Branchiopod nauplii
Above: Cluster of at least five branchiopod
larvae or nauplii in a thin section of Windyfield chert (scale bar = 200µm).
Copyright owned by the University of Aberdeen.
Introduction
Occurrence
Morphology
Identity
As
well as the Lepidocaris rhyniensis, Scourfield
1926, the Windyfield chert has sourced other crustacean remains, namely
numerous specimens of branchiopod nauplii. All have been found within clotted
and coprolitic chert textures, almost exclusively within brecciated, nodular cherts.
These are currently being described for publication in the scientific
literature.
Above: A single branchiopod larva or
nauplius viewed from the anterior aspect (anterior-most portion of the
head, first antennae and right (left as viewed) second antenna truncated by the thin section).
For the meaning of the abbreviations see the insets below (scale bar = 100mm).
Copyright owned by the University of Aberdeen.
The
branchiopod nauplii have been found either as aggregates of numerous individuals
(see inset at top of page) or as isolated specimens (see inset above), in close
association with very well preserved specimens of the charophyte Palaeonitella cranii, and suspended within clots and cumuli of
amorphous organic matter and coprolites. The organic-rich clots and cumuli,
faunal elements and charophyte remains locally form discrete aggregates, which
are in turn enveloped by meniscate mucilages of microbial filaments crossed by
fungal hyphae. The association of charophytes, coprolites, and a clotted
organic-rich matrix with these crustaceans, together with their morphology (see
below) indicates these were freshwater organisms. The clotted and filamentous
organic framework appears open, and the remarkable preservation of the
charophytes, fungi, coprolites, and the crustaceans indicates that
silicification occurred whilst the material was still in an aquatic environment.
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The crustacean remains are almost
certainly larval stages since only the cephalic (head) region of the
animal, the first and second antennae, and mandibles are present (see
insets right), there being no evidence of any maxillary appendages, a
post-cephalic trunk with delineated segments, or any trunk appendages.
The total length of the nauplii has not been fully
ascertained, mainly due to the orientation of the specimens in the thin
sections, and also because of their otherwise fragmentary nature, but on
the whole it appears to be at least 300mm. Excluding the length of the appendages, the
body width is up to 250mm, corresponding to the width of the cephalic shield (CS).
The dorsal area of the cephalic region comprises a
broad, dome-like cephalic shield, made of very thin, smooth cuticle (see
insets right). The anterior of the head has not been fully discerned, but
it generally appears to be rounded. Ventrally a broad labrum (La)
extends from between the position of the second antennae (A2),
posteriorly towards, and overlapping the position of the mandibles (Md).
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Above: Views of the same nauplius at different levels
of focus, showing the first antennae or antennules (A1), second
antennae (A2), cephalic shield (CS), labrum (La)
and mandibles (Md) (scale bar = 50mm).
Copyright owned by the University of Aberdeen.
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Above: Interpretative
drawing of the specimen in the insets above right, viewed from the anterior,
showing the first and second antennae (A1 and A2), the latter
comprising the bifid seta of the proximal endite (BS), endopod (En)
and multi-segmented exopod ramus (Ex). The cephalic shield (CS),
labrum (La) mandible (Md) and mandible palp (Plp) are
also shown (scale bar = 100mm). Copyright owned
by the University of Aberdeen.
Appendages
Of the appendages, only the first antennae (A1),
second antennae (A2) and mandibles (Md) are present. The first antennae appear
uniramous and simple in their morphology (see inset above), comprising six discrete segments. The sixth, most distal
segment possesses at least three long hairs or setae on the distal extremity.
The remaining segments are variably setose.
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are the
second antennae (A2), each comprising a long multi-segmented outer ramus or
exopod (Ex), a smaller endopod (En) and a large proximal endite with bifid seta
(BS) (see inset above). The multi-segmented exopod comprises between eleven and twelve tubular segments of similar
morphology. The most distal article is rounded or blunted and bears up to three
long terminal setae, up to 250mm in length. All the other articles possess a single, medially
positioned, long seta up to 250mm in length, which together with the three setae on the most distal
element gives the exopod a very plumose appearance. The base of each long seta
and the distal margin of each article display an array of smaller setae (see inset right).
The endopod (En) comprises two or possibly three
articles, and is up to 100mm in length. Distally, on what may possibly be a
third article, are up to four setae.
The exopod ramus appears to be articulated directly
with a large proximal endite. This appears
large and robust, and bears a
pronounced bifid seta (BS) (see insets above).
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Above: Close-up of an exopod segment on a
second antenna showing the single long seta, broken in this case (above
centre) and the disposition of the finer setae surrounding it (scale bar =
50mm). Copyright owned by the University of
Aberdeen.
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Posterior to the second antennae are the mandibles (Md),
comprising a proximal endite or coxa, and an articulated ‘palp’ (Plp)
. The morphology of the mandibular palp is similar to
that of the exopod of the second antennae, forming a multi-segmented ramus, comprising at least seven to
eight tubular segments. The setation of the segments is the same as that for the
exopods of the second antennae.
The mandibular coxa, or proximal endite is around
half the size of the proximal endite of the second antenna, and is broadly
rectangular in shape. The proximal endite typically appears lobate, supporting
an array of spine-like setae (see inset right).
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| Right: Isolated mandible coxa showing the
array of spine-like setae (scale bar = 50mm).
Copyright owned by the University of Aberdeen. |
The nauplii are undoubtedly branchiopodan in their
design, especially when comparing the appendage morphology to those of the
nauplii of the Euanostraca, or Brine shrimp (e.g. Walossek 1993, Fig.
44A). The
remains in the Windyfield chert are undoubtedly true naupliar or larval stages
since only the first and second antennae and mandibular appendages occur, and at
most only a small, undivided hind body is present. This is comparable, for
example, with the first or possibly second instar of the euanostracan Artemia
salina (Walossek 1993 and references therein).
Scourfield (1940c) described
two young individuals of Lepidocaris
rhyniensis, both showing a post-cephalic trunk, delineated into segments and
bearing at least rudimentary trunk appendages. The younger of these two
individuals (Scourfield 1940c, Fig. 2) shows six trunk appendages (not including
the modified, trunk limb-like second maxilla), which equates to the tenth instar
of Artemia salina. Therefore these new crustacean remains represent the
earliest ontogenetic stages or instars of a branchiopod found to date in either
the Windyfield or Rhynie cherts.
No post-naupliar instars or adult forms that
belong to the same animal are present in the same chert samples. This suggests the
nauplii may have died very early on in their development, possibly due to a
sharp change in water conditions, such as the introduction of erupted hot-spring
fluids into their aquatic environment. If the nauplii are of Lepidocaris,
the smaller number of articles in the exopods of the second antennae and
mandibles in the post-naupliar stages could be explained by a reduction in the
number of articles from the larval stage during the following ontogenetic
stages. At present, we cannot say with any conviction if the Windyfield chert
nauplii are of Lepidocaris, or if they
may pertain to another form of anostracan branchiopod hitherto unknown in the
cherts, but the similarities observed between these nauplii, extant euanostracan
nauplii (e.g. Artemia), and the known post-naupliar stages of Lepidocaris
suggests they almost certainly belong in the anostracan lineage.
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