Amy Falcão
Ichthyology
Fish Orders
2/1/2013
Semionotiformes
Semionotiformes is an order of fish from the geologic periods of the late Permian to the late
Cretaceous, which elapses through the Paleozoic and Mesozoic eras 1. It is commonly believed that this
order and the order Lepisosteiformes are one in the same taxonomically, with the primitive members
being places in the Semionotiformes and the living fish, Gar, in the Lepisosteiformes 1. For the purposes
of this assignment, I will keep the two orders separate. This order is a part of the superclass
Osteichthyes, the class Actinopterygii, and the subclass Neopterygii 2. Many sources list different
families under this order, but the only families that were listed consistently were Semionotidae and
Dapediidae with the genuses Acentrophorus, and Semionotus under the former and Dapedium under
the latter 3. Those who consider the Lepisosteiformes part of the same order include its families as part
of the Semionotoformes, but I am not including those because they are listed as separate orders.
Members of this family have thick ganoid scales and maxilla bones that do not connect with the
opercular series 1. There is no supramaxilla bone but there is a interopercular 1. The mouths of these fish
are small with teeth that are particularly strong and their caudal fins are hemiheterocercal 3. The shape
of the body varies from fusiform to compressiform 3 with stocky bodies and blunt snouts 4. One aspect
of them that is distinguishing is their opisthocoelous vertebrae 4. Their bodies are part of what causes
people to separate them from the order that some others consider part of this order, Lepisosteiformes.
The modern Gar in this other order are different in their morphology with elongated bodies unlike the
primitive form 4. Fish in the Semionotiformes are known to be "cosmopolitan" species, with fossils being
found all over the world, which is another difference from their modern relatives which only live in
North to Central Americas, although both live in mostly fresh to brackish, shallow, weedy water such as
a lake or pond 4. An interesting fact about this order is that it includes the genus Acentrophorus, which is
the oldest known genus of fish in the Neopterygian subclass 1. I could not find anything on their ecology,
likely because they are such primitive fish that not much is known about their feeding habits or foraging
modes. In 1999 there was an article published entitled "Chromosomal Characteristics of Ribosomal DNA
in the Primitive Semionotiform Fish, Longnose Gar Lepisosteus osseus" where different staining
techniques and fluorescence techniques were used to examine the DNA of the extant species and
discusses what this meant in an evolutionary context 5. This is another example of the overlap of the two
aforementioned orders, but is interesting because determining the genetic differences, or lack-there-of,
would be helpful in coming to a more certain conclusion about whether these two orders are the same
or completely different.
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Perkins, R., Santos, R., Kazlev, M. 2011. Neopterygii: Semionotiformes. Paleos: Life Through DeepTime.
Accessed February 2013. http://palaeos.com/vertebrates/neopterygii/semionotiformes.html
Integrated Taxonomic Information System. 2013. Semionotoformes. Taxonomic Heirarchy. Accessed
February 2013.
http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=161090
Encyclopedia Britannica. 2013. Holostean. Extant Groups. Accessed February 2013.
http://www.britannica.com/EBchecked/topic/269640/holostean/63445/Extant-groups#ref1019985
Bailey, R. Access Science. Semionotiformes. Accessed February 2013.
http://www.accessscience.com/abstract.aspx?id=614300&referURL=http%3a%2f%2fwww.accessscience.
com%2fcontent.aspx%3fid%3d614300
Rab, P., Rabova, M., Reed, K., Phillips, R. 1999. Chromosomal Characteristics of Ribosomal DNA in the
Primitive Semionotiform Fish, Longnose Gar Lepisosteus osseus. Chromosome Research. 7 (6):475-480.