Ammonite fossils

Ammonites

The Origin and Evolution

Based on an evolutionary deep time timeline, ammonites are an extinct group of marine mollusks belonging to the subclass Ammonoidea, which is part of the class Cephalopoda. Their origin dates back to the Devonian period, around 400 million years ago. Ammonites thrived in the oceans until their extinction at the end of the Cretaceous period, about 66 million years ago, coinciding with the same event that wiped out the dinosaurs. Based on an evolutionary deep time timeline, modern cephalopods, such as squids, octopuses, and cuttlefish, are the closest living relatives of ammonites. These contemporary cephalopods share several characteristics with ammonites, including their basic body plan, advanced nervous systems, and predatory lifestyle. However, unlike their shelled ancestors, many modern cephalopods have either internalized their shells or lost them entirely, an adaptation that has allowed for greater mobility and a wider range of ecological niches. Based on an evolutionary deep time timeline, ammonites belong to the phylum Mollusca, which includes a diverse range of organisms such as gastropods (snails and slugs), bivalves (clams and oysters), and other cephalopods. Within Mollusca, cephalopods are distinguished by their bilateral body symmetry, prominent head, and set of arms or tentacles. Ammonites, as part of the subclass Ammonoidea, are a branch of the cephalopod class, separate from the lineages that led to modern cephalopods. Ammonites are important for understanding the geological and evolutionary history of marine environments. Their widespread presence make them excellent index fossils, helping geologists to date rock layers and understand the paleoenvironmental conditions of different geological periods. The study of ammonite fossils continues to provide valuable insights into the diversity and adaptability of life in ancient oceans.

History of discovery

The earliest known ammonite fossils were discovered in the early 19th century by researchers like William Buckland and William Conybeare in Europe. In 1836, Mary Anning, a famous fossil hunter from Lyme Regis, England, discovered a complete and well-preserved ammonite fossil that became a significant find. Throughout the 19th and 20th centuries, numerous ammonite fossils were discovered in various parts of the world, including Europe, North America, South America, Asia, and Africa. Some of the most significant ammonite fossil finds have been made in locations such as the Jurassic Coast of Dorset, England, the Cretaceous rocks of the Western Interior Seaway in North America, and the Cretaceous deposits of northern Africa.


These extinct mollusks, related to modern-day octopuses and squids, are often found in mountain ranges like the Alps, Himalayas, and Andes.

Evolutionary Timeline of Ammonites

Devonian Period (419 - 359 million years ago): Based on an evolutionary deep time timeline, ammonites would have originated during the Devonian period. They evolved from earlier cephalopods known as bactritids, which had straight shells.
Carboniferous Period (359 - 299 million years ago): Ammonites would have continued to diversify and develop more complex suture patterns on their shells, which provided increased strength and buoyancy control.
Permian Period (299 - 252 million years ago): During the Permian period, ammonites would have faced significant extinction events, but they managed to survive and adapt, continuing their evolutionary trajectory.
Triassic Period (252 - 201 million years ago): Ammonites would have experienced a major evolutionary radiation after the Permian-Triassic extinction event. They rapidly diversified into numerous species and genera, becoming dominant marine predators.
Jurassic Period (201 - 145 million years ago): The Jurassic period would have marked a high point in ammonite diversity and abundance. They developed a wide range of shell shapes and sizes, adapting to various ecological niches.
Cretaceous Period (145 - 66 million years ago): Ammonites would have continued to thrive throughout the Cretaceous period, with many species developing intricate and highly ornamented shells. However, they faced extinction at the end of this period, around 66 million years ago, likely due to the same catastrophic event that caused the demise of the dinosaurs.
Post-Cretaceous Period (66 million years ago - Present): Based on an evolutionary deep time timeline, ammonites would not have survived the Cretaceous-Paleogene extinction event. However, their closest relatives, modern cephalopods like squids, octopuses, and cuttlefish, continue to inhabit the world's oceans, carrying forward the evolutionary legacy of this fascinating group.

Challenges and Gaps in the Evolutionary Timeline of Ammonites: An Alternative Perspective

The proposed evolutionary timeline of ammonites, an extinct group of marine mollusks, is riddled with significant gaps, abrupt appearances, and a lack of transitional forms that challenge the conventional gradualist model of evolution. From their earliest origins during the Devonian period to their eventual extinction at the end of the Cretaceous, the fossil record presents numerous instances where the evolutionary narrative falls short in providing satisfactory explanations. The sudden emergence of complex ammonite forms without clear ancestral lineages or transitional fossils during the Devonian period raises doubts about their gradual evolution from earlier cephalopods. Similarly, the suture patterns in the Carboniferous period lack intermediate forms, suggesting that other mechanisms might be at play beyond slow, incremental changes. As we move through the subsequent geological periods, the resilience of ammonites in surviving mass extinction events like the Permian-Triassic crisis remains unexplained within the evolutionary framework. The fossil record does not provide a clear understanding of how these creatures managed to adapt and thrive while other species perished. The major evolutionary radiation of ammonites after the Permian-Triassic extinction event is marked by a sudden proliferation of species in the fossil record, challenging the gradualist model of evolution. The rapid diversification and emergence of numerous new forms without clear precursors pose a significant challenge to the conventional explanations. Throughout the Jurassic and Cretaceous periods, ammonites exhibited a remarkable diversity and abundance, adapting to various ecological niches. However, the fossil record lacks transitional fossils between different ammonite species, making it difficult to reconstruct their evolutionary pathways. The abrupt development of intricate shell ornamentation during these periods further compounds the enigma. The sudden extinction of ammonites, along with the dinosaurs, at the end of the Cretaceous period lacks a comprehensive explanation within the evolutionary paradigm. Moreover, the absence of transitional forms connecting ammonites to modern cephalopods like squids, octopuses, and cuttlefish suggests a discontinuity that challenges the expected smooth transition in evolutionary theory. In light of these numerous gaps, abrupt appearances, and the lack of transitional forms, the proposed evolutionary timeline of ammonites appears incomplete and inadequate in accounting for the observed fossil evidence. Alternatively, a sudden creation event, followed by a global catastrophic event like a worldwide flood, better explains the abrupt appearances, diversifications, and eventual extinction of these remarkable creatures. The scientific evidence seems to align more closely with a model of separate creations and a single, catastrophic extinction event rather than a gradual, continuous evolutionary process.



The Mysterious Origin of Ammonite Morphology

The remarkable diversity of ammonite shell shapes, suture patterns, and ornamentation is truly astounding. One of the most perplexing aspects is the lack of clear transitional forms or intermediate fossils in the geological record that could bridge the gap between ammonites and their hypothesized ancestors. This absence of transitional fossils raises questions about the proposed step-by-step evolutionary pathways that supposedly gave rise to the ammonite morphology. Ammonites appear to have emerged relatively abruptly in the fossil record, without clear precursors or ancestral forms exhibiting simpler, more primitive features. This sudden appearance, often referred to as the "explosive radiation" of ammonites, challenges the traditional notion of a slow, gradual evolution from simpler life forms. The sheer complexity and interdependence of ammonite shell features, such as the suture patterns and chambered structures, further compound the difficulty in envisioning how such highly coordinated and integrated systems could have arisen through random, undirected processes.  The incredible diversity of ammonite forms, spanning a wide range of ecological niches and environments, raises questions about the mechanisms that could have driven such rapid and extensive morphological diversification within a relatively short geological timeframe.
In light of these observations, it becomes evident that the conventional explanations relying solely on neo-Darwinian mechanisms are insufficient to account for the enigmatic nature of ammonite morphology and its abrupt emergence in the fossil record.

Distinct Organismal Architecture 

Cephalopods, including ammonites, possess a number of unique features that set them apart from other mollusks.

According to a research reported in a paper in 2021, an exceptionally well-preserved Middle Jurassic ammonite fossil revealed unprecedented details about the soft-body organization of these extinct cephalopods through correlative neutron and X-ray tomography imaging. 1  


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Paired dorsal muscles were identified that likely allowed the ammonite to withdraw its body deep into the shell for protection. This is different from the modern Nautilus, which uses its funnel for propulsion. Strong paired muscles were also discovered that allowed ammonites to retract deep into their shells for protection against predators.  The findings came from analyzing an exceptionally well-preserved 5cm ammonite fossil found in 1998 in Gloucestershire, UK. By combining high-resolution X-ray and neutron imaging, the researchers created detailed 3D reconstructions of the ammonite's muscles, organs and their orientations. The muscle arrangement suggests ammonites used jet propulsion for swimming, unlike the modern nautilus which they were previously thought to resemble. 

Based on the arrangement and orientation of the muscles observed in the fossil, it appears that ammonites had a specialized muscle system dedicated to rapidly expelling water from their body chambers. 

Ammonites possessed a large, muscular hyponome (a funnel-like structure) at the base of their body, similar to modern cephalopods like squids and nautiluses. The newly discovered muscle arrangement suggests the presence of powerful circular and radial muscles surrounding the hyponome. By contracting these muscles in a coordinated manner, ammonites could rapidly constrict the hyponome, forcefully expelling water from their body chambers through the narrow opening. This sudden expulsion of water would create a powerful jet of thrust, propelling the ammonite forward or in a desired direction. The chambered shell structure of ammonites, with gas-filled compartments, likely provided buoyancy and stability during this jet-propelled swimming. The arrangement of muscles around the hyponome also implies the ability to adjust the direction of the jet by contracting different muscle groups, enabling maneuverability. This jet propulsion mechanism would have allowed ammonites to rapidly accelerate through the water column, potentially for predation, escape, or migration purposes. The discovery of these preserved muscle tissues provides direct evidence for this propulsion system and offers valuable insights into the paleobiology and locomotion strategies of these remarkable ancient marine creatures.

The lack of an ink sac means the muscle-powered retraction was likely an important anti-predator defense mechanism. The study, published in Geology, provides the first 3D visualization of an ammonite's soft parts, marking a breakthrough in understanding their paleobiology. It suggests ammonites may be closer to modern coleoids (squid, octopus, cuttlefish) than previously thought based on comparisons to nautilus. The preservation of these soft tissues provides key insights into the functional morphology and biology of ammonites, an iconic but poorly understood fossil group in terms of their soft anatomy.

This remarkable fossil specimen, coupled with the advanced tomographic imaging techniques, has allowed an unprecedented look into the internal soft body parts of ammonites, shedding new light on their locomotion, defensive strategies, and evolutionary relationships to other cephalopod groups like the coleoids (squids, cuttlefish, etc.).

The exceptionally preserved Jurassic ammonite fossil 2 is supposedly from the Middle Jurassic period, approximately 166-176 million years old. The soft tissues, including muscles and organs, were preserved in remarkable three-dimensional detail, which is extremely rare for ammonoid fossils. The key factors that enabled such exceptional soft tissue preservation were: Early rapid mineralization (phosphatization) of the soft tissues, which prevented their decay.  The fossil was encased in an external mineral concretion, protecting the soft tissues from further degradation by groundwater. Infusion and coating of the soft tissues with iron and iron compounds at a critical point in the decay process protected the cells from autolysis (self-destruction). Cross-linking of collagen fibers and their association with bone minerals further preserved the soft tissues over geological timescales. The combination of these protective mechanisms, facilitated by the specific oxidizing depositional environment, allowed for the extraordinary preservation of the ammonite's soft anatomy. The authors used cutting-edge correlative neutron and X-ray tomography techniques to visualize and reconstruct the 3D internal structures in unprecedented detail.

While the exceptional preservation is truly remarkable, there are several issues that call into question the claimed age of around 166-176 million years: The preservation of soft, undecayed tissues over hundreds of millions of years is highly improbable because these structures tend to degrade and disintegrate relatively quickly, even under ideal conditions. The presence of intact muscle fibers, organs, and other soft tissues in such an allegedly ancient fossil is highly unusual and challenges the conventional age estimation. The dating methods used to assign an age of 166-176 million years to this fossil rely on various assumptions about initial conditions, decay rates, and the lack of contamination or disturbances over time. These assumptions are difficult to verify, especially for such an ancient specimen. The preservation of details without significant degradation or alteration over alleged timescales of hundreds of millions of years is surprising. Even under exceptional conditions, some level of degradation would be expected to occur, yet the fossil appears remarkably well-preserved. There are no known modern analogs that can preserve soft tissues over such vast timescales. Even in exceptional cases of preservation, such as in amber or in anoxic environments, the timescales involved are orders of magnitude shorter than the claimed age of this fossil. The evolutionary assumptions regarding the long timescales involved deserve to be questioned, as they are based on a particular interpretation of the fossil record rather than direct evidence. Rather than relying on these assumed vast timescales, the remarkable preservation of soft tissues in this ammonite fossil is better explained by a more recent origin and a shorter timescale consistent with observed rates of fossilization and preservation. The evidence suggests that this fossil is not as ancient as claimed and formed relatively recently, perhaps even within the past few thousand years or so. While the preservation of soft tissues in this fossil is undoubtedly remarkable, it does not necessarily provide definitive proof of its alleged age of hundreds of millions of years. Instead, a more cautious and critical examination of the evidence is warranted, considering the challenges posed by such an extraordinary claim.



Reconstructions of two Late Volgian ammonites belonging to the Craspeditidae family: Kachpurites fulgens (Trautschold, 1861) (A) and Garniericeras catenulatum (Fischer, 1830) (B). The reconstructions show ten arms, reflecting the observation that modern-day Nautilus and coleoid cephalopods exhibit five pairs of arms in their embryonic stages, suggesting this could be the base number of arms in Cephalopoda (Kröger et al., 2011). Two long tentacles are included, although speculative, as such tentacles could have been useful for external-shelled cephalopods to catch prey from a distance, given their potential difficulty in rapid forward movement. The large hyponome (funnel-like structure) is depicted based on evidence of a funnel-locking apparatus, hyponomic retractors, and the shape of the aperture edges with lateral apertural sinuses and the presence of a large round opening between lappets in some ammonites (Westermann, 1990). The eyes are drawn similar to those of coleoids, as Ammonoidea and Coleoidea were sister taxa (Jacobs and Landman, 1993). The dark transverse bands on the shells correspond to the most common color pattern observed in ammonites (Keupp, 2000), with the presence of such a pattern in the Craspeditidae family confirmed by findings of shells with transverse dark bands (unpublished material). The reconstruction was drawn by Andrey Atuchin, based on a sketch by the author of the article. 3

The Enigma of Ammonite Eyes: A Evolutionary Puzzle

Cephalopods have camera-like eyes similar in structure to vertebrate eyes, a striking example of convergence. 


The internal anatomy of Subplanites has been reconstructed to illustrate how it might have appeared when it came to rest on the sediment. It should be noted that the interpretations of certain organs, such as the reproductive organ, the central nervous system, the hyponome, and the gills, are still a matter of debate. The reconstruction presented here represents just one possible interpretation among several. In this reconstruction, the coiling of the soft parts corresponds to the coiled body chamber (approximately 300°). It's likely that this coiling was altered when the soft parts settled onto the sediment. The diagram is presented in two parts:

Figure a: Shows the reconstructed anatomy as observed in the fossil.
Figure b: Depicts the arrangement of organs according to the conch structure. Link 

Ammonites are thought to have had complex eyes, though the details of their vision and eye structure are not as well-preserved in the fossil record as other aspects of their anatomy, like their shells. However, based on comparisons with their close relatives, such as modern cephalopods (squids, cuttlefish, and octopuses), it is reasonable to infer that ammonites likely had well-developed eyes that could have been similar to the camera-type eyes seen in modern cephalopods. The fossil record does include some evidence of the eye structures of ammonites. Fossilized impressions and remains of the soft tissues sometimes show hints of where the eyes were located and their possible structure. These eyes would have been important for their survival, aiding in navigation, prey detection, and predator avoidance in their marine environments.

Comparison with Modern Cephalopod and Human Eyes


The diagram you provided shows the differences between the eyes of an octopus (a modern cephalopod) and a human. Both are examples of camera-type eyes, but they have evolved differently:

Octopus Eyes: Octopuses have a camera-type eye that is structurally similar to the human eye but lacks a blind spot. Their eyes are highly adapted for underwater vision and can detect polarized light, which is useful for seeing contrasts in the ocean environment.
Human Eyes: Human eyes are also camera-type eyes but have a blind spot where the optic nerve exits the retina. Human eyes are adapted for detecting color and detail, suited for life in a terrestrial environment.

While direct evidence of ammonite eyes is rare in the fossil record, it is reasonable to infer that they had complex, well-developed eyes similar to those of their modern cephalopod relatives. The earliest known camera-type eyes in the fossil record are found in ancient cephalopods and date back to the Ordovician period, highlighting the long evolutionary history of sophisticated visual systems in marine organisms.

The development of such advanced organs without clear intermediate stages is difficult to explain through gradual processes. One of the most remarkable features of ammonites is the presence of highly advanced camera-like eyes, strikingly similar in structure to the eyes of vertebrates. This striking example of convergence, where distantly related organisms have similar traits, poses significant challenges to the conventional evolutionary narrative. Ammonite fossil evidence reveals the existence of these sophisticated eye structures, complete with a lens, a retina, and a cornea. The complexity and sophistication of these eyes are truly remarkable, rivaling the visual capabilities of modern cephalopods and vertebrates. However, the fossil record lacks clear intermediate forms or transitional stages that could shed light on the gradual evolution of such advanced visual organs from simpler precursors. The abrupt appearance of these intricate camera-like eyes in ammonites, without any obvious ancestral forms possessing more primitive eye structures, presents a significant challenge to the traditional concept of gradual, step-by-step evolution. Furthermore, the development of such highly specialized and interdependent structures as the ammonite eye, with its lens system, light-sensitive retina, and supporting musculature, is difficult to reconcile with the idea of random, undirected mutations and natural selection acting incrementally over vast timescales. The sheer complexity and interdependence of the various components in the ammonite eye suggest the presence of an overarching design and coordination, rather than the result of a series of fortuitous, unguided events. The seamless integration of these components raises questions about the plausibility of their gradual assembly through random processes. Consequently, the existence of these advanced camera-like eyes in ammonites, without clear precursors or transitional forms in the fossil record, poses a significant challenge to the conventional evolutionary timeline and the proposed mechanisms driving the gradual emergence of such complex structures. This enigma surrounding ammonite eyes underscores the limitations of our current understanding and highlights the need for a more comprehensive explanation that can account for the rapid appearance and sophistication of these remarkable visual organs in ancient marine creatures.

Nervous system

The nervous system of ammonites, extinct cephalopods is not well understood due to the limitations of the fossil record. Soft tissues, including neural structures, rarely fossilize, making it challenging to directly study their nervous system. What we know about ammonite nervous systems is largely inferred from comparisons with their living relatives, such as nautiluses, and from what can be deduced from their shell structure and behavior. Ammonites likely had a nervous system more advanced than many other invertebrates of their time, but probably not as complex as modern coleoid cephalopods like octopuses and squids. Their nervous system would have included a centralized brain, likely similar to that of modern nautiluses. This brain would have been responsible for processing sensory information, controlling movement, and coordinating basic behaviors. The complexity of ammonite shells and their ability to regulate buoyancy suggest a reasonably sophisticated nervous control. However, the evolutionary pathway to this neural complexity in ammonites is not well-understood. The fossil record provides limited direct evidence of their soft tissue structures, including the nervous system. Researchers must rely on comparative studies with living cephalopods, careful examination of rare well-preserved fossils, and inferences from shell morphology and likely behavior patterns to piece together our understanding of ammonite neurology. This area remains an active field of paleontological research, as scientists continue to search for better-preserved specimens and develop new techniques to glean more information from existing fossils about the soft tissue structures of these ancient cephalopods.

Shell structure

These extinct cephalopods possessed highly complex shells, typically coiled in a planar spiral and divided into chambers by septa. The edges where these septa met the outer shell wall formed elaborate suture patterns. This complexity in shell structure, including the phragmocone for buoyancy control and the siphuncle for fluid regulation, appears suddenly in the fossil record without clear precursors showing gradual development. The growth of ammonite shells often followed precise mathematical patterns, such as logarithmic spirals, and many species displayed elaborate ornamentation including ribs, knobs, and spines. This mathematical regularity and the seeming "design" of these structures raise questions about how undirected evolutionary processes could repeatedly produce such orderly and complex features. Furthermore, the convergent evolution of similar shell structures in different ammonite lineages complicates our understanding of the mechanisms driving their evolution. The shells also exhibited remarkable developmental plasticity, adapting to various environmental conditions, which suggests sophisticated underlying genetic and developmental mechanisms. These characteristics of ammonite shells - their complexity, rapid appearance, mathematical precision, and convergent evolution - present significant challenges to traditional, gradualistic models of evolution. They suggest that much of the genetic information for generating this complexity may have been present early, a concept sometimes referred to as "front-loading." The origin of the biological information required for these complex systems, including the gene regulatory networks and developmental pathways necessary for shell formation, remains a fundamental challenge for evolutionary theory to explain. The study of ammonite shells continues to drive research in paleontology, evolutionary biology, and developmental biology, as scientists seek more comprehensive models to explain the emergence and diversification of these remarkable structures. These investigations may ultimately lead to new insights into the processes of biological innovation and the generation of complexity in living systems.

Buoyancy control

Buoyancy control in ammonites was a sophisticated mechanism integral to their survival and lifestyle. The key to this control was their phragmocone, the chambered portion of the shell. As the ammonite grew, it would seal off the rear portion of its living chamber with a new septum, creating a new chamber. These chambers, connected by a tube-like structure called the siphuncle, played a crucial role in buoyancy regulation. The siphuncle allowed the ammonite to adjust the amount of fluid in the chambers, effectively altering the overall density of the shell. By pumping fluid in or out of the chambers, the ammonite could ascend, descend, or maintain its position in the water column. This system bears similarities to modern nautiluses. The complexity of this buoyancy control system poses questions for evolutionary biology. Its appearance in the fossil record is abrupt, without a clear series of intermediates showing gradual development. This rapid emergence of a sophisticated physiological mechanism challenges traditional models of gradual evolutionary change.

Moreover, the precise control required for this system suggests the presence of complex neural and physiological adaptations. The integration of shell growth, chamber formation, and fluid regulation would have required coordination of multiple biological systems. The origin of such an integrated system through undirected evolutionary processes presents a significant puzzle. The convergent evolution of similar buoyancy control mechanisms in different cephalopod lineages further complicates the picture. This repeatability raises questions about the sufficiency of random mutation and natural selection to explain the independent development of such complex traits. The buoyancy control system of ammonites also demonstrates remarkable developmental plasticity, adapting to various environmental pressures throughout their evolutionary history. This adaptability suggests the presence of sophisticated gene regulatory networks capable of producing varied phenotypes in response to environmental cues. The buoyancy control mechanism of ammonites represents a prime example of the challenges faced by evolutionary theory in explaining the origin of complex, integrated biological systems. Its study continues to drive research into evolutionary developmental biology and systems biology, as scientists seek to understand the processes behind the emergence of such intricate physiological adaptations in ancient organisms.

Integrated Complexity: The shell formation and patterning in ammonites involve an interplay of gene regulation networks, epigenetic codes, and signaling pathways. This complexity is further compounded by the precise coordination required for calcium carbonate deposition, organic matrix formation, and suture patterns. The interdependence of these processes with cell-cell adhesion and cell migration during shell growth demonstrates a level of integrated complexity that is difficult to account for through a series of small, independent mutations. The challenge here lies in explaining how such a tightly integrated system could evolve gradually. Each component seems to rely on the proper functioning of the others, creating a "chicken-and-egg" problem: which came first, and how did the system function before all components were in place?

Irreducible Complexity: The buoyancy regulation system in ammonites provides an excellent example of potential irreducible complexity. This system requires the coordinated action of cellular communication, ion channels, gas exchange mechanisms, and pressure regulation within the shell chambers. 


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The siphuncle, a tube connecting all chambers, plays a crucial role in this regulation. From an evolutionary perspective, it's difficult to envision how this system could have evolved in small, functional increments. What selective advantage would a partially formed chamber or an incomplete siphuncle confer? The system seems to require all components to be present and functional to provide any benefit, challenging gradualistic explanations.

The siphuncle, a defining feature of ammonites and other shelled cephalopods, is a testament to nature's biological engineering. This remarkable tubular structure, threading through the entire coiled shell from its innermost chamber to the aperture, serves as the lynchpin of the ammonite's sophisticated buoyancy regulation system. Its primary function—controlling the gas and liquid content within each chamber—allowed these ancient mariners to navigate the seas with astounding efficiency, rising or descending in the water column with minimal energy expenditure.

But the siphuncle's role extends far beyond mere buoyancy control. As the ammonite grew, sealing off its previous living chamber with a new septum and moving forward, the siphuncle would extend through this wall, maintaining an unbroken connection with all previous chambers. This continuity enabled it to participate in the formation of new chambers and even facilitate the transport of minerals like calcium carbonate, essential for shell growth and repair. In a display of resourceful economy, the siphuncle could remove these minerals from old chambers and redistribute them where needed, a feat of microscopic recycling. The importance of this system for ammonites cannot be overstated. Their predatory lifestyle and their ability to inhabit diverse marine environments hinged on the precision of their buoyancy control. The siphuncle granted them the power to move vertically with ease, maintain specific orientations, compensate for changes in shell size and weight as they grew, and possibly even achieve neutral buoyancy—an important advantage for their hunting strategies.

However, when we peer closer at the siphuncle, its elegant simplicity gives way to reveal a world of stunning complexity. The siphuncle exhibits both internal and external irreducible complexity, along with high interdependence with other physiological systems, presenting a formidable challenge to gradualistic evolutionary explanations. Internally, the siphuncle is far from a simple tube. Its wall is a marvel of selective permeability, allowing for the controlled diffusion of gases and liquids—a feature that requires a specific microstructure. The siphuncular epithelium acts as an osmotic pump, actively moving ions to create concentration gradients that drive the movement of liquids in and out of chambers. Evidence even points to a countercurrent exchange system within the siphuncle, where blood vessels running in opposite directions enable efficient gas exchange. Externally, the siphuncle's effectiveness is contingent on a host of other precisely calibrated structures. It requires well-formed, sealed chambers with impermeable septa; any breach would render the system useless. The outer shell, too, must be impermeable; otherwise, the most efficient siphuncle would be powerless against the influx of seawater. Special muscles attached to the siphuncle brace against the pressure differentials between chambers, another layer of complexity.

The siphuncle doesn't operate in splendid isolation but is deeply interconnected with other bodily systems. It's intimately linked with the cardiovascular system, as blood vessels course through its length. The nervous system provides essential regulation, fine-tuning the osmotic pumping to the animal's immediate needs. Specialized excretory organs often accompany the siphuncle, handling the waste products of its metabolic labors. And its gas exchange processes are inextricably tied to the animal's overall respiration. This interdependence poses profound questions for evolutionary theory. How could such a system, where each component seems to rely on the proper functioning of all others, have evolved through a series of small, gradual steps? What selective advantage would a partially formed chamber or an incomplete siphuncle confer? The system appears to demand the simultaneous presence and functionality of all its components to provide any benefit at all.

Epigenetic codes and "languages" that likely existed in ammonites

In exploring the epigenetic codes and "languages" that likely existed in ammonites, we can draw parallels with their living relatives and the complexity of their biology. While we cannot directly observe the molecular processes in extinct ammonites, we can make informed inferences based on modern mollusks and the structure of ammonite fossils. The Chromatin Code in ammonites would have governed how DNA was packaged and accessed within their cell nuclei. This code involves histone modifications and DNA methylation patterns that determine which genes are accessible for transcription. Closely related to this is the Histone Code, which involves specific chemical modifications on histone proteins, such as methylation, acetylation, and phosphorylation. These modifications would influence gene expression by altering chromatin structure. Another layer of complexity would be added by the DNA Methylation Code, which involves the addition of methyl groups to DNA, typically on cytosine bases. In ammonites, DNA methylation likely played a role in gene silencing and genomic imprinting. This would be crucial for ensuring that only the necessary genes are expressed at the right times. The Transcriptional Regulatory Code would encompass the binding of transcription factors to specific DNA sequences, controlling when and where genes are expressed. This code would be vital for processes such as shell formation, buoyancy regulation, and neural development. Post-transcriptional modifications to RNA molecules, governed by the RNA Modification Code, would influence RNA stability, localization, and translation efficiency.

Furthermore, the Protein Modification Code would involve various post-translational modifications, such as phosphorylation and glycosylation, altering protein function and allowing rapid responses to environmental changes. The Cytoskeleton Code would govern the dynamic organization of the cytoskeleton, critical for processes like cell division, intracellular transport, and maintaining the structure of the siphuncle. Communication between cells in ammonites would be managed by the Cell-Cell Communication Code, involving molecular signals essential for coordinated development and physiological processes. In their nervous system, the Neurotransmitter Code would involve specific combinations of neurotransmitters and receptors, enabling complex behaviors and sensory processing. A particularly crucial aspect for ammonites would be the Biomineralization Code, which involves the molecular processes guiding the deposition of calcium carbonate to form their intricate shells. This code would be essential for the formation and maintenance of their protective and functional shell structures. These codes and languages do not operate in isolation but are deeply interconnected. For example, the chromatin code, histone code, and DNA methylation code work together to regulate gene accessibility. Changes in histone modifications can lead to changes in DNA methylation and vice versa, collectively determining which genes are expressed. The transcriptional regulatory code is influenced by the chromatin state—only accessible genes can be bound by transcription factors. Additionally, the RNA modification code can be directed by the chromatin state of the gene from which the RNA is transcribed, creating another layer of regulation. The protein modification code often responds to cell-cell communication signals. For instance, a signaling molecule might trigger a cascade of protein phosphorylation, rapidly changing cellular behavior. The cytoskeleton code is intimately linked with the protein modification code, as many cytoskeletal rearrangements are initiated by phosphorylation events. The biomineralization code relies on precise spatiotemporal gene expres​sion(controlled by the transcriptional regulatory code) and specific protein modifications to guide shell formation. The neurotransmitter code depends on all the above codes for the proper development of the nervous system, synthesis of neurotransmitters, and expression of receptors. In the context of the ammonite's siphuncle, these interdependent codes would work together to ensure its proper development and function. The transcriptional regulatory code would guide the expression of genes necessary for siphuncle formation at the right time and place during development. The cytoskeleton code would direct the shaping of the siphuncular epithelium, while the cell-cell communication code would coordinate the activities of different cell types within the siphuncle. The protein modification code would regulate the activity of ion pumps involved in buoyancy control, and the biomineralization code would manage the deposition of new shell material as the siphuncle extends. This interplay of molecular "languages" highlights the remarkable complexity of life, even in organisms that existed a long time ago. It underscores how every aspect of an organism, from its biochemistry to its behavior, is the result of layers upon layers of regulated processes, all working in concert.

The fossil record, rather than illuminating a clear path of incremental development, seems to present the siphuncle as a fully formed structure, appearing with relatively little preamble. And despite their shared functional requirements, siphuncles display a perplexing diversity of forms across different ammonite groups, a variety that emerges early in their evolutionary history. In contemplating the siphuncle, we're confronted with biological complexity that rivals the most sophisticated human engineering. Its precise microstructural design, active physiological control, and seamless integration with multiple body systems underscore the inadequacy of simplistic, stepwise evolutionary narratives. The siphuncle beckons us toward more comprehensive models of evolution—models that grapple with systems-level interactions, developmental constraints, and underlying design principles in nature. Far from diminishing its wonder, the challenges the siphuncle poses to our understanding only deepen our appreciation for this structure. It stands as a humbling reminder of the complexity of life, prompting us to expand our investigative horizons and refine our theoretical frameworks. In the end, the siphuncle of the ammonite, though long fossilized, continues to propel us forward in our quest to fathom the depths of biological complexity.

Rapid Diversification

The fossil record shows periods of rapid diversification of ammonites, followed by mass extinction. This pattern where significant changes appear concentrated in rapid bursts rather than gradual transformation, challenges traditional views of gradual evolution. While we can't study the molecular biology of extinct ammonites directly, research on living cephalopods reveals unique features in their genome and development. For instance, extensive RNA editing (a process where RNA is modified after being transcribed from DNA) is common in cephalopods, a characteristic not shared by many other animal groups.

Extensive RNA editing

Extensive RNA editing, particularly the type involving adenosine-to-inosine (A-to-I) editing, is indeed common in cephalopods, such as octopuses, squids, and cuttlefish. This characteristic is quite unique among animals. Most other animal groups do not exhibit RNA editing to the same extent. While RNA editing occurs in vertebrates (including humans), it is generally less extensive and typically focused on specific genes or regulatory regions, rather than being widespread across the genome. Similar to vertebrates, insects have some RNA editing, but it is not as pervasive or as functionally significant as in cephalopods. In Cnidarians (e.g., jellyfish, corals) there is limited evidence of extensive RNA editing in these animals, and any editing that does occur tends to be much less complex than in cephalopods. Even within the mollusk phylum, cephalopods are unique. Most other mollusks, such as snails and bivalves, do not exhibit the same level of RNA editing.In Nematodes and Arthropods which include species like roundworms and crustaceans, show some RNA editing activity, but again, it is not as extensive or as critical for their adaptation and function as it is in cephalopods.

Alps

The Dolomites in Italy, part of the Southern Limestone Alps, are renowned for their ammonite fossils. The Raibl Beds near the Austrian-Italian border have yielded exceptionally well-preserved ammonites. The Dolomites in northern Italy are renowned for their rich fossil record, particularly the ammonites found in the Raibl Beds (also known as the Raibl Formation or Pordoi Formation) near the Austrian-Italian border. These sedimentary rocks have yielded exceptionally well-preserved ammonite fossils.

The Spiti Valley in the Indian Himalayas

The Spiti Valley, located in the Indian state of Himachal Pradesh, is indeed renowned for its rich ammonite fossil deposits, particularly within the "Spiti Shales" formation. The Spiti Shales are part of the Tethyan Himalayan sequence, representing sediments deposited in the ancient Tethys Ocean before the collision of the Indian and Eurasian plates. While ammonites are the most famous fossils from this formation, it also contains other marine invertebrates such as belemnites, brachiopods, and bivalves. The fossils in the Spiti Shales are often exceptionally well-preserved, sometimes retaining original shell material or showing fine details of the suture patterns. The Spiti ammonite fauna has been crucial for biostratigraphic correlations across the Tethyan realm, helping geologists understand the paleogeography and evolution of life during this time period.  The presence of abundant ammonites suggests that the Spiti region was once covered by a relatively deep marine environment. Paleontologists continue to study the Spiti Shales, often discovering new species or gaining insights into ancient ecosystems.

Ammonite fossils in the Los Molles Formation of the Neuquén Basin, Argentina

The discovery of exceptionally well-preserved ammonite fossils in Argentina provides an invaluable window into the marine ecosystems in this region.

The Neuquén Basin, situated in the foothills of the mighty Andes mountain range, is renowned for its rich paleontological heritage. The Los Molles Formation, a sedimentary rock unit within this basin, has proven to be an exceptional repository of ammonite fossils, offering researchers a rare opportunity to study these extinct marine creatures in remarkable detail. Ammonites were a group of highly diverse and widespread cephalopods that thrived in the ancient oceans. Cephalopods are a group of marine mollusks that includes squids, octopuses, cuttlefishes, and nautiluses. They are characterized by having a distinct head, large eyes, and tentacles arranged around their mouth. Their coiled shells, adorned with patterns and ridges, have become iconic fossils for paleontologists. The exceptional preservation of the Los Molles ammonites means that not only are the shells intact, but also delicate features like the suture patterns and even remnants of the soft body tissues have been preserved.

This exceptional preservation is attributed to the unique depositional environment of the Los Molles Formation, which was likely a deep, anoxic (oxygen-depleted) marine basin. The lack of oxygen slowed the decomposition process, allowing the ammonite remains to be entombed and fossilized with remarkable fidelity. The diversity of ammonite species found in the Los Molles Formation is equally remarkable. Researchers have identified various genera, including Arietites, Oxynoticeras, and Uptonia, among others. This diversity provides insights into the radiation, as well as the ecological niches they occupied in the ancient oceans. Furthermore, the abundance of ammonite fossils in the Los Molles Formation suggests that these creatures were thriving in the region, indicating favorable environmental conditions and a rich marine ecosystem. The study of associated fossil assemblages, including other invertebrates and marine reptiles, can shed light on the web of life that existed in the seas of the Neuquén Basin. The Los Molles ammonite fossils not only contribute to our understanding of these extinct creatures but also provide a glimpse into the broader paleoenvironmental and paleoclimatic conditions. By studying the sedimentary rocks, geochemical signatures, and fossil assemblages, researchers can reconstruct the ancient landscapes, ocean currents, and climatic patterns that shaped the region during this pivotal period in Earth's history. These ammonite fossils high in the mountains provide compelling evidence for the massive geological changes that have occurred over millions of years, demonstrating how ancient seabeds have been uplifted to form some of the world's highest mountain ranges.


The Agrio Formation is an Early Cretaceous geologic formation that is up to 1,500 metres (4,900 ft) thick and is located in the southern Mendoza Province and northern-central Neuquén Province, in the Neuquén Basin of northwestern Patagonia, Argentina  Link 

The ammonite fossils found in the Los Molles Formation of the Neuquén Basin in Argentina are indeed fascinating and provide valuable insights into ancient marine ecosystems. However, your question touches on some of the challenges in explaining their origins and diversity through gradual evolutionary processes. Let's explore some of these aspects:

Brachiopods: shelled marine animals found in the Rocky Mountains and the Appalachians

Brachiopods are remarkable shelled marine animals. These creatures, often mistaken for clams or other bivalve mollusks, have a rich history. Despite their superficial resemblance to bivalves, brachiopods are a distinct group of animals, characterized by their two unequal valves and a unique feeding structure called a lophophore. Brachiopods dominated the world's oceans, thriving in an astonishing array of shapes and sizes. Their abundance and diversity in the fossil record is a testament to their success, with over 12,000 known fossil species gracing the annals of prehistoric life. This profusion of forms makes them invaluable to geologists as index fossils, helping to unravel the relative ages of rock layers in which they're entombed. Today, when we find brachiopod fossils nestled within the majestic folds of mountain ranges like the Rockies or the Appalachians, we're glimpsing a world long past—one where these very peaks were the floor of ancient seas. It's a humbling reminder of our planet's ceaseless transformation, as tectonic forces have lifted these former ocean beds high into the sky. Though their glory days are behind them, brachiopods didn't vanish entirely. They weathered the cataclysmic extinction event, albeit with greatly diminished numbers. In our modern oceans, around 350-500 species persist, often overlooked due to their modest size and penchant for colder, deeper waters. These living representatives, though few, offer invaluable insights into the biology and ecology of their long-gone relatives. Attached to the seabed by a fleshy stalk called a pedicle, contemporary brachiopods continue their ancient tradition of filter feeding, their lophophores sifting microscopic morsels from the surrounding water. While they may lack the diversity of their Paleozoic ancestors, their endurance is a testament to the resilience of life. From the rock-strewn slopes of mountains to the dusky depths of today's oceans, brachiopods weave a tale of survival, adaptation, and the inexorable passage of time. They invite us to ponder the vast changes our planet has undergone and serve as humble yet eloquent narrators of Earth's dynamic history.


Brachiopods are sessile marine invertebrates characterized by their bivalve shells and ciliated tentacles surrounding the mouth. Though their shells resemble those of bivalve mollusks, their body structure is distinctly different, placing them in a separate phylum, Brachiopoda. Only a limited number of species exist today.

References

1. Wilby, P.R., Hudson, J.D., Clements, R.G. and Hollingworth, N.T.J. (2004). Taphonomy and origin of an accumulate of soft-bodied cephalopods in the Oxford Clay Formation (Jurassic, England). Palaeontology, 47(5), 1159-1180. Link. (This paper describes the discovery and analysis of exceptionally preserved soft-bodied cephalopod fossils, including ammonites, from the Jurassic Oxford Clay Formation in England, providing insights into their taphonomy and potential causes of their accumulation.)

2. Cherns, L., Spencer, A.R.T., Rahman, I.A., Garwood, R.J., Reedman, C., Burca, G., Turner, M.J., Hollingworth, N.T.J., & Hilton, J. (2022). Correlative tomography of an exceptionally preserved Jurassic ammonite implies hyponome-propelled swimming. Geology, 50(4), 397-401. Link. (This paper reports on an exceptionally preserved Middle Jurassic ammonite fossil that revealed unprecedented details about its soft-body organization, including muscles and organs, through correlative neutron and X-ray tomography imaging.)

Natural History Museum. (2021, December 16). Exceptionally preserved ammonite shows its inner soft tissue in 3D. Link. 

3. Mironenko, A.A. (2015). The Soft-Tissue Attachment Scars in Late Jurassic Ammonites from Central Russia. Acta Palaeontologica Polonica, 60(4), 981-1000. Link 

Alps

Lammerer, B., & Weger, M. (2011). Field Trip Guide: From the Northern Calcareous Alps to the Southern Alps. Ludwig-Maximilians-Universität München. Link. (This field trip guide provides detailed information on the geology, stratigraphy, and tectonic history of the Dolomites and Southern Alps, including the Permian volcanic event, the Permian-Triassic boundary, and the Werfen Formation.)

Kustatscher, E., & Van Konijnenburg-Van Cittert, J.H.A. (2005). Upper Triassic flora from Raibl beds of Julian Alps (Italy) and Karavanke Mts. (Slovenia). Rivista Italiana di Paleontologia e Stratigrafia, 111(3), 513-523. Link. (This paper discusses the plant fossils found in the Raibl Beds of the Julian Alps in Italy and the Karavanke Mountains in Slovenia, while also mentioning the presence of ammonite fossils in these beds.)

Gianolla, P., Morelli, C., & Cucato, M. (2008). Geology of the Dolomites. Episodes, 31(1), 6-17. Link. (This paper provides an overview of the geological history and formations in the Dolomites region, including the Raibl Beds and their rich fossil record, particularly the well-preserved ammonite fossils.)

Krainer, K., & Lutz, D. (2014). Middle Triassic fish remains from the Raibl Beds of the Karawanken Mountains (Carinthia, Austria). Mitteilungen der Geologischen Gesellschaft in Wien, 104, 97-106. Link. (This paper describes various vertebrate fossils, including fish teeth and scales, found in the Raibl Beds of the Dolomites region, although it does not specifically focus on ammonite fossils.)

The Spiti Valley in the Indian Himalayas 

Krishnan, M.S. (1968). Geology of India and Burma. Higginbothams. Link (This book provides a comprehensive overview of the geology of the Indian subcontinent, including a detailed description of the Spiti Shales and their ammonite fossils.)

Uhlig, V. (1903). The fauna of the Spiti Shales. Memoirs of the Geological Survey of India, Palaeontologia Indica, 15th Series, 4(2), 1-132. Link (This seminal work by Victor Uhlig, a German paleontologist, describes and illustrates the diverse ammonite fauna found in the Spiti Shales, establishing the region's significance in paleontological studies.)

Jain, S. (2017). Fundamentals of Invertebrate Palaeontology: Macrofossils. Springer. Link (This textbook on invertebrate paleontology includes a chapter on ammonites, with specific references to the exceptional ammonite fossils found in the Spiti Shales and their importance in biostratigraphy and paleoenvironmental reconstructions.)

Ammonite fossils from the Los Molles Formation in the Neuquén Basin, Argentina

Parent, H., Garrido, A.C., Brambilla, L., & Alberti, M. (2020). Upper Bathonian ammonites from Chacay Melehué (Neuquén Basin, Argentina) and the chronostratigraphy of the Steinmanni Zone. Boletín del Instituto de Fisiografía y Geología, 90, 1-37. Link

Parent, H., Gómez-Peral, L.E., Poiré, D.G., Sandoval, M.I., & Ruiz-Ortiz, P.A. (2021). A microbialitic bioherm related to possible methane seepage (Los Molles Formation, Neuquén, Argentina). Palaeogeography, Palaeoclimatology, Palaeoecology, 562, 110114. Link. (This paper describes an ammonite assemblage found in association with a microbialitic bioherm in the Los Molles Formation, potentially related to ancient methane seepage.)