Based on medical computed tomography (CT) data, herein I document the ontogenetic changes in conch morphology of Maorites densicostatus and M. seymourianus. For this purpose, 20 specimens were measured in 30° steps starting from a shell diameter of 10 mm, to obtain the ontogenetic trajectories of several conch parameters. Results show that four morphological parameters, aperture height, whorl height, whorl width, and umbilical width have a low variation and can be easily modeled. In contrast, derived indices and whorl expansion rates show a high variation, and their ontogenetic trajectories can be categorized in two stages: the first stage is identified by a rapid morphological change (Perlatum stage), followed by a second stage showing a stable conch morphology (Gibbosum stage). Furthermore, the ontogenetic trajectories revealed that in adulthood, M. densicostatus exhibits similar morphologies to juveniles and sub-adults of M. seymourianus. This phenomenon could indicate a possible sizebased heterochronic shift (paedomorphosis) as a possible evolutionary mechanism.

Findings of ammonoid soft tissues are extremely rare compared to the rich fossil record of ammonoid conchs ranging from the Late Devonian to the Cretaceous/Paleogene boundary. Here, we apply the computed-tomography approach to detect ammonoid soft tissue remains in well-preserved fossils from the Early Cretaceous (early Albian) of NE-Germany of Proleymeriella. The ammonites were found in glauconitic–phosphatic sandstone boulders. Analyses of the high-resolution Ct-data revealed the presence of cameral sheets, the siphuncular tube wall, and the siphuncle itself. The siphuncle is a long, segmented soft tissue that begins at the rear end of the body chamber and comprises blood vessels. Chemical analyses using energy-dispersive spectroscopy (EDS) showed that all preserved soft tissues were phosphatized and are now composed of fluorapatite. The same holds true for preserved shell remains that locally show the nacreous microstructure. We provide a short description of these soft tissue remains and briefly discuss the taphonomic pathway.

Here we study the variation of the Paleozoic to Mesozoic ammonoid whorl profile shape (WPS, in previous contributions referred as the whorl cross-section shape) employing a geometric morphometric approach, virtual modelling, and statistical tools to assess possible biological phenomena. For this approach, a dataset covering 300 individuals (each belonging to a different genus) was generated from revised specimens and published literature data. From this dataset, we proposed several predictors (e.g., several size parameters) that could be applied in linear models using the principal components (resulting from the geometric morphometric analyses) as response variables. The whorl profile area from the specimens was calculated from virtual models of their original whorls. The results show that the variation in the whorl profile in this sample is resumed by 10 principal components, of which the first three cover 87% of the total variation. PC1 (54.5%) illustrates transformations mainly from the degree of compression and, to a lesser extent, the degree of whorl overlap. Furthermore, the statistical results indicate a significant association between PC1 with several ammonoid subtaxa and whorl size. A correlation between PC1 and the whorl area is also recognised. PC2 (24.8%) reflects morphological transformations that emulates stretching and compression of the whorl profile shape; it is likely related to the hydrodynamic performance of the resulting conch morphology. PC3 (7.4%) encompasses changes on the shape of the venter (from flat to rounded), and the curvature of the flanks, and it seems to be related to minor modifications required to achieve some extreme shapes.

Linear morphometrics is the most widely applied technique to study the variation of the conch morphology in ammonoids and other ectocochleate cephalopods. However, because this method frequently relies upon a few linear measurements, it lacks the explanatory power to accurately characterize the shape of the whorl cross-section, which is instead discussed solely in descriptive terms, e.g., elliptical, triangular, or subquadrate. Here, we introduce a landmark-based geometric morphometric approach to study ammonoid whorl cross-sections, derived from the regularly used morphometric parameters in cephalopods. This new technique uses virtual modelling to generate semilandmark configurations and virtual models of whorl cross-sections. We applied it to study 50 ammonoid specimens belonging to 48 genera exhibiting a wide range of morphologies and ages. Results indicate that this new method is appropriate to describe the shape of ammonoid whorl cross-sections, allowing us to construct a morphospace showing several biological patterns (e.g., clustering and homeomorphy), and complex morphological transformations that, in some cases, correlate with evolutionary tendencies described by previous authors. Further, this technique can be used to generate the basic segment required for the elaboration of the virtual models employed in hydrostatic and hydrodynamic studies.

We report on remains of the buccal apparatus and possible associated structures for the Late Cretaceous ammonite Maorites seymourianus from the López de Bertodano Formation (Antarctic Peninsula). This is the first description of these structures for the family Kossmaticeratidae. Further, we discuss the most likely taphonomic processes taking place that allowed this exceptional preservation. Reflectance Transformation Imaging (RTI) technique was employed in this work to better document the faint structures. We briefly provide a review of this method and its potential importance for paleontological studies because it seems that this powerful technique has been largely overlooked by paleontologists.