Branchiopods are primitive crustaceans whose history extends back at least to the Devonian, yet some are highly successful today. Most fossil branchiopods do not show many features that lead to an understanding of function, but the Devonian lipostracan Lepidocaris is an exception. Recent work on living branchiopods, especially anostracans and anomopod cladocerans, enables Lepidocaris to be considered as a living animal. Work on extant anostracan nauplii also makes it possible to deduce how its larval stages swam and collected food.

Other early crustaceans are briefly considered and comments are made on the concept of “living fossils”. While rates of morphological and genomic evolution are often discordant, the concept is still useful. Some branchiopods, such as the Notostraca, display remarkable morphological stasis: on the basis of morphology, a Triassic and a present-day form appear to be conspecific. The latter may be referred to as a “living fossil”.

The outstanding morphological stasis of the crustacean nauplius is noted. Its existence in Cambrian times, deducible from its distribution among modern taxa, has been confirmed by Müller's finds. Such stasis, involving part of the genome of organisms whose adults display enormous adaptive radiation, has persisted with relatively small modifications since before the commencement of the entire radiation of vertebrates.

An analysis of swimming in living crustaceans is presented in order to elucidate the range of ways this function has been achieved, and to reveal the principles which constrain it. The study focuses on Gnathophausia ingens, a primitive, bathypelagic malacostracan that swims with thoracic exopods and pleopods. These structures consist of a muscular peduncle and one or two flagella that are fringed with setulate setae. The basic motion is rowing with the limb and setal fan extended on the power stroke and flexed on recovery.

A survey of other crustaceans shows that rowing with this type of swimming structure dominates throughout, although paddles often replace the flagella. Particularly pervasive is the large relative area of setae, whose effectiveness must stem from the ability to extend and flex passively and from the high drag generated on the power stroke by the setules at low Reynolds numbers.

A review of reconstructions of Palaeozoic trilobites and marrellomorphs reveals the likelihood that if swimming was the function of the exites, they operated inefficiently or were employed in other methods as well. Sculling and drag reduction on the recovery stroke through feathering rather than flexion are possible alternatives. The more common occurrence of paddle-like limb shafts and blade-like marginal structures in other Palaeozoic arthropods is also noted.

Living Crustacea possess many types of sensory receptors that detect environmental signals. These structures allow the speculation that similar organs existed in fossil arthropods and were involved in similar ways in the fossil forms when they were extant.

Posture, for example, is difficult to reconstruct in fossils without the understanding that internal proprioceptors monitoring position and movement are crucial in determining the placement of limbs, and the position of the abdomen and other jointed portions of the body. These receptors are associated also with autotomy reflexes through soft cuticle detectors that respond to strain not associated with joints.

Many receptors are placed on the outer surface of the animal, the eyes being the most obvious. The eyes receive no attention in this paper, but descriptions are given of a number of different types of setae, serving both mechanical and chemical sensors. The possession of a pore at the apex (or sub-apex) of a seta or setule probably indicates a chemoreceptor function, but its absence does not necessarily indicate a lack of such sensitivity. A range of forms is illustrated.

The dorsal organ is a recently discovered structure in decapod larvae. It is composed of a central single gland cell opening via an aperture in the midline just behind the rostrum. It is bounded by four depressions each containing a central ‘nipple’. These prove to be sensors, with ciliated endings (bifid) that end in a highly modified cuticle at the depression nipple. The function is unknown, but it is proposed that it represents an unusual chemoreceptor.

A survey is given of the functional morphology of extant Chelicerata with special reference to segmentation of the body, segmentation and articulation of the legs, coxal glands, mouthparts and ingestion, respiratory organs, sperm transfer, and development. This is followed by a discussion of primitive types, evolution and diversity, the transition from aquatic to terrestrial life, and the affinities of extant chelicerate groups.

The modes of life of twenty three species of arthropods from the Shale are reviewed, with special reference to locomotion and feeding. Six groups are recognised, predatory and scavenging benthos, deposit-feeding benthos, scavenging and possibly predatory nektobenthos, deposit-feeding and scavenging nektobenthos, nektonic filter-feeders, and a miscellaneous sixth group that includes a questionable example of parasitism and a species that doubtfully grazed on algae. These animals had but limited powers of walking, digging, raking or swimming. Within these limitations a range of morphological adaptations and modes of feeding had been evolved, by Middle Cambrian time, a range comparable to that found in Recent marine forms. Arthropods in the Shale dominated, in numbers of individuals and possibly in biomass, a fauna dwelling above, on and in a muddy substrate at a depth of about 100 m.

Extensive preparation since 1975 has yielded thousands of specimens of various minute arthropods, mainly phosphatocopid ostracods and other crustaceans. The whole exoskeleton is exceedingly well preserved in three dimensions, showing even delicate mo1 phological details. The material permits not only the description of the morphology and systematic status of the animals but also interpretation, with a high degree of confidence, of the function, life habit, and ontogeny of these early arthropods.

Tealliocaris is probably one of the best preserved of all Carboniferous crustaceans. It occurs scattered through the 12 cm thick laminated Gullane ‘shrimp-bed’. The cuticle is exceptionally well preserved and the specimens, which are normally complete, are infilled with fluorapatite. The carapace has an elongate rostrum, a defined frontal area, a median ridge and paired gastric and inner and outer lateral ridges. The eyes are borne on a stout peduncle. The first antenna bears a pair of flagella, the second an antennal scale and long flagellum. The mandible is large and heavily sclerotised with molar and incisor process. The maxillae and anterior thoracopods are poorly preserved. The posterior six thoracopods each have a long segmented endopod and annulated setose exopod. More than half the specimens preserve lamellae which may represent branchial epipods, or floor a brood chamber beneath the carapace on either side of the thorax. The pleopods are biramous. The telson and uropods form a tail fan. In the absence of oostegites Tealliocaris cannot be assigned to the order Mysidacea but its affinities lie close to these Eumalacostraca. Tealliocaris walked on the thoracic endopods, but could also swim using the pleopods and thoracic exopods. The mode of feeding is uncertain. Tealliocaris probably inhabited a brackish lake or lagoon, but its distribution elsewhere suggests that it may have been tolerant of a range of salinities.

Macrurous decapod crustaceans are generally uncommon and are often incompletely preserved. However, at certain horizons in the Speeton Clay (Ryazanian–Albian) and Atherfield Clay (Lower Aptian) of Yorkshire and the Isle of Wight respectively, macrurans form the dominant element of the fauna. Two species in particular, Meyeria ornata (Phillips) and Meyerella magna (M'Coy), are not only very common in these formations, but are also very well preserved. Recent collections have yielded a large number of specimens of both species. This material is examined with the aid of SEM techniques, gross morphology is discussed, and the details of the exoskeletons are compared. In the case of M. magna, examples are figured which show traces of a colour pattern and the effects of regeneration on the first pereiopods. A discussion is included of the possible function of the appendages in M. magna and M. ornata, and of their other, unique features. Finally, the mode of life of both species is inferred from a consideration of their morphology, and of the evidence from their preservation, distribution and relationships with associated faunas.

This paper reviews the methods which have been used to deduce the life habits of trilobites. The most reliable conclusions are those that satisfy three independent criteria of evidence: (1) functional morphology of the exoskeleton; (2) analogy with living arthropods; and (3) geological evidence, as from facies relationships, or palaeogeography. Pelagic trilobites are one of the better examples for life habit reconstruction. The Ordovician trilobites that were most probably pelagic are those with hypertrophied eyes—Telephinidae, Opipeuteridae, Bohemillidae and Cyclopygidae—which satisfy the threefold requirements for evidence. Opipeuter, Carolinites (and other telephinids) were probably epipelagic, whereas cyclopygids and Bohemilla were likely to have been mesopelagic, living in exterior sites around the perimeter of Gondwana in the earlier Ordovician. These pelagics divide into two morphological groups: poorly streamlined, sluggish species somewhat like the living hyperiid Parathemisto, and well-streamlined species which are larger, and are considered to have been fast and active swimmers, with predatory habits. The streamlining of this group has been demonstrated by experiment. Other trilobites may also have been pelagic, but the evidence is less complete: Irvingella and its homeomorphs, and Remopleurides. Most Olenidae, however, which have been supposed to have been pelagic, were benthic forms inhabiting a specialised environment low in oxygen and possibly below the thermocline; only the leptoplastines may have been pelagic.

It is also shown that there are cases where ventral terrace ridges did not function to grip sediment during filter feeding. This applies not only to pelagic trilobites, but also to benthic ones in which the width and disposition of the doublure renders engagement with the sediment unlikely.

A model suggesting that the trilobite carapace served primarily as a tight hood, under which sandy sediment could be processed for food, is consistent with morphological and ichnological data. It helps us to understand not only the origin of trilobite design, but also its modification for other habitats and modes of life.

Phacopid trilobites, unlike most trilobites, lack functionally articulated libregenae. This distinction presumably dictated ecdysial strategies which were fundamentally different from those forms with sutured cephala. Phacops rana (Green) and Greenops boothi (Green) from the Middle Devonian Hamilton Group (New York) are preserved in circumstances which illustrate certain aspects of their moulting behaviour. Undisturbed exuviae display five different moult patterns; these suggest two basic modes of exuviation in phacopid trilobites (Body-upright and Body Inversion Moult Procedures).

Certain Hamilton beds yield clusters of Phacops comprising complete and/or moulted remains. Complete trilobites in body clusters display varied orientations which, when considered in conjunction with exuvial patterns in moult clusters, suggest ecdysial procedures that involved body inversion and erratic tergal contractions (Body Inversion Moult Procedures). Ensembles where in thoracopygidia are upright and outstretched indicate hyperextension of the thorax and disarticulation of the cephalo-thoracic suture (Body-upright Moult Procedures). Phacops moult ensembles are characterised by inverted and tightly recurved thoracopygidia, whereas Greenops ensembles comprise outstretched, usually upright thoracopygidia with cephala that are frequently upright. Classic ‘Salterian’ patterns are uncommon in Phacops rana and have not been recognised in Greenops boothi. Variations in moult procedure are attributed to differences in functional morphology and substrate consistency.

There is abundant evidence of injury, teratology and pathological conditions in trilobites although it is commonly difficult to distinguish between their morphological results. Many injuries were probably sustained during moulting, with spines, bilamellar fringes and narrow gaps between dorsal exoskeleton and doublure being most vulnerable. Some injuries were the result of predaceous attack and provide important clues to the trophic structure of Palaeozoic communities. Many injuries show evidence of repair. Some supposedly teratological conditions, most notably the number of paradoxidid thoracic segments, are so common that they must be considered part of normal variation. Others represent genetic mutation or ontogenetic malfunction, the latter possibly externally induced. Undoubted pathological conditions are restricted to swellings and borings presumably caused by parasitic infestation.

Abnormalities affecting lateral areas are more common than those involving the rachis and glabella and this probably reflects both greater mortality when the axial region is involved, and greater vulnerability of peripheral regions to predaceous and moulting damage. The first explanation also accounts for the rarity of cephalic abnormalities compared to those of the pygidium.

The commonest arthropod trace fossils from Triassic aquatic red beds are the bilobate traces Isopodichnus and trackways known as ‘Merostomichnites’ triassicus. These trace fossils were probably produced by notostracan branchiopods, similar to Triops. Four arthropod ichnocoenoses from Lower Triassic fluvial sandstones have been analysed in terms of size frequency distribution, behavioural variation and relationship to sedimentary structures and depositional environment. One Isopodichnus ichnofauna associated with flute moulds (Dumfries-shire, Scotland) shows a normal age-structured population of arthropods responding with strong rheotaxis within shallow fluvial channels. The second Isopodichnus assemblage associated with ripple marks (Worcestershire, England) also shows strong rheotaxis but is bimodal in size and morphotype, possibly suggesting change in arthropod behaviour with age. Two ichnocoenoses of trackways with less pronounced rheotaxis associated with ripples (Cheshire, England) and flute moulds (Württemberg, Germany) were produced by larger arthropods than the resting traces. These arthropods probably possessed 6 to 9 pairs of walking limbs.

The conclusions derived from these notostracan trace fossils are compared with data on palaeoecology, population size-frequency, morphology and behaviour of Triops cancriformis derived from the analysis of three Triassic body fossil faunas and literature on living populations. Taxonomic consideration favours retention of the name Isopodichnus but the trackways should be included in Acripes Matthew. Brief review of late Palaeozoic Isopodichnus assemblages which appear to predate known notostracan fossils is inconclusive as regards both identifying producers or infallible means of separation from Cruziana assemblages.

Sediments which occur frequently within Lower Devonian andesites of the Midland Valley of Scotland have so far yielded trace fossils in only two areas: Dunure, Ayrshire and Montrose, Angus.

Smith (1909) interpreted the trace fossils from Dunure as representing an “Upland Fauna” and he named 23 new ichnogenera. Re-examination of his localities and specimens suggests that the environment was a shallow ephemeral lake and when sedimentological and preservational factors, size, behavioural variation and synonymy are taken into account, only 8 ichnogenera are here considered valid. These are redefined as Siskemia, Stiaria, Danstairia, Keircalia, Stiallia, Isopodichnus (=Archatomia Smith), Mermia and Merostomichnites. The Montrose ichnofauna of 5 ichnogenera includes a large arthropod trackway Mitchellichnus ferrydenensis gen. and sp. nov.

The trackways from both localities appear to have been formed by various merostomes, crustaceans and possibly early myriapods. The habitat was essentially aquatic although some arthropods may have been amphibious.

The fossil record of unquestionable decapod remains dates back to Permo Triassic times. Evaluation of all available data from the fossil record, trace fossils and facies reveals various trends within the evolution of Mesozoic decapods, and its relation to their ecology. One of these trends is the protection of the long vulnerable abdomen. The most successful method, gradual reduction and inflection of the abdomen against the ventral cephalothorax, led to the Brachyura. Brachyurans exhibit the broadest spectrum of habitat adaptions.

The life habit of Neseuretus is reconstructed on the basis of functional analysis and compared with that of other Calymenacea. Neseuretus, like all Calymenidae, was probably a filter feeder, burrowing downward into the sediment, where it rested hidden in a U-shaped attitude. Homalonotidae developed frontal burrowing and Bathycheilidae probably lived on the sediment. These calymenid families have different enrolment and behavioural patterns. Enrolment structures are developed, analysis of which leads to an understanding of phylogeny. Bathycheilidae maintained an unspecialised spiral enrolment. Flexicalymeninae and Calymeninae developed an advanced spiral enrolment, while a pseudosphaeroidal enrolment was achieved along independent lines in Kerfornellinae and Homalonotidae.

Many Ordovician trilobites possess morphological structures on the cephalon, the pygidium and sometimes also on the thoracic segments, which interlock during enrolment. These coaptative devices, also known in living arthropods, are functional, inherited, and seem to remain stable at the species level. Significant evolutionary changes in coaptative devices occur throughout time within the genera Placoparia (Pliomeridae) and Crozonaspis (Dalmanitidae). New data are presented which have relevance for the details of the coaptative structures in Placoparia (Coplacoparia), and for the classification of the dalmanitid species morenensis and dujardini. In some species of Crozonaspis and in Placoparia (Coplacoparia), interlocking structures prevent the possibility of shearing between cephalon and post-cephalic exoskeleton.

The striking morphological similarity that exists between appendages of the extant portunid crabs, such as Callinectes sapidus, and those of the extinet eurypterids has long been noted. The fifth pair of pereiopods in blue crags and other portunids are modified to form the broad, flat, highly mobile ‘swim paddles.’ A nearly identical modification is seen in the sixth pair of prosomal appendages of many eurypterids. The similarities are due to convergence and not to shared descent.

The kinetics of blue crab swimming were studied using high speed films. The animals are capable of slow upwards locomotion (‘hovering’) and rapid sideways swimming. The blue crab paddles apparently act as reciprocating hydrofoils, employing well-understood principles of lift and thrust generation to overcome the animal's weight and drag. Experimental studies indicated that the paddles are capable of producing appreciable amounts of lift. Drag on the body and paddles was also determined. Resxults are similar to those obtained in previous studies of bird and insect flight.

The physical principles employed to study blue crab swimming can be applied to the study of eurypterid locomotion. The eurypterid paddles may have functioned as hydrofoils, producing lift and thrust on forestroke and backstroke. Eurypterids were probably highly agile and manoueverable swimmers, capable of hovering and of high speed swimming. This model predicts observed morphological correlates. Predicted morphological correlates of earlier models (often based on analogies with Limulus) were not found.

The observed convergence between eurypterids and blue crabs may have resulted from similar functional constraints and parallel phylogenetic histories.

Cyrtoctenus wittebergensis sp. nov. is described from a unique holotype from the Witteberg Group of the Cape Supergroup. It is a giant hibbertopteroid eurypterid having combs and specialised movable spines of crytoctenid type (Størmer & Waterston 1968) on the more distal podomeres of the second to fourth prosomal appendages. The function of the combs and their associated movable spines is discussed and it is suggested that together they formed a unique adaptation of eurypterid structures to sweep filter-feeding, the combs forming the filters and the spines the cleaners. The digestive tract is remarkably preserved and shows a spiral valve, posterior to the stomach, which is interpreted as an adaptive feature in this large arthropod to increase the absorptive area of the gut.

The new evidence provided by the South African specimen has required the re-interpretation of the disarticulated Cyrtoctenus specimens previously described from Europe. Disjecta membra recently obtained from the Tournaisian of Foulden, Berwickshire, which may belong to Cyrtoctenus, are described and show characters previously unknown in Scottish material but similar to certain features in the South African specimen.

The taxonomic relationships within the Hibbertopteroidea are discussed in the light of the new combination of characters found in C. wittebergensis. Two families are recognised in the superfamily, the Hibbertopteridae, including Hibbertopterus and Campylocephalus, and the new family Cyrtoctenidae which is here erected to include Cyrtoctenus, Dunsopterus and possibly also Hastimima.