Lepidocaris
Above: The holotype of Lepidocaris
rhyniensis. This is a half-grown individual showing the multi-segmented
trunk (a) and tail or 'telson'
(t) (scale bar = 200µm) (Copyright
owned by The Natural History Museum).
Introduction
Relationships
Morphology
Palaeoecology
One of the better-known faunal elements from the Rhynie chert, Lepidocaris
rhyniensis (see inset below) was originally described by Scourfield
in 1926. Until recently, Lepidocaris or 'the beautiful shrimp', was the
only crustacean known from the chert. It is the only species of an extinct order
of branchiopod crustaceans called the Lipostraca and its like has not
been found anywhere else than in these Early Devonian rocks at Rhynie. The
relationships between Lepidocaris and other branchiopods, its morphology
and palaeoecology are outlined below.
Above: Reconstruction of Lepidocaris
rhyniensis (in this case a female) showing dorsal (left), lateral (centre)
and ventral (right) views of the animal. The egg pouch (e) is also
shown (after Scourfield
1926) (scale bar = 1mm).
As a branchiopod, the lipostracan Lepidocaris is related to extant
forms such as the tadpole shrimp Triops, the water-flea Daphnia,
the clam shrimp Cyzicus and the
fairy shrimp Artemia. It is an important fossil because occurring as it
does in the Early Devonian Rhynie chert it represents one of the earliest known
freshwater crustaceans. As such, an understanding of this early branchiopod may
give us more of an insight into the timing and mechanism by which these
crustaceans adapted to a continental freshwater habitat.
There are a number of differences which help us distinguish this fossil
lipostracan from the other four main orders of living branchiopods.
Superficially Lepidocaris is most similar to fairy shrimps, the anostracans, Scourfield (1926) noted fifteen points
of difference. The
following is a list some of the more important distinguishing features between
all five branchiopod orders:
| Lipostracans
e.g.: Lepidocaris |
Notostracans
(tadpole shrimps) |
Cladocerans
(water fleas) |
Conchostracans
(clam shrimps) |
Anostracans
(fairy shrimps) |
| No cephalic shield in adults |
Single, broad cephalothoracic shield covering head and
thorax |
Single cephalothoracic shield enclosing body but not head |
Bivalved cephalothoracic shield enclosing body |
No cephalic shield in adults (present only in hatching nauplii) |
| Biramous 2nd antennae |
Reduced uniramous 2nd antennae in adults |
Biramous 2nd antennae |
Biramous 2nd antennae |
Uniramous 2nd antennae |
| 11 pairs of trunk limbs, of which the first pair are
modified maxillae |
35 - 71 pairs of trunk limbs; 29 - 52 of which are post
genital |
4 - 6 pairs of trunk limbs |
10 -32 pairs of trunk limbs; 1 - 16 of which are post
genital |
11 - 19 pairs of trunk limbs |
The body of Lepidocaris can be broadly divided into three parts; a
distinct head region, a body or 'trunk' and a tail. No cephalic shield is
present, except in the youngest stages (Scourfield
1940c). In many specimens the
fossils appear disarticulated or comprise isolated fragments or clusters of
material. This suggests that many of the specimens probably represent exuviae
or moults.
Forms in various stages of development have been found from probable egg
cases, through juvenile stages (Scourfield 1926,
1940c) to fully grown adults. Adults
attained a maximum length of 3mm. Consequently we are able to establish much of
the ontogeny of Lepidocaris. Both
females and males have also been identified.
Head Region
| The head of Lepidocaris comprises a conspicuous domed
capsule. On the ventral side the capsule passes into a
posteriorly directed plate, the labrum, that protects the
mouthparts. The head lacks eyes, but possesses two pairs of appendages,
the first antennae (or antennules) and second antennae, anterior to the mouthparts. The
first antennae are small, and uniramous comprising three segments or articles, the most
distal bearing very fine setae or 'hairs'. The second antennae are characteristic,
being long and biramous (see inset right). The main 'branch' comprises
eight articles, the three most distal articles bearing long plumose setae.
The second 'branch' is attached to the third proximal article of the main
branch and comprises two or three articles in female and male forms
respectively. The mouthparts comprise a pair of robust mandibles
(see inset right) and posterior to these the maxillulae. In females the maxillulae are small,
consisting of a single plate bearing a single spine and rows of setae. In
males, however, these are developed into large clasping organs (see inset
below and below right). The maxillae are greatly modified and
'leg-like' (Walossek 1993), forming
what Scourfield (1926) originally
described as the first pair of trunk limbs (see insets below). These
clearly belong to the head region, because the corresponding segment is
fused with the head capsule. The junction between the maxillae segment and
the rest of the head capsule is marked by a 'mandibular groove' (see
reconstruction towards top of page). The osmoregulatory 'neck organ', seen in
extant branchiopods, and a shared feature or synapomorphy of
Branchiopoda and Maxillopoda (Walossek
1995), has not been observed in Lepidocaris.
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Above: A slightly oblique longitudinal
section from the dorsal aspect showing the head and anterior of the body
of an articulated specimen of Lepidocaris. This is an adult
female. Head capsule (h), dislocated first antenna (n),
second antenna (a), mandibles (m),
followed by the maxillulae, the leg-like maxillae, and at least six pairs of foliaceous biramous trunk limbs (l) are shown
(scale bar = 500µm).
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Body Region
The body or trunk of Lepidocaris comprises a thorax and an apodous
abdomen. Excluding the head and tail, the trunk in adult forms comprises
seventeen segments or somites. The first fourteen ring-like somites, following
the back of the head capsule, bear lateral scales. The final three somites form
apodous, simple rings. Lepidocaris
has ten pairs of foliaceous trunk limbs with a disposition of
one pair per somite. These form a complex filter feeding system, a feature
which, together with the presence of a sternal food-groove, is characteristic of
the Branchiopoda (Walossek 1993, 1995)
The first two pairs of trunk limbs are similar in morphology to the maxillae.
These and the maxillae are modified with the terminal
articles (endopods) being slightly robust, palmate and comb-like (see
inset below and below right).
Above: Fragmentary remains of a male Lepidocaris
showing the right clasping organ (c) and parts of the biramous
maxillae and first pair of trunk appendages (t) (click on the image for a close-up of
these appendages!). A transverse section through a disarticulated somite is
also visible (s) (scale bar = 300µm) (Copyright owned by The Natural History Museum).
| The following eight pairs of appendages are far more foliaceous and delicate
(see inset right), and are rather copepod-like in their appearance. Posterior to this series of appendages in females occurs an
egg pouch and cover that may have developed from modified trunk limbs. In
males, however, the last two pairs of appendages are slightly modified,
the exopods and endopods appearing broader than those on the preceding appendages.
Right: Various lipostracan appendages (not shown to
scale): A: A clasper of a male; B: Modified maxilla (1st
trunk limb of Scourfield (1926)); C: Trunk limb of
sixth to tenth pairs; D:
Trunk limb of third - fifth pairs. The basic elements of the biramous crustacean appendage
are labeled: protopod (pr), exopod (ex), endopod (en)
and endites (ed) (after Scourfield 1926).
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Tail
| The tail region of Lepidocaris is quite distinctive. The caudal somite
or the 'telson' is slightly rectangular and elongate, being about equal
in length to the two preceding somites of the trunk. The telson possesses two
small posteriorly directed 'lateral processes' positioned just posterior to the
middle of the somite (see inset right). Behind these, emerging almost from the posterior of the
telson is a pair of long caudal furcae, each comprising a single article
ending with a cluster of four long setae. These are the secondary furcae. Between these long furcae on the
posterior margin of the telson are two smaller knob-like furcae, each bearing a
single spine-like seta (see inset right). These are the primary furcae.
Right: Tail of Lepidocaris showing a lateral process (lp),
the short primary furcae (pf) and the longer secondary furcae (sf)
(scale bar = 200µm) (after Scourfield
1926).
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Lepidocaris is perhaps one of the more abundant faunal elements found
in the Rhynie chert. It is typically found associated with charophytes,
coprolites, algal filaments and often occurs in cherts displaying a clotted or 'mulm-like'
texture and was undoubtedly an aquatic organism. The delicate, foliaceous nature
of the trunk appendages indicates Lepidocaris was an active swimmer, the
more pectinate anterior trunk appendages and maxillae quite probably being used for feeding
and food gathering.
The general ecology of Lepidocaris may have been somewhat
similar to that of modern anostracans, perhaps thriving in small temporary
ponds, feeding off algae and organic particulates. The common association of the
animal with the charophyte Palaeonitella, assuming the ecology of the
latter was similar to modern day Nitelleae, could suggest Lepidocaris, as
with many modern anostracans, preferred to inhabit alkaline waters.
One final point can be made about the palaeoecology of this enigmatic
branchiopod in comparison with modern anostracans. Based mainly on the mixture
of primitive and modified features Scourfield (1926)
observed in Lepidocaris, together with the overall lack of
contemporaneous aquatic invertebrates in the chert, he suggested that the animal
must have adapted to live in a 'very peculiar' environment. In fact he
postulated the lipostracan may have adapted to live in hot, silica-charged
water. However, there is no evidence to suggest that prior to silicification
temperatures in these inhabited pools were any different to similar freshwater
environments elsewhere, particularly considering the aquatic flora present.
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