EVOLUTION - THE TRANSITIONAL FOSSILS
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neopterygians

Introduction

The neopterygians (clade Neopterygii in clade Actinopteri) represent a group that contains most of the fish species alive today. Their crown group, whose node is represented as a red dot below, comprises the teleostean and holostean fishes:
Picture
The terminal clades shown ​  at the right-hand side of the above tree each have a separate page that can be reached through the menu bar at the top of the page (under Evolution of Life/Animals/Vertebrates).

The stem group

A fairly recent phylogenetic tree (from Latimer and Giles, 2018) of the neopterygian stem group is shown below:
Picture
The earliest known fossil from the neopterygian stem group is Discoserra pectinodon, found in the Early Carboniferous (Serpukhovian) Bear Gulch Member of the Heath Formation at Bear Gulch in Fergus County, Montana, USA (Lund, 2000; Friedman et al, 2013). This fossil, together with other members of the stem group, is shown below (click on any image for a larger view):
Names in    red indicate   that the fossil is younger  than the oldest known crown-group fossil.
​* after name indicates that the image represents a life restoration
The above images are numbered in order from the most basal to those closest to the crown group, but no obvious evolutionary changes can be seen. All of these fossils, at least superficially, appear quite modern.

The time frame of the evolution of the neopterygian stem group is shown below:
Picture
Most of the stem-group fossils represented above are younger than the oldest crown group fossil and thus cannot be direct ancestors of the crown group. Rather, they are descendants of older, currently unknown, stem neopterygians that branched off the stem line (shown as a red line in the phylogenetic tree above) before the crown group appeared and were more similar to the ancestors of the crown group. However, some characteristics are shared within any given stem branch, such that the stem group fossils can be considered as generally representative of the conceptual ancestor at the base of that stem branch. For instance, Arduafrons prominoris, although much younger than the oldest crown-group fossil, has features in common with the last ancestor before the crown-Neopterygii appeared.

The crown group

The neopterygian crown group appeared when the stem lines of the Teleostei (most extant ray-finned fishes) and the Holostei (the bowfin and the gars) separated from one another:
Picture
The oldest known fossil representative of the crown-Neopterygii is the Early Triassic (Induan) Watsonulus eugnathoides, a crown holostean found in the Middle Sakamena Group at Ambilombe Bay, Madagascar (Olsen, 1984; Benton et al, 2015). This fossil, for which no public-domain image is available, is older than the earliest known stem teleostean species, Prohalecites porroi. This implies a ghost lineage of at least 9.2 million years. The relationship between these ages and that of the earliest stem neopterygian is shown in the figure below. Also shown are representative images of species from the respective stem groups (note that in the case of the Holostei the oldest crown group fossil,   Watsonulus eugnathoides, is older than all the known stem group fossils).

The figure below depicts the total uncertainty in the age of the Neopterygii crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the neopterygian stem group and the initiation of the crown group could have been as long as 80 million years:
Picture

Image credits - Neopterygians
Header: Banded archerfish (Toxotes jaculatrix (banded archerfish)  By James St. John [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]
Ebenaqua ritchei  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Bobosatrania groenlandica  By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Discoserra pectinodon  By Gyik Toma (Tommy the paleobear) from Romania [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]
Brembodus ridens  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Gibbodon cenesis  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Mesturus verrucosus  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Mesturus sp.  By Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons)
Arduafrons prominoris  By Ghedoghedo [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]
Eomesodon gibbosus  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Peltopleurus lissocephalus  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Dipteronotus ornatus  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
Prohalecites  sp.  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Dapedium stollorum  By Daderot [CC0]
  • Home
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  • Time scale
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        • Chimaeras
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