Larval and juvenile gastropods from a Carboniferous black shale: palaeoecology and implications for the evolution of the Gastropoda

A horizon in the late Visean Ruddle Shale from Arkansas contains the oldest well-preserved gastropod protoconchs known from the Americas. The gastropod fauna consists of a diverse larval shell assemblage and a low diversity assemblage of juvenile gastropods that probably had a benthic life habit. Gastropod larval shells are always isolated, i.e. the gastropods did not complete their life cycle (no metamorphosis) and were unable to become benthic. This was caused by unfavorable environmental conditions on the soft muddy bottom that was probably due to anaerobic to exaerobic conditions. The absence or scarcity of bioturbation caused by invertebrate detritus or sediment feeders in both shale and concretions (formed before compaction) favored preservation of the delicate larval shells. The lack or scarcity of infauna and bioturbation as well as the low diversity of the presumed benthos supports an interpretation of a quasi-anaerobic to exaerobic benthic environment. The superbly preserved larval shells demonstrate that there are more caenogastropod clades present in the late Palaeozoic than suggested previously. Some larval shell types have an openly coiled first whorl followed by a planktotrophic larval shell; openly coiled initial whorls are unknown from modern caenogastropods. The vetigastropods have a smooth protoconch of two whorls clearly demarked from the following whorls - a pattern unknown in modern vetigastropods which have a protoconch of less than one whorl and build no larval shell during their planktonic stage. This could indicate a link between Palaeozoic vetigastropods and the caenogastropods.     

Internal embryonic brooding and development in the southern Australian micro-snail Tricolia rosea (Angas, 1867) (Vetigastropoda: Phasianellidae: Tricoliinae)*

ABSTRACT

Internal embryonic brooding has been suggested as an adaptation to enhance reproductive success in minute gastropods. It is rare in vetigastropods, previously known in only two species of Spectamen Iredale, 1924 (Solariellidae) from South Africa. Herein it is confirmed in the temperate Australian micro-snail Tricolia rosea (Angas, 1867), with up to 46 embryos observed within a gravid female. Embryos are brooded to an advanced stage, possessing a translucent, calcified shell and a calcified operculum. The initial protoconch is colourless, spirally sculptured and delineated by a consistent axial demarcation at the 0.75 whorl mark, when it measures 320?µm across. Beyond this, the second part of the protoconch is tinted pink, the strong spiral sculpture continues but the ribs and interstices are broader, with smoother surface microsculpture. At the 1.1 to 1.125 whorl mark the protoconch measures 400?µm across when transition to smooth teleoconch sculpture occurs. No brooded embryos possessed teleoconch sculpture. The potential relationship of protoconch morphology to embryonic development, hatching, feeding and release are considered. The mechanism of fertilisation is unknown, but embryos in a brood are at the same developmental stage. Unanswered questions in embryonic development and problems with protoconch terminologies in vetigastropods are also discussed.     

Shell structure and affinity of vermiform 'gastropods'

Vermiform ‘gastropods’ are reported from a variety of rocks ranging from Givetian to Lower Triassic age. Examples encrusting shells and plants have been identified in non-marine shales, in addition to previously recognized occurrences in shallow marine microbial bioherms and stromatolites. SEM studies of the planorbiform or trochiform protoeonch reveal a shell wall comprising three calcite layers: an outer, initial acicular layer: a blocky prismatic layer: and an inner irregular micro-lamellar layer. Minor irregularities and microstructural details suggest a high original organic content. allowing flexibility for attachment. The sinistrally coiled (or hyperstrophic dextral) calcitie teleoconch is composed of an outer simple prismatic layer and inner micro-lamellar layer comprising sheets of irregular, platy, sometimes fused calcite tablets. displaying ridges and grooves similar to those of cross-bladed fabric. Repetition of layers may occur. Regular closely-spaced punctae. passing through and disrupting the micro-lamellar layer. are unlike any mollusean tubulation. Punetation may he a shell-strengthening response to uncoiling. Septa bear a centrai, anteriorly-projecting, probebly perforate protrusion. reminiscent of the siphunele of cephalopods and similar structures in tentaculitoida. The micro-lamellar layer in the protoconch, the micro-lamellar layer with distinctive ridge and groove structure and punetation in the teleoconch, and the structure of the septa point to a close affinity between vermiform ‘gastropods’ and the Tentaculitoidea. The three-layered microstructure of the protoconch and the coiled nature of the shell distinguish vermiform ‘gastropods’ from tentaculitoids. A consideration of the shared characters indicates that the tentaculitoids and vermiform ‘gastropods’ should be regarded as a sister group to the molluses. ***Vermiform gastropods: microstructure, protoconch, teleoconch, acicular laver, prismatic laver, micro-lameller layer. cross-bladed structure. punetation, septation. Tentaculitoidea     

Inflated protoconchs and internally dissolved, coiled protoconchs of nudibranch larvae: different developmental trajectories achieve the same morphological result

Abstract. Light and scanning electron microscopy were used to examine protoconch form in eight species of planktotrophic heterobranch larvae, including four nudibranch species with a coiled (type 1) protoconch, two nudibranch species with an inflated (type 2) protoconch, and two cephalaspid species with a coiled protoconch. The coiled protoconchs of the cephalaspids and nudibranchs have a similar form at hatching, and shell growth up to metamorphic competence is hyperstrophic. Shell added to coiled protoconchs during the larval stage overgrows all but the left wall of the initial protoconch that exists at hatching. The entire protoconch of cephalaspids, including overgrown areas, is retained through metamorphosis. However, during later larval development in nudibranchs with a coiled protoconch, overgrown shell is completely removed by dissolution. As a result, regardless of whether nudibranch larvae hatch with an inflated or coiled protoconch type, the protoconch is a large, hollow cup at metamorphic competence. The protoconch of nudibranchs is shed at metamorphosis and absence of a post-metamorphic shell is correlated with absence of visceral coiling in this gastropod group. Internal dissolution of the coiled protoconch in nudibranchs allows the left digestive gland to uncoil prior to metamorphic shell loss. Retention of overgrown protoconch whorls in cephalaspids allows the attachment plaque of the pedal muscle to migrate onto the parietal lip of the post-metamorphic shell. Release from this constraint in nudibranchs, in which the larval pedal muscles and the entire protoconch are lost at metamorphosis, may have permitted internal protoconch dissolution and precocious uncoiling of the visceral mass, as well as evolutionary emergence of the inflated larval shell type.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.     

Decline in diversity of early Palaeozoic loosely coiled gastropod protoconchs

Quantitative data on molluscan larval conch fossil assemblages of ages ranging from the Ordovician (Argentina and the Baltic region), through Silurian (Austria), Devonian (Poland) to Carboniferous (Texas) supplement knowledge of early planktonic gastropods communities transformations. They show that larval shells of the bilaterally symmetrical bellerophontids and dextrally coiled gastropods with a hook-like straight apical portion of the first whorl initially dominated. Their relative frequency, as well as that of the sinistrally coiled ‘paragastropods’, diminished during the Ordovician and Silurian to virtually disappear in the Late Devonian and Early Carboniferous. Already during the Ordovician, diversity of larvae with gently loosely coiled first whorl increased, to be replaced then with more and more tightly coiled forms. Both the aperture constrictions and mortality peaks, probably connected with hatching and metamorphosis, indicate that the Ordovician protoconchs with hook-like first coil represent both the stage of an embryo developing within the egg envelope and a planktonic larva. The similarity of the straight apex to larval conchs of hyoliths and advanced thecosome pteropods is superficial, as these were not homologous stages in early development.     

Morphology and mode of life of the polychaeteRotularia

The serpulidRotularia built rather regular, planispiral or trochospiral tubes with an uncoiled adult portion. Juveniles ofRotularia were cemented to small substrates, and later in ontogeny grew into secondary, reclining epifaunal soft-bottom dwellers in medium- to high-energy environments. The uncoiled adult portion, together with the coiled portion of the tube, constituted a stabilizing structure increasing the effective area of contact with the substrate, and was only exceptionally bent out of the coiling plane, a situation common in other sessile soft-bottom dwellers among polychaetes and gastropods.     

Zygopleuroidea (Gastropoda) aus dem Lias und Dogger Deutschlands und Nordwestpolens

16 taxa of gastropods are described from the Lower and Middle Jurassic of Germany and northwestern Poland. They belong to seven genera. Two species (Pommerozygia aspera, Costazygia bilzi) and two genera (Brevizygia, Costazygia) are new. The family Pommerozygiidae is new as well. Compared to the Zygopleuridae, the Pommerozygiidae have a rather short and broad shell with only few teleoconch whorls. The protoconch is broad conical with a rounded apex (because the first whorls are nearly planispiral). From the Zygopleuridae only members of the Zygopleurinae have been found. Most Jurassic species have a smooth protoconch. Within the Zygopleuridae, the development possibly began with protoconchs carrying collabral axial ribs (and spirals) and led to smooth protoconchs. The genera of the Pommerozygiidae are rather similar to each other. The planktotrophic larval shell has a subsutural row of nodes like many species of Triassic Zygopleuridae. Therefore, both families are closely related. The Pommerozygiidae are possibly a separate branch of the Zygopleuroidea without descendants. The main branch is probably the evolutionary line Zygopleuriidae — Janthinoidea.     

Some heterostrophic gastropods from Triassic St. Cassian Formation with a discussion on the classification of the Allogastropoda

Fifteen species of Heterostropha are described, 12 of them for the first time. All are newly interpreted with regard to their taxonomic relation to fossil and living gastropods. The Streptacidoidea with long Paleozoic history are represented in the Late Triassic St. Cassian Formation by several genera that can be differentiated into four families. The Ebalidae are represented byEbala, with smooth protoconch, Cassianebalidae byCassianebala andLoxebala with axially ornamented protoconch. The Donaldinidae of St. Cassian are represented by one species ofDonaldina and two ofNeodonaldina that stand in the continuation of Paleozoic species ofDonaldina. Architectonicoidea with shells coiled in a plane and Valvatoidea appear in the St. Cassian fauna without known Paleozoic relation. In the former superfamily the Architectonicidae can be recognized in the genusRinaldoconchus with two species. Cassianaxidae withCassianaxis, Amphitomariidae withAmphitomaria, Stuoraxidae withStuoraxis andAmpezzogyra have a sinistral protoconch and planispirally coiled dextral teleconchs. They all resemble different modern species that have similarly small shells. Modern Hyalogyrinidae have withAlexogyra a new representative from the Triassic. The Valvatoidea are represented with the generaCarboninia andBandellina of the Cornirostridae in the Triassic representatives. The relation of described species in the system of the Heterostropha is discussed.     

Morphogenetic significance of the conchal furrow in nautiloids: evidence from early embryonic shell development of Jurassic Nautilida

As reported by many workers over the past two centuries, the inner part of the shell of various straight and coiled Palaeozoic to tertiary nautiloid taxa bears a continuous mid-ventral furrow that extends into the phragmocone and the body chamber nearly to the aperture. Study of the early embryonic shell development of Jurassic Nautilida shows that the most apical part of this so-called conchal furrow originates from the inner part of the initial, calcified shell apex, in line with the inner ventral termination of the central linear depression of the cicatrix, the initial site of shell deposition. The conchal furrow corresponds to a morphological feature arising as a developmental by-product. Rare specimens of scattered ammonoid species (and possibly of bactritoids) display a similar feature, whereas their protoconch lacks a cicatrix. However, the protoconch of recent cuttlefish, Sepia officinalis, often displays a longitudinal fold of the primary shell epithelium. A longitudinal groove or a pair of grooves appears connected with this cicatrix-like structure. Although the mid-ventral ridge in ammonoids must probably be viewed as an incidental 'fabricational noise', whether or not it originates from a so far undocumented optional ridge on the protoconch or from some other structure related to shell development remains an open question.     

Appearance of morphological novelty in a hybrid zone between two species of land snail

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.     

Sectorial expansion and shell morphogenesis in molluscs

Checa, A. 1991 01 15: Sectorial expansion and shell morphogenesis in molluses. Lethaia . Vol. 24, pp. 97–114. Oslo. ISSN 0024–1164.
Any coiled shell can be described as a series of independent helicospirals that join homologous points along the shell surface. The cross-section is therefore seen as a set of points. obtained at its intersection with the helicospirals. Any cross-sectional sector contained between two adjacent points is capable of expanding or contracting during the development and a differential parameter ( L'/L ) has been devised to quantify this expansion rate. The morphometrics so obtained is here called sectorial expansion analysis. This analysis and other related procedures have been applied on cross-sections and apertures in selected Molluxa with the aim of evaluating the incidence of sectorial expansions on shell shape. Those parameters affecting whorl expansion rate and whorl overlapping may be directly modified by sectorial expansions. Changes in the mode of coiling (curvature and torsion) often, but not always, involve sectorial expansions, perhaps as a constructional feature. This approach reveals the advantages and drawbacks of the present analysis as compared to previous theoretical models. Sectorial expansion. morphogenesis. ornamentation, septal suture, coiled shell, gastropods, bivalues. ammonites .     

Ontogenetic torsion in two basal gastropods occurs without shell attachments for larval retractor muscles

Results of this study on two species of vetigastropods contradict the long-standing hypothesis, originally proposed by Garstang (1929), that the larval retractor muscles power the morphogenetic movement of ontogenetic torsion in all basal gastropods. In the trochid Calliostoma ligatum and the keyhole limpet Diodora aspera, the main and accessory larval retractor muscles failed to establish attachments onto the protoconch (larval shell) when the antibiotics streptomycin sulfate and penicillin G were added to cultures soon after fertilization. Defects in protoconch mineralization were also observed. Despite these abnormalities, developing larvae of these species accomplished complete or almost complete ontogenetic torsion, a process in which the head and foot rotate by 180 degrees relative to the protoconch and visceral mass. Analysis by using phalloidin-fluorophore conjugate and transmission electron microscopy showed that myofilaments differentiated within myocytes of the larval retractor muscles and adherens-like junctions formed between muscle and mantle epithelial cells in both normal and abnormal larvae. However, in abnormal larvae, apical microvilli of mantle cells that were connected to the base of the larval retractor muscles failed to associate with an extracellular matrix that normally anchors the microvilli to the mineralized protoconch. If morphogenesis among extant, basal gastropods preserves the original developmental alteration that created gastropod torsion, as proposed by Garstang (1929), then the alteration involved something other than the larval retractor muscles. Alternatively, the developmental process of torsion has evolved subsequent to its origin in at least some basal gastropod clades so that the original alteration is no longer preserved in these clades.     

Attached vermiform gastropods in Carboniferous marginal marine stromatolites and biostromes

Tubiform fossils conventionally referred to Serpula cf. advena Salter and species of Spirorbis Lamarck from the British Lower Limestone Shales and Border Group (Lower Carboniferous) are re-examined. They occur in peritidal carbonate environments of schizohaline aspect. These fossils superficially resemble calcareous polychaete tubes but have skeletal characters, including molluscan wall structure, numerous internal septa, and protoconch, which indicate that they represent a new group of substrate-attached, disjunctly coiled gastropods. They resemble archaeogastropods in internal morphology of the skeleton but show parallels in external form and occurrence with the extant Vermetidae. There are two principal modes of occurrence: (1) erect tubes forming intertidal biostromes associated with non-skeletal algal laminites, and (2) prostrate discoidal tubes encrusting subtidal skeletal stromatolites or occasionally forming larger irregular bioherms. These biostromes and bioherms are comparable in structure to Recent vermetid reef developments.     

From near extinction to recovery: Late Triassic to Middle Jurassic ammonoid shell geometry

The Triassic–Jurassic extinction resulted in the near demise of the ammonoids. Based on a survey of ammonoid expansion rates, coiling geometry and whorl shape, we use the Raup accretionary growth model to outline a universal morphospace for planispiral shell geometry. We explore the occupation of that planispiral morphospace in terms of both breadth and density of occupation in addition to separately reviewing the occurrence of heteromorphs. Four intervals are recognized: pre‐extinction (Carnian to Rhaetian); aftermath (Hettangian); post‐extinction (Sinemurian to Aalenian) and recovery (Bajocian to Callovian). The pre‐extinction and recovery intervals show maximum disparity. The aftermath is marked by the disappearance of heteromorphs and a dramatic reduction in the range of planispiral morphologies to a core area of the morphospace. It is also characterized by an expansion into an evolute, slowly expanding part of the morphospace that was not occupied prior to the extinction and is soon abandoned during the post‐extinction interval. Aftermath and post‐extinction ammonoid data show a persistent negative correlation whereby rapid expansion rates are associated with narrow umbilical widths and often compressed whorls. The permanently occupied core area of planispiral morphospace represents generalist demersals whose shells were probably optimizing both hydrodynamic efficiency and shell stability. All other parts of the planispiral morphospace, and the pelagic modes of life the shells probably exploited, were gradually reoccupied during the post‐extinction interval. Planispiral adaptation was by diffusion away from the morphospace core rather than by radical jumps. Recovery of disparity was not achieved until some 30 Myr after the extinction event.     

ADAPTATION FROM RESTRICTED GEOMETRIES: THE SHELL INCLINATION OF TERRESTRIAL GASTROPODS

The adaptations that occur for support and protection can be studied with regard to the optimal structure that balances these objectives with any imposed constraints. The shell inclination of terrestrial gastropods is an appropriate model to address this problem. In this study, we examined how gastropods improve shell angles to well‐balanced ones from geometrically constrained shapes. Our geometric analysis and physical analysis showed that constantly coiled shells are constrained from adopting a well‐balanced angle; the shell angle of such basic shells tends to increase as the spire index (shell height/width) increases, although the optimum angle for stability is 90° for flat shells and 0° for tall shells. Furthermore, we estimated the influences of the geometric rule and the functional demands on actual shells by measuring the shell angles of both resting and active snails. We found that terrestrial gastropods have shell angles that are suited for balance. The growth lines of the shells indicated that this adaptation depends on the deflection of the last whorl: the apertures of flat shells are deflected downward, whereas those of tall shells are deflected upward. Our observations of active snails demonstrated that the animals hold their shells at better balanced angles than inactive snails.     

The Paleozoic evolution of the gastropod larval shell: larval armor and tight coiling as a result of predation‐driven heterochronic character displacement

Early and middle Paleozoic gastropod protoconchs generally differ strongly from their corresponding adult morphologies, that is, most known protoconchs are smooth and openly coiled, whereas the majority of adult shells are ornamented and tightly coiled. In contrast, larval and adult shells of late Paleozoic gastropods with planktotrophic larval development (Caenogastropoda, Neritimorpha) commonly resemble each other in shape and principle ornamentation. This is surprising because habitat and mode of life of planktonic larvae and benthic adults differ strongly from each other. Generally, late Paleozoic to Recent protoconchs are tightly coiled. This modern type of larval shell resembles the adult shell morphology and was obviously predisplaced onto the larval stage during the middle Paleozoic. The oldest known planktonic‐armored (strongly ornamented) larval shells are known from the late Paleozoic. However, smooth larval shells are also common among the studied late Paleozoic gastropods. The appearance of larval armor at the beginning of the late Paleozoic could reflect an increase of predation pressure in the plankton. Although there are counter examples in which larval and adult shell morphology differ strongly from each other, there is statistical evidence for a heterochronic predisplacement of adult characters onto the larval stage. Larval and adult shells are built in the same way, by accretionary secretion at the mantle edge. It is likely that the same underlying gene expression is responsible for that. If so, similarities of larval and adult shell may be explained by gene sharing, whereas differences may be due to different (planktic vs. benthic life) epigenetic patterns.     

Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods

The early ontogeny of gastropods (i.e., planktotrophic vs. nonplanktotrophic) may be inferable from the morphology of the protoconch in adult shells. The protoconch consists of both embryonic and larval shells in species with planktotrophic development; the embryonic shell forms in the intracapsular period and the succeeding larval shell gradually develops during the larval period. In nonplanktotrophic species, on the other hand, there is no additional growth of the larval shell and the protoconch consists exclusively of a relatively large embryonic shell formed prior to hatching. This "shell apex theory" has been applied to many species of shell-bearing gastropods, but biotic and abiotic erosion of the apex often prevents detailed examination of the protoconch and subsequent inferences about ontogeny. I examined the gastropod operculum to test its utility for predicting developmental mode, drawing on the Neritimorpha as model taxa. Most aquatic members of Neritimorpha were found to bear an operculum with a clearly demarcated nucleus; SEM observations reveal four types of nuclei, which correspond to different types of protoconch morphologies and observed ontogenies for the study species. The nucleus is secreted before metamorphosis, fits into the shell aperture of the larva, and reflects early ontogeny as morphology, as does the protoconch. Moreover, the apparently organic (rather than calcareous) composition of the nucleus makes it nearly invulnerable to erosion and very advantageous, compared to the protoconch, in this ecologically diverse group, whose habitats range from freshwater streams and mangrove swamps to rocky shores and deep-sea hydrothermal vents. The measurements of the nucleus are also valuable for taxonomic purposes, especially in the species identification of veliger larvae and juvenile snails. On the other hand, the opercular nuclei of the Caenogastropoda and Heterobranchia are often eroded away in adult individuals; even if present, the morphology of the nuclei does not seem to clearly reflect early ontogeny in those groups.     

Genetic separation of third and fourth whorl functions of AGAMOUS.

AGAMOUS (AG) is an Arabidopsis MADS box gene required for normal development of the third and fourth whorls of the flower. In previously described ag mutants, the third whorl stamens are replaced by petals, and the fourth whorl is replaced by another (mutant) flower. We describe two new ag alleles, ag-4 and AG-Met205, retaining partial AG activity. Both produce flowers with stamens in the third whorl and indeterminate floral meristems; however, ag-4 flowers contain sepals in the fourth whorl, and AG-Met205 produces carpels. The ag-4 mutation results in partial loss of the C terminus of the K domain, a putative coiled coil, and AG-Met205 contains a site-directed mutation that causes a single amino acid change in this same region of the K box. Two models that might explain how these changes in AG result in the separation of different AG activities are discussed.     

HETEROSTOPHIC SHELLS AND PELAGIC DEVOLOPMENT IN TROCHOIDEANS: IMPLICATIONS FOR CLASSIFICATION, PHYLOGENY AND PALAEOECOLOGY

Four species of archeogastropods, presumed members of threesubfamilies of the trochidae, exhibit significant differencesin developmental modes and shell coiling. All four species havelecithotrophic development which is reflected in their inflatedpausispiral protoconchis; however, Margarites marginatus andLirularia succincta have benthic development in gelatinous masses,while Margarites pupillus and Calliostoma ligatum have pelagicembryos and swimming larvae with a potential for dispersal overa period of a week or longer. These modes cannot be deducedfrom the size of the egg, the size or shape of the protoconch,or the size or relative prominence of female pallial reproductivestructures. The protoconch of C. ligatum is orthostrophicallycoiled, but the protoconchs of the other three species are hyperstrophicallycoiled although their teleconchs are orthostrophic. These threetrochoidean species thus share with architectonicoideans, pyramidelloideans,opisthobranchs, and pulmonates the distinctive shell characterof heterostrophy, previously unreported for archaeogastropods. These observations, considered together with others reportedin the literature, show: (1) that developmental mode (pelagicor benthic) cannot be inferred from protoconch appearance ortaxonomy in major trochoidean ganera; (2) that significant dispersalpotential is present in the histories of some trochoidean archeogastropoids;and (3) that character sets (pallial reproductive structures,pairing during spawning, heterostrophic shell coiling) thoughtnot to occur below the mesogastropod level are found in theTrochoidea. These conclusions bring into question the usefulnessof these characters in defining higher gastropod taxa and raiseadditional questions concerning the ancestry of the higher gastropods. (Received 16 February 1989; accepted 25 June 1989)