In 2000, field surveys conducted by the University of Aberdeen resulted in
the discovery of several chert float blocks near Castle Hill farm approximately
1.5km east of the known Rhynie and Windyfield chert sites (see Rice
et al. 2002). The cutting and preparation of one of these chert
blocks led to the discovery of a new branchiopod crustacean, named Castracollis
wilsonae and recently described by Fayers
and Trewin (2003). Numerous specimens were discovered in one 50mm thick
interval of the chert block, comprising near complete and fragmentary remains. This is the first new crustacean described from the
chert since Lepidocaris rhyniensis (Scourfield
Comprising a long multi-segmented thorax and abdomen, the thorax bearing two
distinct sets of numerous appendages; and possibly possessing a large
cephalothoracic shield, this animal belongs to the phyllopod lineage of the
Branchiopoda and appears to be closely related to the Calamanostraca (a
phyllopod group that includes the modern Tadpole Shrimps (the Notostraca)
and the extinct Kazacharthra). It also shows some
similarities in terms of its head morphology with the Onychura (the other
phyllopod group that includes the Water Fleas or Cladocera and the Clam Shrimps
or Conchostraca). For reviews of these other branchiopods and their
relationships refer to Walossek (1993, 1995).
Castracollis is the earliest known calmanostracan-like branchiopod in
the fossil record. Although track-ways attributed to similar animals are known
from the Lower Devonian, until now the earliest known fossil Calmanostraca have
been recorded from the Carboniferous.
The following table shows some of the differences and similarities between
the morphologies of Castracollis,
the Notostraca, Kazacharthra, Lipostraca and Conchostraca:
|Possible cephalothoracic shield covering head and thorax
||Single, broad cephalothoracic shield covering head and
||Single, broad, often ornamented, cephalothoracic shield
covering head and thorax
||No head shield in adults
||Bivalved cephalothoracic shield enclosing body
|Long biramous 2nd antennae
||Reduced uniramous 2nd antennae in adults
||Reduced biramous 2nd antennae
||Long biramous 2nd antennae
||Long biramous 2nd antennae
|Distinct anterior and posterior series of trunk limbs: 21 -
26 pairs of which the posterior 10 -15 are relatively small (and possibly
||Distinct anterior and posterior series of trunk limbs: 35 -
71 pairs of which the posterior 29 - 52 are relatively small and post
genital with variable number of leg pairs per body segment
||11 pairs of trunk limbs (no distinct posterior series)
||11 pairs of trunk limbs, of which the first pair are
||10 -32 pairs of trunk limbs; 1 - 16 of which are post
|Lateral scales on leg-bearing body segments
||Lateral scales absent
||Lateral scales absent
||Lateral scales on leg-bearing body segments
||Lateral scales absent
Castracollis was a small branchiopod, the largest specimens found
being 8mm in length. The majority of the specimens are fragmented, and the more
complete specimens show damage to the anterior of the trunk and the head is
often detached. This suggests most are exuviae or the moults of the
The crustacean has a large head, possibly bearing a broad cephalothoracic
shield covering the thorax, with a long multi-segmented trunk ending with a
telson bearing two prong-like furcal rami. The multi-segmented trunk comprises a
thorax and abdomen of near identical ring-like somites (see insets below).
Above: An almost complete specimen of
Castracollis wilsonae with interpretative drawing, showing the multi-segmented thorax
(Th) and abdomen (Abd), with the remains of the head (H).
Also shown is the anterior series of long, raptorial thoracic appendages (Th.aa)
and the posterior series of short, phyllopodous, thoracic appendages (Th.pa)
(scale bar = 1mm).
|The head is often damaged, particularly the
dorsal area, and therefore 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 this animal. Some specimens
exhibit what may be a detached cephalothoracic shield (see inset below). The head appears to
be rounded towards the anterior where it bears a conspicuous notch,
possibly representing the position of the naupliar eye (see inset below). First
antennae (antennules) have not been observed, but the second
antennae are distinctive in that they are long and biramous (see inset
right), with symmetrical multi-segmented rami - a feature seen in extant
Onychura rather than Calmanostraca (Fayers and
Trewin 2003). Ventrally a large spatulate labrum extends
posteriorly from between the second antennae and covers the mouth parts
(inset below). Above this and behind the second antennae are a pair of
large, robust mandibles each comprising a single coxa. The distal,
mesial or grinding surface where they meet bears numerous, fine, parallel
ridges. Small maxillulae are present behind the mandibles.
Above: Near ventral view of a segmented,
biramous second antenna (A2) attached to fragmented remains of the
head (H). A section through the left mandible (Md) is also
shown. Scale bar = 500mm.
Above: Near longitudinal section through the
head and anterior of the body showing the base of the left second antenna (A2),
mandible (M), labrum (L) and anterior notch (N). This
specimen also shows the detached remains of a possible cephalothoracic shield (C).
Scale bar = 1mm.
|The body is long and multi-segmented, comprising a thorax
and abdomen (see inset right). The
thorax, comprising between 21 and 26 segments, bears the leg appendages with a
disposition of one pair per segment. The abdomen is apodous
and variably comprises up to 28 segments. The segments of the body form single
rings, each bearing a row of short spurs on the posterior margin. The
segments of the thorax each bear a pair of lateral scales above where the
legs are attached (see inset right).
|Right: The holotype of Castracollis
wilsonae. Abbreviations: A2 = second antenna, Abd =
abdomen, Fu = furcal rami, H = head, Tel = telson, Th.aa
= anterior thoracic appendages, Th.pa = posterior thoracic
appendages, Tp = lateral scales. Scale bar = 1mm.
The younger growth stages of Castracollis show fewer segments on the
abdomen (see inset below), and were probably added during each successive moult.
Above: Drawing of the body and tail of a
young individual showing very few abdominal segments. Abbreviations: Fu = furcal rami, Tel = telson, Th.pa = posterior thoracic
appendages. Scale bar = 200mm.
The tail of Castracollis comprises a telson with a pair of furcal rami
(see insets above and above right). The telson is oblate to cylindrical and
longitudinal, sub-parallel rows of spurs on the dorsal surface. The furcal rami
are attached to the posterior of the telson where they are surrounded by a
series of small spurs. Each ramus comprises a single,
long spinose article terminating in an articulated spine.
|Each thoracic segment bears a single pair of leg appendages,
and can be divided into an anterior and posterior series, based primarily
on their size (see insets below and right). The leg morphology is very
similar, but not identical, to that of notostracans and kazacharthrans.
The relative number and disposition of the legs differs from these last
two calmanostracan orders (see comparative table above).
series consists of 11 leg pairs, up to 1.5mm in length and each of similar
morphology (see insets right and below). Each leg comprises a basipod, bearing on the inner (mesial) surface 5 lobate
endites with an
array of comb-like setae. The distal 4 endites also bear a single, long
tubular spine. Distally the basipod bears an articulated, raptorial and
paddle-like endopod fringed with sharp spines and setae. Laterally, the
distal portion of the basipod also bears a large, triangular, sheet-like
exopod fringed with a row of long, fine setae.
The endopods would have been used in tearing up food items, and the
setose endites used for moving the food up towards the mouth. The
sheet-like exopod was probably used in locomotion. An epipod (expected to
be positioned proximal to the exopod, and used as a gill) has not been
observed on any of the specimens.
The posterior series of legs, of which there are variably between 10
and 15 pairs with a disposition of one pair per segment, are similar to
the anterior series in their morphology, but are less than half to two
thirds the length of the latter and far more delicate in their appearance
(see inset right). These gradually decrease in length towards the
posterior of the thorax (see insets below).
Above: Reconstructions of typical
thoracic appendages of Castracollis, viewed from the anterior. That on the
left is from the anterior series of appendages and that on the right from
the posterior series. Scale bar = 500mm.
Above: Longitudinal cross section of a near
complete specimen, viewed from the right, showing the differentiation of the
thoracic appendages into an anterior (th.aa) and posterior (th.pa)
series. Also shown are segments of the thorax (ths) and lateral scales
(tp), together with part of the abdomen (abd). One of the
mandibles (md) is also visible. Scale bar = 1mm.
Above: Reconstruction of the crustacean Castracollis
wilsonae (Fayers and Trewin 2003),
shown with a hypothetical cephalothoracic shield. The animal in the lower
right of the image is next to axes of the charophyte Palaeonitella
was undoubtedly an aquatic organism, indicated by the morphology of the leg
appendages and their similarity to those of extant notostracans, namely being
foliaceous with sheet-like exopods. The associated biota of Lepidocaris,
various multi-cellular, filamentous and unicellular cyanobacteria together with
charophytes indicate a freshwater environment. The chert matrix that hosts the Castracollis
material displays a ‘clotted’ texture of aggregates of amorphous organic
matter, coprolites and the filamentous meshworks. These textures are strongly
reminiscent of mulm, the fluffy, amorphous organic-rich material found in many
modern freshwater ponds (Anderson and Trewin 2003;
Fayers and Trewin 2003).
modern notostracans, Castracollis appears to have been
predominantly a detritivore and probably also a facultative predator. It may
well have had a short life cycle, similar to notostracans, developing and
moulting rapidly in temporary freshwater ponds.