably used in
actively hunting prey items - so this was not a slow-moving filter-feeder.
All of this evidence therefore resulted in the palaeontologists reclassifying Ainiktozoon as
an arthropod - specifically in a class called Thylacocephala. This grouping contains a number
of other very weird-looking members and lived from potentially as early as the Cambrian until
the Late Cretaceous. The exact classification of this clade within the arthropods is itself
a bit of a mystery too, but it’s possible that they represent a lineage of crustaceans.
And that’s about the last bit of major research that’s been done on this organism.
The relatively little amount of work that’s been done on this bizarre animal has honestly surprised
me as I researched this video, I thought that significantly more would have been done on such
a mysterious and extraordinary-looking animal, but I guess a combination of
its obscurity and the difficulty involved in determining its true anatomy and affinities
has led to Ainiktozoon slipping under people’s radar or possibly even caused them to just be
put off studying such a difficult animal. Maybe this video will help to change things,
I certainly hope it’s able to bring some more attention to this prehistoric enigma.
I have no doubt that eventually more research will be done on this animal and hopefully more
specimens will be found too, and I honestly wouldn’t be surprised if other palaeontologists
end up interpreting it differently again. Whatever happens, it’ll be very interesting to see.
Anyway, I really hope you enjoyed this video and learned something new.