Conch Morphology Research Articles

Morphometric analysis of the Late Cretaceous Placenticeras of Alabama, USA: sexual dimorphism, allometry, and implications for taxonomy

AbstractA traditional typological approach to taxonomy often does not adequately account for intraspecific variation and can result in taxonomic oversplitting. For many groups, including ammonoids of the Placenticeras genus, intraspecific variation documented in recent studies (e.g., ontogenetic changes, sexual dimorphism, polymorphism) challenges the historic proliferation of species names. Here, we used a population approach to taxonomy and quantitatively evaluated morphometric variation in a sample of Late Cretaceous (Santonian–Campanian) Placenticeras from Alabama and adjacent counties.We used linear mixed models (LMMs) to characterize how morphological variables scale with conch size across the sample, exploiting mixed longitudinal data to evaluate individual variation in growth and inform interpretations of multivariate analyses. Extended LMMs incorporating geological formation evaluated morphological changes through time. Principal component analysis and clustering analysis were then used to evaluate the number of distinct clusters that emerged in multivariate morphospace independent of previous taxon name assignments.Discontinuous scaling relationships and distinct clusters in multivariate space suggest sexual dimorphism characterized by differences in adult size and, secondarily, shape. Previous Stantonoceras and Placenticeras assignments broadly overlap in our morphospace, failing to justify this historic distinction (as sexual dimorphs or as genera or subgenera). Placenticeras conch morphology and ornament placement changed through time, suggesting a potential utility for coarse (stage-level) biostratigraphy. However, temporal changes were not associated with distinct clusters in morphospace, and our data fail to support the plethora of reported species names. As few as one or two (successive) species may be present in our sample (representing 130 years of collection effort). In addition to highlighting the need for a significant taxonomic revision of the Placenticeras genus, this study demonstrates the utility of LMMs for distinguishing between different sources of morphological variation, improving interpretations of morphospace under a population approach to taxonomy, and maximizing the amount of ontogenetic information that can be obtained nondestructively.

Covariable changes of septal spacing and conch shape during early ontogeny: a common characteristic between Perisphinctina and Ancyloceratina (Ammonoidea, Cephalopoda)

AbstractWe analyzed the ontogenetic trajectories of conch morphology and septal spacing between successive chambers in Cretaceous ammonoids (suborders Perisphinctina and Ancyloceratina) collected from southern India, Madagascar, and Japan. All examined species, except for the family Collignoniceratidae, exhibited similar characteristics during early ontogeny. The common ontogenetic trajectories of septal spacing show a cycle comprising an increase and a subsequent decrease in septal spacing during early ontogeny. The conch diameters at the end of the cycle were estimated to be 1–4 mm. The conch shape (aperture height and whorl expansion rate) covariably changed at this conch diameter. Such covariable changes are commonly recognized in the suborders Perisphinctina and Ancyloceratina. The similarity in the ontogenetic trajectories of conch morphology implies a closer phylogenetic relationship between these suborders compared to Lytoceratina or Phylloceratina.

Resurrecting extinct cephalopods with biomimetic robots to explore hydrodynamic stability, maneuverability, and physical constraints on life habits

Externally shelled cephalopods with coiled, planispiral conchs were ecologically successful for hundreds of millions of years. These animals displayed remarkable morphological disparity, reflecting comparable differences in physical properties that would have constrained their life habits and ecological roles. To investigate these constraints, self-propelling, neutrally buoyant, biomimetic robots were 3D-printed for four disparate morphologies. These robots were engineered to assume orientations computed from virtual hydrostatic simulations while producing Nautilus-like thrusts. Compressed morphotypes had improved hydrodynamic stability (coasting efficiency) and experienced lower drag while jetting backwards. However, inflated morphotypes had improved maneuverability while rotating about the vertical axis. These differences highlight an inescapable physical tradeoff between hydrodynamic stability and yaw maneuverability, illuminating different functional advantages and life-habit constraints across the cephalopod morphospace. This tradeoff reveals there is no single optimum conch morphology, and elucidates the success and iterative evolution of disparate morphologies through deep time, including non-streamlined forms.

Is there more than one species in the genusSpirula(Cephalopoda: Decabrachia): evidence for an Atlantic–Pacific divide

ABSTRACTSpirula spirula (Coleoidea: Decabrachia) is a unique deep-sea squid with an uncertain taxonomic status. Here, we apply geometric morphometric analyses to precisely describe changes in conch morphology during the course of ontogeny of 21 specimens collected from 12 localities worldwide. These data were used to explore whether the genus is monospecific or comprises several species. Different 2D and 3D conch parameters are presented based on micro-computed tomography data, combining noninvasive imaging techniques with a range of morphometric analyses. Our data imply that Atlantic and Indo-Pacific specimens form two distinct morphological clusters, potentially representing two pseudocryptic species or two populations undergoing divergence (i.e. in the process of speciation). Given the evolutionary trend from straight to more coiled forms, we suggest that S. spirula represents a neotenous form that migrated from the Indo-Pacific towards the Atlantic via the Agulhas leakage, which has been active since the closure of the Strait of Panama (10–3 Ma).

The middle Smithian (Early Triassic) ammonoid Arctoceras blomstrandi: conch morphology and ornamentation in relation to stratigraphy

AbstractThe ammonoid genus Arctoceras (Hyatt) occurs across all palaeolatitudes, and is a key genus for middle Smithian biostratigraphical correlations globally. In this study, intraspecific variations in conch morphology, ornamentation and allometry are examined in relation to stratigraphic position. Arctoceras is the most abundant ammonoid genus in the middle Smithian of Svalbard. Originally, seven Arctoceras species were described from Svalbard. Later, as the importance of intraspecific variation was recognized, six of the Arctoceras species from Svalbard were treated as junior synonyms of Arctoceras blomstrandi (Lindström). Yet, the variations in A. blomstrandi conch morphology remain poorly quantified and the dependence on stratigraphic position, unknown. We quantify the intraspecific variation in conch morphology, ornamentation and allometry in relation to stratigraphy of the Svalbard Arctoceras. The results support the assignment of all Arctoceras morphotypes from Svalbard to a single species A. blomstrandi. The new data allow for an updated species description and open the way for the use in biostratigraphy of the endmember morphology A. blomstrandi var. costatus. We document consistent changes in conch morphology and ornamentation in the studied stratigraphic interval, with a distinct shift towards more evolute and ornate conchs in the top of the interval. The trends in the strength of ornamentation are partly explained by covariation with conch morphology (Buckman's law). The most marked shift in the conch morphology and allometric development of A. blomstrandi coincides with the onset of the positive carbon isotope excursion at the end of the middle Smithian, but pre‐dates the mid–late Smithian cooling of the sea surface.

The Late Devonian ammonoid subfamily Paratornoceratinae

Ammonoids of the subfamily Paratornoceratinae are characterised by disc-shaped adult conchs; furthermore they display a wide range of ontogenetic pathways leading to a similar adult conch morphology. The subfamily Paratornoceratinae occurs in the late early Famennian shelf sediments of various regions from the Anti-Atlas of Morocco to the Canning Basin of Western Australia; where endemic species of the genera Paratornoceras and Acrimeroceras appear in stratigraphic succession. We revise the species Paratornoceras lentiforme (Sandberger, 1857) and describe the new species Paratornoceras thuringense Korn, Bartzsch & Weyer n. sp., P. harounense Korn & Ebbighausen n. sp., P. ayense Korn & Buchwald n. sp., P. peterseni Korn & Buchwald n. sp., Acri meroceras hoppeckense Korn & Buchwald n. sp., A. ropicense Korn, Bartzsch & Weyer n. sp. and A. saalense Korn, Bartzsch & Weyer n. sp.

Patterns of intraspecific variation through ontogeny: a case study of the Cretaceous nautilidEutrephoceras dekayiand modernNautilus pompilius

AbstractThe magnitude and ontogenetic patterns of intraspecific variation can provide important insights into the evolution and development of organisms. Understanding the intraspecific variation of organisms is also a key to correctly pursuing studies in major fields of palaeontology. However, intraspecific variation has been largely overlooked in ectocochleate cephalopods, particularly nautilids. Furthermore, little is known regarding the evolutionary pattern. Here, we present morphological data for the Cretaceous nautilidEutrephoceras dekayi(Morton) and the modern nautilidNautilus pompiliusLinnaeus through ontogeny. The data are used to describe conch morphology and to elucidate the evolutionary patterns of intraspecific variation. We discovered a similar overall pattern of growth trajectories and the presence of morphological changes at hatching and maturity in both taxa. We also found that intraspecific variation is higher in earlier ontogeny than in later ontogeny in both taxa. The high variation in earlier ontogeny may imply increased flexibility in changing the timing of developmental events, which probably played an important role in nautilid evolution. We assume that the decrease in variation in later ontogeny reflects developmental constraints. Lastly, we compared the similarity/dissimilarity of ontogenetic patterns of variation between taxa. Results reveal that the similarity/dissimilarity of the ontogenetic pattern differs betweenE. dekayiandN. pompilius. We conclude that this shift in the ontogenetic pattern of variation may be rooted in changes in the developmental programme of nautilids through time. We propose that studying ontogenetic patterns of intraspecific variation can provide new insights into the evolution and development of organisms.

Variability of conch morphology in a cephalopod species from the Cambrian to Ordovician transition strata of Siberia

Variability of conch morphology in a cephalopod species from the Cambrian to Ordovician transition strata of Siberia

The polyphasic ontogeny of the discoidal Late Devonian ammonoid Acrimeroceras

The Late Devonian ammonoid species Acrimeroceras falcisulcatum and A. stella have similar-shaped discoidal adult conchs. Their conch morphology and its ontogenetic development are described and analysed. Despite great similarities in their adult conch morphology, they can be clearly distinguished by differences in the shape of their juvenile whorl profile and by the growth trajectories of their cardinal conch parameters: conch width index, umbilical width index and whorl expansion rate. Quantification of ontogenetic change in the two species demonstrates that the stratigraphically younger species A. falcisulcatum possesses the less complex ontogeny compared to the stratigraphically older A. stella. The closely related genus Paratornoceras (early Famennian) and the unrelated Late Visean Calygirtyoceras show juvenile conch morphology and ontogenetic trajectories very similar to Acrimeroceras stella. This similarity can be seen as iterative independent unfolding of homoplastic characters, possibly linked to size-dependence of the functional morphology of the conchs.

Morphological disparity and ontogeny of the endemic heteromorph ammonite genus Aegocrioceras (Early Cretaceous, Hauterivian, NW-Germany)

Despite the fact that heteromorph ammonites often show a large range of intraspecific variation, that variation has rarely been quantified. Here, we quantify the intraspecific variation and ontogeny of 85 newly collected specimens of the Hauterivian endemic ammonite genus Aegocrioceras. All specimens were collected from concretions in the recently exposed bed 82 in the clay pit Resse in NW Germany (Lower Saxony Basin). Data for the conch morphology (diameter, whorl height, whorl interspace, rib spacing) were collected in 45° steps using the new software CONCH©. The CONCH© software will be made officially available with this publication. All ammonites were a priori identified as A. bicarinatum or A. semicinctum. An assessment of the intra- and interspecific variation as well as the ontogeny of this assemblage demonstrates the presence of just one species, i.e., A. bicarinatum. Comparison with the type material of A. bicarinatum and A. semicinctum reveals a large morphological overlap of different conch parameters and does not support a separation of both forms as distinct species. The analyzed assemblage of A. bicarinatum has a high variation in conch morphology including whorl interspace, umbilical width, and whorl expansion. Furthermore, the changes in intraspecific variation through ontogeny were quantified and the presence or absence of sexual dimorphism within the assemblage is discussed.

Morphology of the Early Jurassic Arietitidae and the effects of syn vivo serpulid infestations

Abstract. Selected specimens from the Jurassic ammonoid family Arietitidae were investigated using morphometric methods of transverse and longitudinal conch section analysis. The family Arietitidae is characterized by similarities in the conch geometry, but variation can be demonstrated by means of differences in conch morphology. Our study focuses on a specimen of the arietitid Pararnioceras sp., which revealed striking changes in conch morphology due to a syn vivo growth through a parasitic serpulid. Changes in its ontogenetic development are compared with specimens without epizoans. The ecological interpretation of the morphometric data allows the conclusion that the host possessed the ability to counteract the parasitic conch abnormalities by adapting its housing growth, thus ensuring survival.

The old and the new plankton: ecological replacement of associations of mollusc plankton and giant filter feeders after the Cretaceous?

Owing to their great diversity and abundance, ammonites and belemnites represented key elements in Mesozoic food webs. Because of their extreme ontogenetic size increase by up to three orders of magnitude, their position in the food webs likely changed during ontogeny. Here, we reconstruct the number of eggs laid by large adult females of these cephalopods and discuss developmental shifts in their ecologic roles. Based on similarities in conch morphology, size, habitat and abundance, we suggest that similar niches occupied in the Cretaceous by juvenile ammonites and belemnites were vacated during the extinction and later partially filled by holoplanktonic gastropods. As primary consumers, these extinct cephalopod groups were important constituents of the plankton and a principal food source for planktivorous organisms. As victims or, respectively, profiteers of this case of ecological replacement, filter feeding chondrichthyans and cetaceans likely filled the niches formerly occupied by large pachycormid fishes during the Jurassic and Cretaceous.

Goniatites sphaericus (Sowerby, 1814), the archetype of Palaeozoic ammonoids: a case of decreasing phenotypic variation through ontogeny

The Late Visean (Early Carboniferous) ammonoid species Goniatites sphaericus (Sowerby, 1814) is revised with the use of the type material. With respect to conch morphology, suture line and particularly shell ornament, it is regarded as a senior synonym of the species Goniatites fimbriatus (Foord and Crick, 1897) and must therefore be re-established as an important index fossil for the ammonoid stratigraphy of the Rhenohercynian and Subvariscan shelves. The Goniatites fimbriatus Zone has been re-named Goniatites sphaericus Zone on this basis. The study of ontogenetic changes in the conch parameters shows that intraspecific variation occurs within a wide range. In the whorl profile, variation is very high in small juveniles, while in the coiling rate, intermediate growth stages are more strongly affected.

Exploring the limits of morphospace: ontogeny and ecology of late Viséan ammonoids from the Tafilalt, Morocco

Early late Visean ammonoid assemblages in Morocco are composed of diverse and well-preserved specimens. The material was found in a plain in the Tafilalt (eastern Anti-Atlas). Here, we describe mass-occurrences of juvenile specimens, in which subadult and adult specimens occur in low numbers. The juveniles of some species display a conch morphology that differs fundamentally from the adult stages. Accordingly, we emend the species diagnoses of Goniatites lazarus as well as Calygirtyoceras darkaouaense, introduce the species Entogonites bucheri sp. nov., and discuss possible ecological implications of the morphologic changes throughout ontogeny. In particular, we compare the changes in conch morphology through ontogeny in the light of Pareto Optimiality according to which the morphology of organisms would fill a polygon or polyhedron in morphospace. Data points in one of the vorteces of the polyhedron indicate optimisation for the corresponding task. Although shape is not a proof of function, it appears pl...

The Late Devonian ammonoid Mimimitoceras in the Anti-Atlas of Morocco

Mimimitoceras is the most abundant late Famennian goniatite genus in the Anti-Atlas of Morocco and the Saoura Valley of Algeria. Based on conch morphology, conch ontogeny, suture line and ornament, six new species can be separated, which occur in three successive rock intervals. The new species M. carnatum, M. comtum, M. endocuboide, M. taourirtense, M. alidrisii and M. ibnishaqi are compared with time equivalent Central European species.

Late Cenomanian–Early Turonian ammonites of the southern Tethys margin from Morocco to Oman: Biostratigraphy, paleobiogeography and morphology

The Cenomanian–Turonian ammonite biostratigraphical framework for the southern Tethys margin (North Africa, Middle East and the Arabian Peninsula) is becoming better understood. A first attempt at a synthetic range chart is presented, with 85 taxa and precise correlations for ammonites along a west–east transect from Morocco to Oman, inclusive of the Trans-Saharan Seaway as far south as northern Nigeria. On the basis of a critical review of ammonite taxonomy, 13 bioevents can be identified in the interval from the Late Cenomanian to the Early Turonian (c. 3.5 myr) with each bioevent corresponding to a time interval of approximately 270,000 years, on average. They are consistent throughout several regions along the southern Tethys margin, though some gaps remain, at least at the stage boundary. These bioevents are correlated with the zonation defined for the stratotype (GSSP) of the base of the Turonian in the Western Interior (USA). The paleobiogeographic distribution of ammonites reveals some endemism but the predominant picture is that of a homogeneous fauna throughout the area, even though distinct Boreal and Western Tethys (Atlantic domain) marine influences are evident. An interpretation of the evolution of conch morphology and ornamentation through the zones of the Late Cenomanian–Early Turonian is proposed.

Cephalopod ancestry and ecology of the hyolith “Allatheca” degeeri s.l. in the Cambrian Evolutionary Radiation

Pyritized, elongate, conical conchs of “Allatheca” degeeri s.l. are common in dysoxic, dark gray mudstone intervals in the Early Cambrian (upper Terreneuvian–Series 2 boundary interval) Cuslett Formation at Keels, eastern Newfoundland. Wave-oriented, horizontal specimens are most abundant in this cool-water, high latitude, off-shore shelf facies of the Early Palaeozoic Avalon microcontinent. Based on conch morphology, shell microstructure, and the operculum, the species is an orthothecid hyolith. Comparison with the sizes of the early shells of planktic gastropods indicates a non-planktic life mode of “A.” degeeri s.l. hatchlings, although buoyancy calculations show that small juveniles with septate conchs to ca. 17mm long could have been nektic/planktic. If smaller “A.” degeeri s.l. individuals had a non-benthic mode of life, they and pseudoconodonts were the oldest skeletalized pelagic/nektic animals in the Cambrian Evolutionary Radiation. Most “A.” degeeri s.l. conchs at Keels are horizontally embedded and show a bimodal, wave-determined orientation, but about 10% of the large conchs are vertically embedded with their aperture down. As larger shells were not neutrally buoyant, the vertical orientations of about 10% of the conchs suggests an infaunal, likely detritivore, life mode suggestive of a scaphopod. Available morphologic and taphonomic evidence suggests that the vertically embedded conchs are in situ remains of dead benthic animals that colonized the bottom in better oxygenated intervals. Based on the current knowledge of Early Palaeozoic hyolith and cephalopod larval and adult morphologies, existing hypotheses of a planktic origin of cephalopods from hyolith ancestors are evaluated, and no evidence for such an evolutionary relationship is concluded to exist.

A key for the description of Palaeozoic ammonoids

A key for the description of Palaeozoic ammonoids is presented. It contains a catalogue of descriptive terms to characterise conch morphology, ornament, and suture line particularly of Carboniferous ammonoids. A number of examples of tables and illustrations are given for the description of conch characters, their ontogeny and intraspecific variability. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.200900008" target="_blank">10.1002/mmng.200900008</a>

A key for the description of Palaeozoic ammonoids

Abstract. A key for the description of Palaeozoic ammonoids is presented. It contains a catalogue of descriptive terms to characterise conch morphology, ornament, and suture line particularly of Carboniferous ammonoids. A number of examples of tables and illustrations are given for the description of conch characters, their ontogeny and intraspecific variability. doi: 10.1002/mmng.200900008

Ammonoid assemblages from the Asbian B 2b (Early Carboniferous: Mississippian) buildups of the Peak District, England

SUMMARY Ammonoids from the late Asbian (B 2b Assemblage Biozone) Cracoean buildups or reefs of the Peak District (Derbyshire and Staffordshire, England) are revised. Utilizing new collections and additional museum material, enhanced descriptions and illustrations are possible. Details of conch morphology, growth characteristics, suture development and shell ornament are presented for species belonging to the genera Bollandites, Bollandoceras, Beyrichoceras, Goniatites, Sygambrites, Nomismoceras and Prolecanites. The new species Bollandites rileyi sp. nov., Bollandoceras fordi sp. nov., Bollandoceras crowdecotense sp. nov. and Sygambrites mundyi sp. nov. are described. A neotype is proposed for Bollandites castletonensis and a lectotype for Beyrichoceras delicatum.