Buckman's Paradox: variability and constraints on ammonoid ornament and shell shape

Buckman's Law of Covariation states that ammonoid shell shape and ornamentation are typically correlated, such that compressed, involute forms have light ornament while more inflated, evolute forms have heavier ornament. Such covariation has been observed in many ammonoid groups, and implies a link between the morphogenesis of shell shape and ornamentation. However, other evidence suggests that while ornament growth is controlled by the genetic‐developmental program of the ammonoid, shell shape is strongly influenced by environmental factors. These differing viewpoints lead to Buckman's Paradox – are ornamentation and shell shape tightly linked, as implied by Buckman's covariation, or is the morphogenesis of ornament controlled genetically, while shell shape is controlled environmentally? To address this issue, the variability of shell shape and rib morphology has been compared for a group of endemic acanthoceratid ammonites from the Cretaceous Western Interior Seaway of North America. If Buckman's Law holds due to a morphogenetic connection between shell shape and ornamentation, we would expect taxa with more variable shell shapes to also show more variable rib features and growth. Morphometric analysis of seven shell shape and two rib characters for the Western Interior acanthoceratids finds no such correlation, suggesting that shell shape and rib growth are controlled by different processes. Indeed, rib growth appears to be more constrained than shell shape, consistent with the view that ornamentation is more tightly controlled by the developmental‐genetic growth program of the ammonoid. These results emphasize the complexity of ammonoid morphogenesis and highlight our limited understanding of the causes underlying Buckman's Law.     

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.     

Analysis of a Carboniferous embryonic ammonoid assemblage implications for ammonoid embryology

An embryonic ammonoid assemblage was discovered in a carbonate concretion recovered from a dysoxic, relatively offshore marine shale of Virgilian (Upper Pennsylvanian) age in Kansas, USA. The assemblage consists primarily of two species of the Goniatitina, Aristocerassp. and Vidrioceras sp., whose initial chambers (protoconchs) differ in size and shape. Microscopic observations of serial thin sections of specimens at different growth stages reveal the sequence of embryonic shell development starting with the formation of the initial chamber and ending with the synchronous secretion of a prismatic proseptum and nacreous swelliig (primary varix) at the aperture. The mode of occurrence of the embryonic shells of the two species in the concretion suggests that these ammonoids produced numerous small offspring, a reproductive strategy similar to that in many extant coleoids. □ Ammonoids, embryonic shells, development, Carboniferous, Kansas.     

First direct evidence of ammonoid ovoviviparity

Ammonoids had high evolutionary rates and diversity throughout their entire history and played an important role in the high‐resolution sub‐division of the Mesozoic, but much of their palaeobiology remains unclear, including the brooding habitat. We present our study of the first recorded ammonite embryonic shell clusters preserved with calcified embryonic aptychi in situ within the body chambers of mature macroconch shells of the Early Aptian (Early Cretaceous) ammonite Sinzovia sazonovae. The following support the idea that the clusters are egg masses, which developed inside ammonite body chambers: the absence of post‐embryonic shells and any other fossils in these clusters, the presence of the aptychi in all embryonic shell apertures and peculiarities of adult shells preservation. These facts confirm earlier speculations that at least some ammonoids could have been ovoviviparous and that, like many modern cephalopods, they could have reproduced in mass spawning events. The aptychi of ammonite embryonic shells are observed here for the first time, indicating that they were already formed and calcified before hatching. Our results are fully congruent with the peculiar modes of ammonoid evolution: quick recovery after extinctions, distinct evolutionary rates, pronounced sexual dimorphism and the nearly constant size of embryonic shells through ammonoid history. We assume that adaptation to ovoviviparity may be the reason for the presence of these features in all post‐Middle Devonian ammonoids.     

Recent advances in heteromorph ammonoid palaeobiology

Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, heteromorphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles, a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontogeny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro- and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40–500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.     

Revision of the Permian ammonoid family Cyclolobidae

The development of views on the phylogeny of the family Cyclolobidae Zittel, 1895 is discussed. The generic composition is emended, phylogenetic links are indicated, a new version of the phylogenetic scheme is proposed, and phylogeny of the key genera is traced at species level. The sutural ontogeny of the genus Mexicoceras is examined, its placement in the family Cyclolobidae is supported, and its position in the phylogenetic reconstruction of the family is determined.     

Early Carboniferous (Mississippian) ammonoid biogeography

Cluster analyses and non-metric multidimensional scaling of ammonoid assemblages from six successive Early Carboniferous time intervals (two Tournaisian, Viséan, and Serpukhovian) show that increasing provincialism is the main biogeographic trend through the period. While the assemblages from the Tournaisian to the early Late Viséan are globally similar in the tropical occurrences, dissimilarities appear in the middle Late Viséan and become pronounced in the latest Viséan, Pendleian, and Arnsbergian. From the latest Viséan, two major biogeographic realms are recognisable. A north-western realm consists of the four North Variscan regions (Rhenish Mountains, North England, South Portugal, and Moroccan Meseta) as well as the American Midcontinent and the Antler Foreland Basin. The second realm includes the South Urals, which is the most diverse region and thus has an isolated position, as well as the Jadar Block of Serbia, the Cantabrian Mountains of Spain, the Anti-Atlas of Morocco, and the Saoura Valley of Algeria.     

Post-pathological keel-loss compensation in ammonoid growth

Among the various pathologies documented in ammonoids, impairs affecting the apertural margin may have long-lasting sequelae on subsequent shell geometry. An interesting healing pattern, known as sculptural compensation, led to the permanent replacement of an ornament by adjacent sculptural elements. Moreover, in several ventrally impaired individuals the symmetry was preserved. Those developed annular ribs in place of any previous ventral ornamentation (keel, sulcus or smooth area). This phenomenon is known from diverse ammonite families. Monestieria resouchei (Monestier 1931), type species of 'Monestieriinae' Sapunov 1965, displays exactly that type of annularly-ribbed morphology and has been shown to be otherwise similar to species of Grammoceratinae Buckman 1904 occurring in the same beds, thus corroborating its pathological nature and leading to the rejection of that taxon. Now, keel absence in Praehaploceras Monestier 1931 and Buckmanites Guex 1973 cannot be explained by the same process as they do not have annular ribs. Moreover, the absence of any clue of malformation, their relative frequency and specific characteristics exclude the previously suggested synonymies with Pseudolioceras Buckman 1889 as equivalent pathological forms. In consequence, their rehabilitation is herein proposed. They should be included within Harpoceratinae Neumayr 1875.     

Tithonian ammonoid biostratigraphy in eastern Himalayan Tibet

The Tithonian and Lower Berriasian sediments in the eastern Himalayas of Tibet contain an extensive sequence of ammonoid fauna. New collections in situ through the Lanongla, Pure, Gucuo and Jiapeila sections have facilitated a major revision of the ammonoid assemblages. Probably due to depositional facies segregation, the Belemnopsis galoi-bearing beds can be regarded as the oldest Tithonian sediments in which the basal Tithonian ammonoid Kossmatia is not present. The Lower Tithonian includes the Virgatosphinctes-Aulacosphinctoides and Uhligites-Aulacosphinctes assemblages; the Upper Tithonian includes the Haplophylloceras pingue, Blanfordiceras wallichi and Haplophylloceras strigile-Corongoceras-Himalayites assemblages. The Spiticeras assemblage is suggested to be from the Lowermost Berriasian. The new ammonoid assemblages at well-defined levels in Himalayan Tibet provide some crucial links for correlation with other regions of the SW Pacific domain where these ammonoid genera have been widely distributed.     

Early evolutionary trends in ammonoid embryonic development

During the Devonian Nekton Revolution, ammonoids show a progressive coiling of their shell just like many other pelagic mollusk groups. These now extinct, externally shelled cephalopods derived from bactritoid cephalopods with a straight shell in the Early Devonian. During the Devonian, evolutionary trends toward tighter coiling and a size reduction occurred in ammonoid embryonic shells. In at least three lineages, descendants with a closed umbilicus evolved convergently from forms with an opening in the first whorl (umbilical window). Other lineages having representatives with open umbilici became extinct around important Devonian events whereas only those with more tightly coiled embryonic shells survived. This change was accompanied by an evolutionary trend in shape of the initial chamber, but no clear trend in its size. The fact that several ammonoid lineages independently reduced and closed the umbilical window more or less synchronously indicates that common driving factors were involved. A trend in size decrease of the embryos as well as the concurrent increase in adult size in some lineages likely reflects a fundamental change in reproductive strategies toward a higher fecundity early in the evolutionary history of ammonoids. This might have played an important role in their subsequent success as well as in their demise.     

Mechanical significance of ammonoid septa with complex sutures

Pre- and post-19th century hypotheses relating hydrostatic pressure to the mechanical function of sutural complexity are compared. The old ideas gave rise to the 19th century 'Buckland hypothesis', which is in turn largely synonymous with the 'Westermann model'. Buckland (1836) postulated that fluted septa buttressed the weak flanks of the phragmocone wall. Two new parameters are introduced to define the covariation between the strength of cylindrical segments of the wall flank bounded by the distance between adjacent lobe and saddle-flutes in transverse sections. The product of the index of wall strength (IWS) and this inverse support angle (ISA) predicts the buckling pressure in a cylinder of infinite length, and it implies that coiled nautiloids were more likely to be imploded via their whorl flanks than the apparently weaker oxyconic ammonoids. The widely used index of sutural complexity (ISC) measures the marginal corrugation which obscures this trend and acts as an elastic bed for both strong and weak walls. However, the ISC is more proportional to habitat depth than the buckling pressure when all other factors are constant. The central thickness of each fluted septum was increased in direct proportion to the distance spanned by the septum and the hydrostatic pressure on it in the 'last septum' position. The marginal thickness was maintained at a more constant value, which permitted the suture to increasingly act like a spring or shock absorber, as the wall thickness was enlarged during ontogeny. Both the ratios, between the central and marginal thicknesses and the closely related ISC, therefore, increased with shell diameter and habitat depth.     

Revision of the permian ammonoid family thalassoceratidae Hyatt

The paper discusses various interpretations of the taxonomy and phylogeny of the Late Paleozoic ammonoid family Thalassoceratidae, including our own interpretation of the composition and evolution of this group. We propose a revised classification and phylogenetic reconstruction of thalassoceratids, and provide updated information on the stratigraphic and geographical distribution of thalassoceratid species. Examination of the inner whorls of Thalassoceras gemmellaroi, Aristoceras appressum, and Prothalassoceras bashkiricum has shown that the siphuncle was nonventral throughout the first whorl.     

Extensive intraspecific variation in a Triassic ammonoid from Siberia

The ammonoid species Czekanowskites rieberin.sp . from the Lower Anisian of Arctic Siberia (River Olenek) shows an enormous range of intraspecific variability. The 700 investigated specimens are packed in one single concretion. They are all adult with preserved living chambers and apertural structures. Morphologically they range from keeled smooth suboxycones with narrow umbilicus, through feebly ribbed platycones with a little wider umbilicus, to subcadicones with relatively wide umbilicus and straight ribs with bullae. The morphological variation is continuous; all specimmh belong to a single genetically linked population, variants of one biospecies. This phenomenon, which is quite common in boreal Triassic ammonoid faunas, has in the past led to serious taxonomic oversplitting. It presents a serious challenge to current ideas about a close correlation between modeof life and shell morphology in arnmonoids.□ Amntonoidea, boreal Triassic, ecology. Siberia, variability .     

Relationships Between Behavioral Rhythms,Plasma Corticosterone and Hypothalamic Circadian Rhythms

Circadian rhythms in physiological processes and behaviors were compared with hypothalamic circadian rhythms in norepinephrine (NE) metabolites, adrenergic transmitter receptors, cAMP, cGMP and suprachiasmatic nucleus (SCN) arginine vasopressin (AVP) in a single population of rats under D: D conditions. Eating, drinking and locomotor activity were high during the subjective night (the time when lights were out in L: D) and low during the subjective day (the time when lights were on in L: D). Plasma corticosterone concentration rose at subjective dusk and remained high until subjective dawn. Binding to hypothalamic α₁- and β-adrenergic receptors also peaked during the subjective night. Cyclic cGMP concentration was elevated throughout the 24-hr period except for a trough at dusk, whereas DHPG concentration peaked at dawn. Arginine vasopressin levels in the suprachiasmatic nucleus peaked in the middle of the day. No rhythm was found either in binding to the α₂-adrenergic receptor, or in MHPG or cAMP concentration. Behavioral and corticosterone rhythms, therefore, are parallel to rhythms in hypothalamic α₁-and β-receptor binding and NE-release. Cyclic GMP falls only at dusk, suggesting the possibility that cGMP inhibits activity much of the day and that at dusk the inhibition of nocturnal activity is removed. SCN AVP, on the other hand, peaking at 1400 hr, may play a role in the pacemaking function of the SCN that drives these other rhythms.