Abstract
Assessing the taxonomic importance of the suture line in shelled cephalopods is a key to better understanding the diversity of this group in Earth history. Because fossils are subject to taphonomic artifacts, an in-depth knowledge of well-preserved modern organisms is needed as an important reference. Here, we examine the suture line morphology of all known species of the modern cephalopods Nautilus and Allonautilus. We applied computed tomography and geometric morphometrics to quantify the suture line morphology as well as the conch geometry and septal spacing. Results reveal that the suture line and conch geometry are useful in distinguishing species, while septal spacing is less useful. We also constructed cluster trees to illustrate the similarity among species. The tree based on conch geometry in middle ontogeny is nearly congruent with those previously reconstructed based on molecular data. In addition, different geographical populations of the same species of Nautilus separate out in this tree. This suggests that genetically distinct (i.e., geographically isolated) populations of Nautilus can also be distinguished using conch geometry. Our results are applicable to closely related fossil cephalopods (nautilids), but may not apply to more distantly related forms (ammonoids).
Highlights
Assessing the taxonomic importance of the suture line in shelled cephalopods is a key to better understanding the diversity of this group in Earth history
We investigate the morphology of the suture line in modern nautilids in detail by applying high-resolution computed tomography and geometric morphometrics
We aim to answer the following questions: (1) To what degree does the morphology of the suture line differ among species of modern nautilids? (2) How does the variation in the morphology of the suture line compare to that of other conch parameters such as conch geometry and septal spacing? (3) Is the suture line a useful diagnostic character for cephalopods compared to other conch parameters? (4) How do our results based on these conch parameters compare to those from molecular data?
Summary
Assessing the taxonomic importance of the suture line in shelled cephalopods is a key to better understanding the diversity of this group in Earth history. Results reveal that the suture line and conch geometry are useful in distinguishing species, while septal spacing is less useful. The Cephalopoda are marine mollusks including both extinct (e.g., orthocerids, bactritoids, ammonoids, and belemnoids) and modern taxa (squids, octopuses, and nautiloids) Since their origin in the Cambrian[1], they have played an important ecological role in the oceans, occupying most likely multiple trophic levels[2,3,4,5,6]. The morphology of septa—and, the suture line—varies widely among and within cephalopod taxa at different geological times It is relatively simple in Paleozoic taxa including plectronocerids, oncocerids, and orthocerids, while it is significantly more complex in various clades of ammonoids in the Mesozoic. Geographic region Indonesia Papua New Guinea Palau New Caledonia Papua New Guinea Philippines Philippines Western Australia Lizard Island, Australia
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