Introduction
The holosteans (class Holostei, clade Neopterygii) form a crown group that comprises the extant bowfin (order Amiiformes) and gars (order Lepisosteiformes). These two groups comprise two clades, the Halecomorphi and the Ginglymodi, which are discussed briefly below.
The stem group
The tree shown below represents a recent interpretation (by Latimer and Giles, 2018) of the phylogeny of the stem-Holostei:
This tree is unusual in that the stem group exists as a single sister clade to the crown group. It is thus monophyletic, whereas stem groups are commonly polyphyletic.
The oldest known member of the stem-Holostei is Sargodon tomicus, found in the Late Triassic (middle Norian) Dolomia di Forni Formation at Friuli-Venezia Giulia, Italy (Dalla Vecchia, 1995; Latimer and Giles, 2018). This fossil is illustrated below, together with other stem holosteans for which images exist in the public domain (click on any image to see a larger view):
The oldest known member of the stem-Holostei is Sargodon tomicus, found in the Late Triassic (middle Norian) Dolomia di Forni Formation at Friuli-Venezia Giulia, Italy (Dalla Vecchia, 1995; Latimer and Giles, 2018). This fossil is illustrated below, together with other stem holosteans for which images exist in the public domain (click on any image to see 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
* after name indicates that the image represents a life restoration
Again, these images are numbered in order from the most basal to those closest to the crown group, but no obvious changes can be seen except perhaps for the more elongate shape of Huletta americana compared with the rest.
The time frame of the evolution of the stem-Holostei is shown below:
The time frame of the evolution of the stem-Holostei is shown below:
It is noteworthy that all of the stem-group fossils represented above are younger than the oldest crown group fossil and thus cannot be closely related to direct ancestors of the crown group. Rather, they are all descendants of a single ancestor that itself descended from that part of the stem group, currently unknown in the fossil record, that appeared before the crown group.
The crown group
The crown-Holostei appeared when the stem lines of the Halecomorphi and the Ginglymodi separated from one another:
The earliest known member of the stem-Halecomorphi is Watsonulus eugnathoides, a stem bowfin from the Early Triassic (Induan) Bed 5 of the Middle Sakamena Group at Ambilombe Bay, Madagascar (Olsen, 1984; Benton et al, 2015). No public-domain images are available of this species. It predates the oldest known representative of the stem-Ginglymodi, which is Sangiorgioichthys sui, found in the Middle Triassic (Anisian) Vertebrate Level of the Upper Member of the Guanling Formation near the village of Dawazi, Luoping County, Yunnan Province, South China (López-Arbarello et al, 2011; López-Arbarello and Sferco, 2018). Again, no public-domain image is available.
The relationship between the ages of appearance mentioned above and that of the earliest stem holostean is shown in the figure below. Also shown are representative images of species from the respective stem groups. However, the age of the holostean stem group, being younger than either the stem-Halecomorphi or the stem-Ginglymodi, cannot be used to estimate the total uncertainty in the age of the holostean crown node. For that we can only use the age of the stem-Neopterygii, which gives an estimate of about 80 million years for the maximum period of time for the holostean stem-to-crown transition:
The relationship between the ages of appearance mentioned above and that of the earliest stem holostean is shown in the figure below. Also shown are representative images of species from the respective stem groups. However, the age of the holostean stem group, being younger than either the stem-Halecomorphi or the stem-Ginglymodi, cannot be used to estimate the total uncertainty in the age of the holostean crown node. For that we can only use the age of the stem-Neopterygii, which gives an estimate of about 80 million years for the maximum period of time for the holostean stem-to-crown transition:
Image credits - Holosteans
- Header: Florida gar (Lepisosteus platyrhincus) By Raimond Spekking / CC BY-SA 4.0 (via Wikimedia Commons)
- Hulettia americana By Isaac T Valtez [Public domain]
- Tetragonolepis semicincta By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Hemicalypterus weiri (3a) By Gibson SZ (2016) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Hemicalypterus weiri (3b) By Gibson SZ (2016) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Dandya ovalis By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Sargodon tomicus By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Dapedium noricum By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Dapedium caelatum By Daderot [Public domain]
- Dapedium punctatum By Ghedoghedo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
- Dapedium stollorum By Daderot [CC0]
- Dapedium pholidotum By Daderot [CC0]
- Ophiopsis attenuata By Haplochromis [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Paralepidotus sp. By Ghedoghedo [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Discoserra pectinodon By Gyik Toma (Tommy the paleobear) from Romania [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]