Introduction

The teleosteans (class Teleostei, clade Neopterygii) comprise a clade that contains most of the ray-finned fishes alive in the world today, with nearly 30,000 species (Ravi and Venkatesh, 2018). They consist of two major divisions ("megacohorts"), the Elopocephalai and the Osteoglossocephalai, which are briefly considered below in the section on the crown group.

The stem group

​The telesotean stem group is known as the Teleosteomorpha. Its phylogeny is suggested by the following quite recently published tree (Arratia, 2017):
The oldest known representatives of the stem-Teleostei are Prohalecites porroi and two species of Malingichthys, both of Middle Triassic (Late Ladinian) age. The only one with a public-domain image available is P. porroi, found in the Middle Triassic (Ladinian) Calcare di Perledo-Varenna at a site near Perledo, Como, northern Italy (Tintori, 1990; Arratia, 2017). This fossil is illustrated below (as Prohalecites sp.), together with other members of the teleostean stem group (for a larger view, click on any image):
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 the only obvious change is the disappearance of the pointed snout seen in the Pachycormidae and the Aspidorhynchidae, families located at the most basal part of the phylogenetic tree.

The teleostean stem group evolved from Middle Triassic to Late Jurassic time, as shown below: 

The crown group

The crown-Teleostei appeared when the stem lines of the Elopocephalai and the   Osteoglossocephalae separated from one another:
The oldest known fossil representative of the stem-Elopocephalai is Anaethalion zapporum, found in the Late Jurassic (Late Kimmeridgian) Rögling Formation at Schamhaupten, near Eichstätt, Germany (Arratia, 2000; Arratia, 2017). This predates the first appearance of the stem-Osteoglossocephalai, represented by Lycoptera davidi, found widely in the Jehol Biota of northeast China (Pan et al, 2013) and specifically in the Qiaotou Member of the Early Cretaceous (Barremian) Huajiying Formation at Longfengshan Hill, Hebei Province, China (Ji et al, 2005; Zhang, 2006). This difference in time of appearance implies a ghost lineage of 23 million years for the osteoglossocephalan stem line.

The relationship between the ages of appearance mentioned above and that of the earliest stem teleostean is shown in the figure below. Also shown are representative images of species from the respective stem groups. The figure also depicts the total uncertainty in the age of the teleostean crown node. This uncertainty represents the maximum period of time for the stem-to-crown transition; the time between the origin of the stem-Teleostei and the initiation of the crown group could have been as long as 87 million years:
Image credits - Teleosteans
  • Header:  Clown anemonefish (Amphiprion ocellaris)  By Nhobgood Nick Hobgood [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Pachycormus bollensis  By Daderot [CC0]
  • Orthocormus cornutus (2a)  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Orthocormus cornutus (2b)  By DiBgd [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Hypsocormus insignis (3a)  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Hypsocormus insignis (3b)  By Dmitry Bogdanov [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Vinctifer comptoni  By Hectonichus [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Belonostomus muensteri  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Aspidorhynchus acutirostris  By Daderot [CC0]
  • Prohalecites sp.  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Eurycormus speciosus  By Daderot [CC0]
  • Parapholidophorus nybelini  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Pholidophorus latiusculus  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Pholidorhynchodon malzannii  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Lombardichthys gervasuttii  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Pholidoctenus serianus  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Siemensichthys macrocephalus  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Ankylophorus similis  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Dorsetichthys bechei  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Leptolepis coryphaenoides  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Tharsis dubius  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Ebertichthys ettlingensis  By Martin Ebert [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]
  • Ascalabos voithii  By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Varasichthys ariasi (21a)  By Sergio Soto-Acuña; Rodrigo A. Otero; David Rubilar-Rogers; Alexander O. Vargas [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Varasichthys ariasi (21b)  By Gloria Arratia. Published by Sergio Soto-Acuña; Rodrigo A. Otero; David Rubilar-Rogers; Alexander O. Vargas [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Luisichthys vinalensis  By Nobu Tamura (http://spinops.blogspot.com) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
  • Anaethalion sp.  By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
  • Lycoptera davidi  By Dwergenpaartje [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]