Double function of aptychi (Ammonoidea) as jaw elements and opercula

Lehmann, U. & Kulicki, C. 1990 10 15: Double function of aptychi (Ammonoidea) as jaw elements and opercula. Lethaia , Vol. 23, pp. 325–331. Oslo. ISSN 0024–1164.
Aptychi are calcitic coverings on the outer surface of organic ammonite lower jaws. They are similar in shape to that of the corresponding ammonite apertures. This observation and additional features of many aptychi are in harmony with their former interpretation as protective opercula. We suggest that they served as opercula in addition to functioning as jaws. The primary function of the lower jaws was thus secondarily extended to that of protective shields when they acquired their calcitic covering, while as lower jaws their importance dwindled to that of a more passive abutment. Phylogenetically, this seems to have started slowly in some anaptychi and became obvious with the first aptychi. ▭ Ammonites, aptychus, operculum, jaw apparatus, evolution, function .     

Aptychi microstructure in Late Cretaceous Ancyloceratina (Ammonoidea)

The microstructure of aptychi (bivalved calcareous coverings on lower jaws) of three genera of Late Cretaceous Ancyloceratina, Baculites, Polyptychoceras and Jeletzkytes is described for the first time on the basis of well-preserved and in situ material from the Western Interior of the USA and Hokkaido, Japan. Optical and scanning electron microscope observations of aptychi on polished median and cross-sections reveal some variation in their relative size, shape and microstructure among the three genera. The aptychus of Baculites is composed of two calcitic layers: one with tilted lamellae and the other one with horizontal lamellae, whereas those of Polyptychoceras and Jeletzkytes consist of a thin layer with horizontal lamellae. Comparison with aptychi (e.g. Laevaptychus) of Jurassic Ammonitina shows that the aptychi of Ancyloceratina differ from those of Jurassic Ammonitina in the smaller number of layers and the absence of a sponge-like structure. We propose for the first time growth models for a sponge-like aptychus of Jurassic Ammonitina and the lamellar aptychus of Cretaceous Ancyloceratina. The remarkable microstructural variation of aptychi observed in Mesozoic Ammonoidea is probably related to the diversity of their modes of feeding and the secondary function of the lower jaws as opercula.     

Microstructure of aptychi of Upper Jurassic (Upper Oxfordian) ammonites from Central Russia

To date, bivalve calcitic plates which cover the outer chitinous lamella of the lower jaws of Jurassic and Cretaceous ammonoids (aptychi sensu stricto) have been classified into several morphotypes (form genera) based on shape, surface sculpture and internal microstructure. However, previous works on aptychi microstructure focused mainly on thick morphotypes (e.g. Laevaptychus and Lamellaptychus), whereas little was investigated for thin Praestriaptychi. In this study, the microstructure of Praestriaptychi of Upper Oxfordian Perisphinctes and of recently discovered aptychi of aspidoceratid microconch Mirosphinctes (both belonging to the superfamily Perisphinctoidea) is described. These aptychi are compared with Laevaptychi of Upper Oxfordian macroconchs Euaspidoceras (a dimorphic counterpart of Mirosphinctes). This study demonstrated that the aptychi of Perisphinctes and Mirosphinctes differ from each other and from Laevaptychi in their microstructure, number of calcitic layers and in growth patterns. The aptychi of both aspidoceratid dimorphs are similar in terms of growth pattern, but differ in microstructure and the number of calcitic layers. Neither aptychi of Perisphinctes nor of Mirosphinctes have a tubular layer with a honeycomb surface pattern, which is typical for Laevaptychus. This indicates that aptychi were extremely diverse and their microstructure varied significantly not only within the superfamily, but even within a dimorphic pair of aspidoceratid ammonites. A lack of a tubular layer in Praestriaptychus indicates that it developed independently in Lamellaptychus of Haploceratoidea and Laevaptychus of Aspidoceratidae.     

Aptychi: the myth of the ammonite operculum

Re-examination of ammonite specimens with the aptychus apparently in place closing the aperture has shown that these are accidents of preservation and have been misinterpreted as indicating that aptychi functioned as opercula. There remain no convincing grounds for doubting that aptychi functioned only as lower jaws of ammonites, not as opercula. Chance preservation of exceptional specimens can sometimes be misleading. Ammonoidea, aptychi, anoptychi, functional morphology .     

Über Nahrung und Ernährungsweise von Ammoniten

Until only a few years ago, the majority of ammonites were considered to have been rather swift nectonic swimmers of little ecologic bondage. About their food, nothing was known with certainty. Comparison with recent Coleoids and withNautilus suggests, however, that most ammonoids were poor swimmers. It is not even certain that the hyponome was used for locomotion as much as inNautilus. Slow locomotion made it impossible for ammonites to hunt for fish or other highly agile prey. Their lower jaws, which are identical to the structures known as aptychi and anaptychi (formerly considered to be opercula), are like shovels and make it impossible for their bearers actually to bite and cut with them. Consequently the structure of their jaws also prevented their hunting large and active prey. Their jaws were probably actually used like shovels along the bottom of the sea. They may have gathered small or slow benthonic organisms like foraminifers, ostracodes, small crustaceans, young brachiopods, corals, bryozoa, but also drifting or slowly swimming and dead organisms. Several unusually well preserved stomachs of ammonites contained foraminifers, ostracodes, crinoids, smaller ammonites, and possibly inarticulate brachiopods. It is concluded that, ecologically, ammonites inhabited the place of modern bottom-feeding gastropods.     

First find of aptychi of <Emphasis Type="Italic">Leioceras</Emphasis> and <Emphasis Type="Italic">Bredyia</Emphasis> (Ammonoidea,Hildoceratoidea) in the Aalenian of Northern Caucasus,Russia

Aptychi are reported from the Lower Aalenian Leioceras opalinum Zone and Subzone of the Khussa-Kardonik section in Karachay-Cherkessia (Kuban River Basin). They are associated with conchs of the ammonites genera Leioceras and Bredyia (superfamily Hildoceratoidea) and interpreted as their lower jaws. The Hildoceratoidea aptychi are referred to a form group defined by Trauth as ‘cornaptychi’. Wide valves with a straight apical angle (morph I) probably belong to Bredyia. Aptychi with an acute apical angle (wide in the basal part; morph II) and relatively narrow ones (morph III) belong to macroconchs and microconchs of Leioceras, respectively. Aptychi assignable to Leioceras opalinum are herein also reported from Southern Germany.     

Phylogeny of the aptychi-possessing Neoammonoidea (Aptychophora nov., Cephalopoda)

The different forms of the aptychi (opercula, homologous with lower jaws) of the Ammonoidea are used for the first time in a phylogenetic analysis of part of the classic Ammonoidea phylogeny. The results indicate that the aptychi-possessing ammonoids form a monophylum for which we propose the informal name Aptychophora nov. Among the Jurassic ammonoids, it is possible to recognize several monophyletic groups. In part, our results support existing superfamilies (e.g. Hildocerataceae, Haplocerataceae) by new synapomorphies. However, the Perisphinctaceae can now be much more clearly differentiated than in the previously established phylogenetic tree. The Upper Cretaceous ammonoid superfamilies cannot be derived from the Haplocerataceae, but are descendants of a 'primitive' perisphinctacean possessing a praestriaptychus. Nor can they be derived from the 'higher' perisphinctaceans (family Perisphinctidae) because that clade is characterized by granulaptychi. The consequence of these results is that the quadrilobate primary suture of the 'Ancyloceratina' must have evolved more than once by reduction from an ancestral quinquelobate primary suture. The Ancyloceratidae have praestraptychi or aptychi types which can be derived from praestriaptychi, whereas the Crioceratitinae have longitudinally striated anaptychi.     

Microstructure and mineralogy of the outer calcareous layer in the lower jaws of Cretaceous Tetragonitoidea and Desmoceratoidea (Ammonoidea)

Tanabe, K., Landman, N.H. & Kruta, I. 2011: Microstructure and mineralogy of the outer calcareous layer in the lower jaws of Cretaceous Tetragonitoidea and Desmoceratoidea (Ammonoidea). Lethaia, Vol. 45, pp. 191–199. Based on the differences in their relative size, overall shape, structure and the degree of development of an outer calcified covering, lower jaws of the Ammonoidea have been classified into four morphotypes: normal, anaptychus, aptychus and rhynchaptychus types. However, detailed microstructural and mineralogical comparison of these morphotypes has not yet been addressed. This article documents the results of SEM and XRD observations of the lower jaws of three Late Cretaceous ammonoid species belonging to the Tetragonitoidea (Anagaudryceras limatum) and Desmoceratoidea (Pachydiscus kamishakensis and Damesites aff. sugata), based on excellent material preserved in situ within the body chamber, and retaining an aragonitic shell wall. The lower jaws of the three species are assigned to an intermediate form between anaptychus and aptychus types for the first two species and the rhynchaptychus type for the third species. Their black, presumably originally chitinous outer lamella is wholly covered with a calcareous layer. The calcareous layer is composed of aragonite in D. aff. sugata and A. limatum, and calcite in P. kamishakensis. The microstructure of the outer calcareous layer differs among the three species, i.e. granular in A. limatum, spherulitic prismatic in D. sugata, and prismatic in P. kamishakensis, all of which can be distinguished from the lamellar and spongy structure of the outer‐paired calcitic plates of the typical aptychus‐type lower jaws in some Jurassic and Cretaceous Ammonitina and Ancyloceratina. Our study suggests that most Jurassic and Cretaceous ammonoids possessed an outer calcareous layer in their lower jaws, although its mineralogy, microstructure and relative thickness vary among different taxa. □Ammonoidea, Cretaceous, Desmoceratoidea, lower jaw, microstructure, Tetragonitoidea.     

Ammonite aptychi: Functions and role in propulsion

Seven previous proposals of aptychus (sensu stricto) function are reviewed: lower mandible, protection of gonads of females, protective operculum, ballasting, flushing benthic prey, filtering microfauna and pump for jet propulsion. An eighth is introduced: aptychi functioned to actively stabilize the rocking produced by the pulsating jet during forward foraging and backward swimming. Experiments with in-air models suggest that planispiral ammonites could lower their aperture by the forward shift of a mobile cephalic complex. In the experiments, the ventral part of the peristome is lowered from the lateral resting (neutral) position by the added “ballast” of a relatively thin Laevaptychus to an angle < 25° from horizontal with adequate stability to withstand the counter-force produced by the jet of the recurved hyponome. However, of the shell forms tested, only brevidomes with thick aptychi, e.g., the Upper Jurassic Aspidoceratidae with Laevaptychus and average whorl expansion rates, were stable enough to swim forward by jet propulsion at about Nautilus speed (∼ 25 cm/s). We propose that aptychus function most commonly combined feeding (jaw, flushing, filtering) with protection (operculum), and, more rarely, with locomotion (ballast, pump, diving and stabilizing plane). Aptychi may thus have been multi-functional.     

The jaw apparatuses of Cretaceous Phylloceratina (Ammonoidea)

The jaw apparatuses of two species of Late Cretaceous Phylloceratina (Ammonoidea), Hypophylloceras subramosum and Phyllopachyceras ezoensis, are described on the basis of well‐preserved in situ material from Hokkaido, Japan. Gross morphological and X‐ray CT observations reveal that the upper and lower jaws of the two species are essentially similar in their overall structure. Their upper jaws consist of a shorter outer lamella and a pair of larger, wing‐like inner lamellae that become narrower and join together in the anterior portion, as in those of other ammonoids. The upper jaws of the two phylloceratid species are, however, distinguishable from those of other known ammonoids by the presence of a thick, arrowhead‐shaped calcified rostral tip. The lower jaws of the two species consist of a short, reduced inner lamella and a large, gently convex outer lamella covered with a thin calcareous layer, the features of which are common with the rhynchaptychus‐type lower jaws of the Cretaceous Lytoceratina. In the presence of a sharply pointed, thick calcareous tip on upper and lower jaws, the jaw apparatuses of the Phylloceratina resemble those of modern and fossil nautilids, suggesting that they were developed to serve a scavenging predatory feeding habit in deeper marine environments. This and other studies demonstrate that at least some Mesozoic rhyncholites and conchorhynchs are attributable to the Phylloceratina and Lytoceratina.     

Biological and stratigraphical significance of the Silurian nautiloid Aptychopsis

Turek, Vojtach 1978 04 15: Biological and stratigraphical significance of the Silurian nautiloid
Circular or elliptical structures described as the genus Aptychopsis Barrande 1872 (and mostly referred to the Crustacea) represent the opercula of a specialized group of orthoceratids. Their morphology bears a considerable resemblance to the structure of the lower jaws (aptychi) of some Mesozoic ammonoids, particularly of the genus Physodoceras . Because of its size and structure, however, Aptychopsis cannot have functioned as a jaw apparatus. Observation of an operculumin siru supports this assumption as regards its functional and systematic position. The classification of the genus Aptychopsis as given by Jones and Woodward (1872–1893) is purely typological and hardly applicable in palaeobiology. Continuous gradations exist between the various morphological types. The lack of systematically distinctive features explains the unsuitability of Aptychopsis for biostratigraphical use. Aptychopsids occur in the Silurian of Central Bohemia between the Spirograprus turriculatus Zone and the Neodiversograptus nilssoni Zone, inclusive, with maximum frequency in the uppermost Wenlockian.     

Size and function of ammonite aptychi in comparison with buccal masses of modern cephalopods

Morton, Nicol & Nixon, Marion 1987 07 15: Size and function of ammonite aptychi in comparison with buccal masses of modem cephalopods.
Previous impressions that the size of ammonite aptychi is unusually large, thereby posing a problem to their interpretation as part of the jaw apparatus, are shown to be incorrect. The relative length of the aptychus for at least the Jurassic ammonite groups studied, at approximately 15% of the length of the body chamber, is remarkably constant in different taxonomic groups and sizes. This is well within the range for buccal mass length as percentage of mantle length of living cephalopods, being most similar to Octopus and Sepia and much smaller than Nautilus. The height and width of aptychi relative to whorl height and width are larger but again ammonites are probably not significantly different from modern cephalopods. The size and design, with no obvious structures for biting or crushing, suggest that ammonites were adapted to a particular type of relatively unspecialised feeding in which mostly small animals were ingested, possibly with some external digestion. The large shovel-like lower jaw may have functioned like a scoop for collecting large quantities of water and small prey, and movement of the buccal complex with the upper jaw almost closed against the lower jaw could have expelled water while retaining captured prey. Calcification of aptychi may have been protective, but more likely acted to weight the buccal mass for nektobenthic feeding and to make it more rigid.     

A New Approach for the Determination of Ammonite and Nautilid Habitats

Externally shelled cephalopods were important elements in open marine habitats throughout Earth history. Paleotemperatures calculated on the basis of the oxygen isotope composition of their shells can provide insights into ancient marine systems as well as the ecology of this important group of organisms. In some sedimentary deposits, however, the aragonitic shell of the ammonite or nautilid is poorly or not preserved at all, while the calcitic structures belonging to the jaws are present. This study tests for the first time if the calcitic jaw structures in fossil cephalopods can be used as a proxy for paleotemperature. We first analyzed the calcitic structures on the jaws of Recent Nautilus and compared the calculated temperatures of precipitation with those from the aragonitic shell in the same individuals. Our results indicate that the jaws of Recent Nautilus are secreted in isotopic equilibrium, and the calculated temperatures approximately match those of the shell. We then extended our study to ammonites from the Upper Cretaceous (Campanian) Pierre Shale of the U.S. Western Interior and the age-equivalent Mooreville Chalk of the Gulf Coastal Plain. In the Pierre Shale, jaws occur in situ inside the body chambers of well-preserved Baculites while in the Mooreville Chalk, the jaw elements appear as isolated occurrences in the sediment and the aragonitic shell material is not preserved. For the Pierre Shale specimens, the calculated temperatures of well-preserved jaw material match those of well-preserved shell material in the same individual. Analyses of the jaw elements in the Mooreville Chalk permit a comparison of the paleotemperatures between the two sites, and show that the Western Interior is warmer than the Gulf Coast at that time. In summary, our data indicate that the calcitic jaw elements of cephalopods can provide a reliable geochemical archive of the habitat of fossil forms.     

Unusual kinematics and jaw morphology associated with piscivory in the poeciliid,Belonesox belizanus

Piscivory in fishes is often associated with the evolution of highly elongate jaws that achieve a large mouth opening, or gape. Belonesox belizanus, the pike killifish, has independently evolved this morphology, which is derived from short-jawed poeciliids within the Cyprinodontiformes. Using kinematic analysis of high-speed video footage, we observed a novel aspect of the elongate jaws of Belonesox; the premaxilla rotates dorsally during mouth opening, while the lower jaw rotates ventrally. Anatomical study revealed that this unusual motion is facilitated by the architecture of the premaxillomandibular ligament, prominent within cyprinodontiforms. In Belonesox, it allows force to be transferred from the lower jaw directly to the premaxilla, thereby causing it to rotate dorsally. This dorsal rotation of the premaxilla appears to be assisted by a mediolateral twisting of the maxilla during jaw opening. Twisting maxillae are found in members of the group such as Fundulus, but are lost in Gambusia. Models revealed that elongate jaws partially account for the enlarged gape, but enhanced rotation at the quadrato-mandibular joint was equally important. The large gape is therefore created by: (i) the convergent evolution of elongate jaws; (ii) enhanced jaw rotation, facilitated by loss of a characteristic cyprinodontiform trait, the lip membrane; and (iii) premaxilla rotation in a novel direction, facilitated by the retention and co-option of additional cyprinodontiform traits, the premaxillomandibular ligament and a twisting maxilla.     

Chemico-structural phylogeny of the discinoid brachiopod shell

Stratiform shells of living discinids are composed of membranous laminae and variously aggregated, protein-coated granules of apatitic francolite supported by proteinaceous and chitinous nets in glycosaminoglycans (GAGs) to form laminae in rhythmic sets. The succession is like that of living lingulids but differs significantly in the structure of the periostracum, the nature of baculate sets and in its organic composition. In particular, discinids have a higher level of amino acids although with relatively lower acidic and higher basic concentrations; and their overall lower organic content is owing to lower levels of hydrophilic components, like GAGs and chitin. The organic constituents are not completely degraded during fossilization; but data are presently too meagre to distinguish between linguloid and discinoid ancestries. Many differences among three of the four described extant genera emanate from transformations with a long geological history. Pelagodiscus is characterized by regular, concentric rheomorphic folding (fila) of the flexible periostracum and the plastic primary layer and by sporadically developed hemispherical imprints of periostracal vesicles. Both features are more strikingly developed in Palaeozoic discinids. In the oldest discinid, the Ordovician Schizotreta, and the younger Orbiculoidea and related genera, vesicles were persistent, hexagonal close-packed arrays fading out over fila. They must have differed in composition, however, as the larger vesicles of Schizotreta were simple (possibly mucinous), whereas the smaller vesicles of Orbiculoidea and younger genera were composites of thickly coated spheroids, possibly of lipoproteins, which survive as disaggregated relicts in Pelagodiscus. Baculate sets within the secondary layer are also less well developed in living discinids, being incipient in Pelagodiscus and restricted to the dorsal valve of Discinisca. The trellised rods (baculi) with proteinaceous cores are composed of pinacoids or prisms of apatite, depending on whether they are supported by chitinous nets or proteinaceous strands in GAGs. This differentiation occurred in Schizotreta but in that stock (and Trematis) the baculate set is symmetrical with baculi subtended between compact laminae, whereas in younger and post-Palaeozoic species the outer bounding lamina(e) of the set is normally membranous and/or stratified. The most striking synapomorphy of living discinids is the intravesicular secretion of organsiliceous tablets with a crystalline habit within the larval outer epithelium and their exocytosis as a close- or open-packed, transient, biomineral cover for larvae. Canals, on the other hand, are homologous with those pervading lingulid shells. Both systems interconnect with chitinous and proteinaceous sets and have probably always served as vertical struts in an organic scaffolding supporting the stratiform successions. A phylogenetic analysis based mainly on shell structure confirms the discinoids as the sister group of the linguloids but, contrary to current taxonomic practice, also supports the inclusion of acrotretoids within a ''discinoid'' clade as a sister group to the discinids.     

Redescription of the geologically youngest albanerpetontid (?Lissamphibia): Albanerpeton inexpectatum Estes and Hoffstetter, 1976, from the Miocene of France

Albanerpeton inexpectatum Estes and Hoffstetter, 1976, the type species of Albanerpeton and the geologically youngest albanerpetontid, is rediagnosed and redescribed based on a large collection of jaws and frontals from Miocene fissure fills near La Grive-Saint-Alban, southeastern France. Intraspecific variation is documented in these elements, and is attributed to growth and individual differences. Synapomorphies of the upper jaws indicate that A. inexpectatum a) belongs in a clade whose members are otherwise known from the Upper Cretaceous-Paleocene of North America and b) is the sister species of an undescribed North American Paleocene species. The presence of A. inexpectatum in the Miocene of France is postulated to be the result of an Early or Middle Tertiary dispersal of an unknown ancestral species from North America into Europe. Cranial apomorphies of A. inexpectatum are interpreted as having strengthened the skull for burrowing in rocky soil and feeding.     

Feeding biomechanics in Acanthostega and across the fish–tetrapod transition

Acanthostega is one of the earliest and most primitive limbed vertebrates. Its numerous fish-like features indicate a primarily aquatic lifestyle, yet cranial suture morphology suggests that its skull is more similar to those of terrestrial taxa. Here, we apply geometric morphometrics and two-dimensional finite-element analysis to the lower jaws of Acanthostega and 22 other tetrapodomorph taxa in order to quantify morphological and functional changes across the fish–tetrapod transition. The jaw of Acanthostega is similar to that of certain tetrapodomorph fish and transitional Devonian taxa both morphologically (as indicated by its proximity to those taxa in morphospace) and functionally (as indicated by the distribution of stress values and relative magnitude of bite force). Our results suggest a slow tempo of morphological and biomechanical changes in the transition from Devonian tetrapod jaws to aquatic/semi-aquatic Carboniferous tetrapod jaws. We conclude that Acanthostega retained a primitively aquatic lifestyle and did not possess cranial adaptations for terrestrial feeding.     

Vertebrate head development: Segmentation,novelties, and homology

Vertebrate head development is a classical topic lately invigorated by methodological as well as conceptual advances. In contrast to the classical segmentalist views going back to idealistic morphology, the head is now seen not as simply an extension of the trunk, but as a structure patterned by different mechanisms and tissues. Whereas the trunk paraxial mesoderm imposes its segmental pattern on adjacent tissues such as the neural crest derivatives, in the head the neural crest cells carry pattern information needed for proper morphogenesis of mesodermal derivatives, such as the cranial muscles. Neural crest cells make connective tissue components which attach the muscle fiber to the skeletal elements. These crest cells take their origin from the same visceral arch as the muscle cells, even when the skeletal elements to which the muscle attaches are from another arch. The neural crest itself receives important patterning influences from the pharyngeal endoderm. The origin of jaws can be seen as an exaptation in which a heterotopic shift of the expression domains of regulatory genes was a necessary step that enabled this key innovation. The jaws are patterned by Dlx genes expressed in a nested pattern along the proximo-distal axis, analogous to the anterior–posterior specification governed by Hox genes. Knocking out Dlx 5 and 6 transforms the lower jaw homeotically into an upper jaw. New data indicate that both upper and lower jaw cartilages are derived from one, common anlage traditionally labelled the “mandibular” condensation, and that the “maxillary” condensation gives rise to other structures such as the trabecula. We propose that the main contribution from evolutionary developmental biology to solving homology questions lies in deepening our biological understanding of characters and character states.     

Ouzocerus gracilis n.g., n.sp., Bovidae (Artiodactyla,Mammalia) du Vallésien (Miocène supérieur) de Macédoine (Grèce)

A new antilope, Ouzocerus gracilis n.g., n.sp., isdescribed from a late Miocene layer from Northern Greece. It is known from an incomplete skull, jaws and limb bones. A comparison with other spiral horned antilopes shows it has some affinities with Protragelaphus but it is clearly different. Its known geographic area extends from Greece to Iran.