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Sarti, Carlo (1999). "Whorl Width in the Body Chamber of Ammonites as a Sign of Dimorphism". Advancing Research on Living and Fossil Cephalopods. pp.315–332. doi: 10.1007/978-1-4615-4837-9_23. ISBN 978-1-4613-7193-9. Ward, Peter (1996). "Ammonoid Extinction". Ammonoid Paleobiology. Topics in Geobiology. Vol.13. Springer. pp.815–823. doi: 10.1007/978-1-4757-9153-2_20. ISBN 978-1-4757-9155-6. Scientists also have a more refined sense of what a dinosaur “species” actually is. Paleontologists once gave the name Iguanodon to fossils that spanned tens of millions of years. Re-evaluations now show that Iguanodon is really multiple species, including a new one unveiled in November. Some ammonites have been found in association with a single horny plate or a pair of calcitic plates. In the past, these plates were assumed to serve in closing the opening of the shell in much the same way as an operculum, but more recently they are postulated to have been a jaw apparatus. [14] [15] [16] [17]
a b Peterman, David J; Ritterbush, Kathleen A (2022-12-12). "Stability–Maneuverability Tradeoffs Provided Diverse Functional Opportunities to Shelled Cephalopods". Integrative Organismal Biology. 4 (1): obac048. doi: 10.1093/iob/obac048. ISSN 2517-4843. PMC 9743176. PMID 36518181. Fossils can be very large or very small. Microfossils are only visible with a microscope. Bacteria and pollen are microfossils. Macrofossils can be several meters long and weigh several tons. Macrofossils can be petrified trees or dinosaur bones. Stegouros also fills in an important evolutionary gap. Very few armored dinosaurs have been found within the lands that once made up Gondwana, an ancient supercontinent that started breaking apart during the age of dinosaurs. Before Stegouros, only two armored dinosaurs had been found in what was once southern Gondwana, and neither is as complete as the newly described animal. 8-9. Two huge dinosaurs found in China’s pterosaur gold mineOxycone – Strongly involute and very narrow, with sharp ventral keels and a streamlined, lenticular ( lens-shaped) cross-section. These ammonoids are estimated to be nektonic (well-adapted to rapid active swimming), as their shell form incurs very little drag and allows for efficient, stable coasting even in turbulent flow regimes. [10] Lehmann, Ulrich (1981). The Ammonites: Their life and their world. Cambridge University Press. ISBN 978-0-521-23627-0. However, it is estimated that over 10,000 species of ammonite - possibly even over 20,000- have been discovered. A fossil (from Classical Latin fossilis, lit. 'obtained by digging') [1] is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood and DNA remnants. The totality of fossils is known as the fossil record.
Martin, Hilary C et al. " Insights Into Platypus Population Structure And History From Whole-Genome Sequencing". Molecular Biology And Evolution, vol 35, no. 5, 2018, pp. 1238-1252. Oxford University Press (OUP), doi:10.1093/molbev/msy041 Modern paleontologists have a variety of tools that help them discover, examine, and describe fossils. Electron microscopes allow paleontologists to study the tiniest details of the smallest fossils. X-ray machines and CT scanners reveal fossils' internal structures. Advanced computer programs can analyze fossil data, reconstruct skeletons, and visualize the bodies and movements of extinct organisms. At least 57 species of ammonites, which were widespread and belonged to six superfamilies, were extant during the last 500,000 years of the Cretaceous, indicating that ammonites remained highly diverse until the very end of their existence. All ammonites were wiped out during or shortly after the K-Pg extinction event, caused by the Chicxulub impact. It has been suggested that ocean acidification generated by the impact played a key role in their extinction, as the larvae of ammonites were likely small and planktonic, and would have been heavily affected. [24] Nautiloids, exemplified by modern nautiluses, are conversely thought to have had a reproductive strategy in which eggs were laid in smaller batches many times during the lifespan, and on the sea floor well away from any direct effects of such a bolide strike, and thus survived. [29] Many ammonite species were filter feeders, so they might have been particularly susceptible to marine faunal turnovers and climatic change. [5] Some reports suggest that a few ammonite species may have persisted into the very early Danian stage of the Paleocene, before going extinct. [30] [31] Cultural significance [ edit ] Smith, C. P. A.; Landman, N. H.; Bardin, J.; Kruta, I. (4 June 2021). "New evidence from exceptionally "well-preserved" specimens sheds light on the structure of the ammonite brachial crown". Scientific Reports. 11 (1): 11862. Bibcode: 2021NatSR..1111862S. doi: 10.1038/s41598-021-89998-4. PMC 8178333. PMID 34088905. Ceratosuchops inferodios translates to “horned, crocodile-faced hell heron,” inspired by the theory that spinosaurids were riverbank predators like today’s herons. Riparovenator milnerae means “Milner’s riverbank hunter,” a tribute to British spinosaurid expert Angela Milner. Each dinosaur was probably about 26 feet long and roughly 6.6 feet tall at the hip.Like modern cephalopods, ammonites displayed sexual dimorphism, which is the noticeable difference in appearance between sexes. But when ammonite fossils that looked unique were found in the past, they tended to be recorded as new species instead of as the microconch (male) or macroconch (female) of an existing species, as this difference between the sexes was not yet known about. Kin, Adrian; Błażejowski, Błażej (2014-10-02). "The Horseshoe Crab of the Genus Limulus: Living Fossil or Stabilomorph?". PLOS ONE. 9 (10): e108036. Bibcode: 2014PLoSO...9j8036K. doi: 10.1371/journal.pone.0108036. ISSN 1932-6203. PMC 4183490. PMID 25275563.
The majority of ammonite species feature planispiral shells, tightly coiled in a flat plane. The most fundamental difference in spiral form is how strongly successive whorls expand and overlap their predecessors. This can be inferred by the size of the umbilicus, the sunken-in inner part of the coil, exposing older and smaller whorls. Evolute shells have very little overlap, a large umbilicus, and many exposed whorls. Involute shells have strong overlap, a small umbilicus, and only the largest and most recent whorls are exposed. Shell structure can be broken down further by the width of the shell, with implications for hydrodynamic efficiency. Goniatites, which were a dominant component of Early and Middle Permian faunas, became rare in the Late Permian, and no goniatite is thought to have crossed into the Triassic. [26] a b McGowan, Alistair J.; Smith, Andrew B. (May 2007). "Ammonoids Across the Permian/Triassic Boundary: A Cladistic Perspective". Palaeontology. 50 (3): 573–590. Bibcode: 2007Palgy..50..573M. doi: 10.1111/j.1475-4983.2007.00653.x. Monks, Neale; Palmer, Philip (2002). Ammonites. Smithsonian Institution Press. ISBN 978-1-58834-024-5.
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a b Landman, Neil H; Tanabe, Kazushige; Davis, Richard Arnold (1996). Ammonoid paleobiology. Springer. ISBN 978-0-306-45222-2.
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