Where do sea horses come from?

A little off topic,
but I was curious to see how the odd morphology of the sea horse came to be, who its ancestors were and what transitional taxa went through on their evolutionary journey through deep time. Hope you find this interesting.

The relationship between sticklebacks and sea horses
has been known for many decades. Both are members of the clade Gasterosteiformes, which is in the clade of spiny finned fish, Acanthopterygii, which is in the clade of bony ray-finned fish, Actinopterygii.

A helpful guide
is Gregory 1933, available online as a PDF. Most of the images below come from that book.

No phylogenetic analysis was performed here,
so think of the following images as broad evolutionary brush strokes, not a narrow ladder of succession. Few details are offered because most are apparent at first glance. Precise last common ancestors remain unknown. These are rare derived representatives of deep time radiations.

Even so,
the early appearance of body armor in the stickleback, G. aculeatus; the diminution of the tail (except in the pipefish/fantails, Dunckerocampus and Solenostomus); the gradual loss of the fusiform shape; the elongation of the rostrum and the reduction of the mouth are all apparent in this series of illustrations.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

The skulls of the taxa shown above
(Fig. 2) detail other changes, such as how far anteriorly the quadrate and palate bones shift on these fish with an ever longer rostrum and ever smaller mouth losing tiny teeth. The hyomandibular (hy) is the stapes in tetrapods. Not sure about the homology of the squamosal and the labeled preopercular, but the following is offered. Sometimes fish and tetrapods have different names for the same bones, as we learned earlier here.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

So many bones
are displaced or lost in sea horses distinct from their basal vertebrate locations (e.g. Cheirolepis, Fig. 3) that an evolutionary series illustration (Fig. 2) proves helpful in understanding the lumping and splitting of clade members. Sarcopterygians, like Osteolepis (Fig. 3), split off early from other ray-finned fish, which is why they appear share more traits and proportions with Cheirolepis. Note the jaw hinge remains posterior to the orbit in these two.

Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Figure 3. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Understanding where we came from,
and where our cousins went in their evolutionary journeys are the twin missions of this blogpost in support of ReptileEvolution.com.


References
Franz-Odendaal TA and Adriaens D 2014. Comparative developmental osteology of the seahorse skeleton reveals heterochrony amongst Hippocampus sp. and progressive caudal fin loss. EvoDevo 2014, 5:45
Gregory WK 1933. Fish skulls: a study of the evolution of natural mechanisms. American Philosophical Society. ISBN-13: 978-1575242149 PDF

wiki/Seahorse
diverosa.com/Syngnathidae

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