The search for sterna

This is step 2 in a search for sterna in the Tetrapoda.
Please consider this a work in progress because several hard-to-find appearances of the single sternum or dual sterna in tetrapods (and one fish, Fig. 9) may have been overlooked. Sterna also tend to come and go. They may be poorly ossified if present. We first and last looked at sternum distribution five years ago here with far fewer taxa. 

The sternum
This second anchor for pectoral muscles may be a single medial element (Fig. 1) or side-by-side elements slightly separated by the posterior process of the medial interclavicle, the first anchor for pectoral muscles. Almost universally the sternum is posterior to the interclavicle (Fig. 1). Sometimes the sternal rim anchors sternal ribs.

Since the sternum seems to be missing
more often than present, the various appearances of this bone in disparate clades appears to be largely convergent—with a genetic underpinning based on anterior gastralia in the most primitive reptiles.

Sometimes, as in birds,
the sternum (breast bone) can be the largest bone in the body.

The most primitive appearance of a sternum is in frogs,
like Rana (st in Fig. 1). Other basal tetrapods lack a sternum, but have a large interclavicle.

Figure 1. Rana the frog. Note the tiny sternum in the upper left inset, posterior to the coracoids.

The basalmost reptiles in the LRT,
Gephyrostegus and Silvanerpeton (Fig. 2) have short gastralia in the shape and place of sterna. These are immediately posterior to the coracoids and are precursors that variously evolve into sterna, if they don’t disappear, which happens more often than not.

Immediately following these basalmost reptiles,
the first dichotomy splits the new Lepidosauromorpha and the new Archosauromorpha.

Figure 2. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Sterna in basal Lepidosauromorpha:

1. Thuringothyris has small paired post-coracoid elements not seen in sister taxa.
2. Stephanospondylus (a pareiasaur ancestor) has an anterior ‘procoracoid’ and a  coracoid apparently not homologous to sterna. No gastralia are present.
3. Both soft-shell and hard-shell turtles develop a plastron, a set of bones presently considered not homologous with sterna or gastralia.

Figure 3. Saurosternon, the first taxon in the lepidosauromorph lineage with sterna.

Sterna in basal Lepidosauriformes:

1. Saurosternon (Fig. 3) has paired sterna posterior to the coracoids.
2. Jesairosaurus has a single, posteriorly indented sternum.

Sterna in Rhynchocephalian Lepidosauria:

1. Sphenodon has a diamond-shaped sternum, but sister taxa lack one.

Figure 4. Tritosaur pectoral girdles demonstrating the evolution and migration of the sternal elements to produce a sternal complex.

Sterna in Tritosaurian Lepidosauria:

1. Almost all tritosaurs (e.g. Huehuecuetzpalli through Cosesaurus, Fig. 4)  have a sternum except the hyper-neck taxa, Tanystropheus and Dinocephalosaurus.
2. All pterosaurs and Longisquama fuse the sternum to the clavicles and interclavicle to form a sternal complex (Fig. 4).

Sterna in Protosquamata and Squamata:

1. Homoeosaurus has a single sternum.
2. MFSN 19235 (= ‘Renestosaurus‘) has a single sternum.
3. Lyriocephalus and Chlamydosaurus have a single sternum.
4. Moloch and Trioceros have a single sternum.
5. Eolacerta and Gekko have a single sternum.
6. Varanus and Tylosaurus have a single sternum.

Sterna in basal Archosauromorpha:

1. Eldeceeon and Diplovertebron have paired sterna.

Sternal elements in Synapsida
Following these two, the sternum is absent in basal proto-synapsids and basal synapsids.

Sternal elements in Mammalia
In monotremes a string of one to several articulated sternal elements appear (Fig. 5) where the clavicles are green, the interclavicle is red, the sternal manubrium is blue. The interclavicle disappears in the opossum Didelphis and its descendants, all higher therians. Only the manubrium and sternal elements remain. In many placentals the sternal elements fuse together (Fig. 5 image at right) as they anchor dorsal ribs that wrap all the way around from the back.

Figure 5. The pectoral girdle of basal mammals and their relatives. Note the presence of an interclavicle (red), clavicles (green) and a new bone, the manubrium (deep blue), which develops where the sternum develops in other tetrapods.

Sterna in basal Diapsida

1. Only in Petrolacosaurus and Araeoscelida is a sternum present.

Figure 6. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Sterna in marine Younginiformes and Enaliosauria

1. Thadeosaurus, and Hovasaurus have paired sterna (Fig. 6).
2. Tangasaurus has a single large sternum (Fig. 6). these are all basal taxa in this clade.

Sterna in terrestrial Youngininformes and Protorosauria are not present.

Figure 7. Champsosaurus sternum (yellow).

Sterna in Archosauriformes

1. Champsosaurus has a small, narrow sternum (Fig. 6). Due to its size and shape a closer examination of related taxa is warranted, but currently a sternum has not been identified.
2. Crocodylus appears to have a short, broad ‘sternum’ anchoring elongate coracoids, but this is the inter cruciform interclavicle. Basal archosaurs, including basal dinosaurs lack sterna.

Figure 8. Hummingbird skull and skeleton. Note the large sternum at bottom, anchoring flight muscles. Image courtesy of Digimorph.org and used with permission.

Sterna in Dinosauria

1. Scipionyx. Compsognathus and Struthiomimus have paired sterna.
2. Zhenyuanlong and Tianyuraptor have paired sterna.
3. Velociraptor, Balaur, Haplocheirus, Shuuvia and Mononykus have paired sterna.
4. Limusaurus and Khaan have paired sterna.
5. Microraptor and Sinornithosaurus have paired sterna.
6. Troodontids have paired sterna.
7. Birds (Fig. 8) have large fused sterna, except the enantiornithine, Sulcavism which lacks sterna, replaced with gastralia, as in basalmost reptiles (Fig. 2). Talk about a reversal!!
8. Camarasaurus, Brachiosaurus, Apatosaurus, and other sauropods have paired sterna.
9. Psittacosaurus. and ceratopsians have paired sterna. Hard to find them elsewhere.

Figure 9. Rhombichthys, a tiny Late Cretaceous tarpon with deep scutes creating a sternum.

Sternum in fish

1. Rhombichthys (Fig. 9), is a tiny Cretaceous tarpon that looks like an angelfish. Here the ‘sternum’ is created by fusion of several dozen elongate scales that are not pelvic or anal in origin. This is the only sternum present in a fish taxon in the LRT.

Summary
Paired and median sterna appear and disappear throughout the clade Tetrapoda. Since some sterna are small and/or poorly ossified, their distribution within the Tetrapoda may be greater than currently counted. Primitive gastralia proximal to the coracoids appear to be homologous to derived sternal plates proximal to the coracoids. The sternum fuses to the interclavicle and clavicles in pterosaurs and their allies.

Editor’s Note:
WordPress has recently revised their creative methods, now offering buttons for [EDIT], which delivers a blank page permitting no inputs whatsoever and [CLASSIC EDIT], which permits traditional editing. Unfortunately when you press on the [ADD NEW] button you no longer get a blank format ready to be filled, but another blank page permitting no inputs whatsoever. Let’s hope these ‘bugs’ get fixed soon. I have about a week of posts ready to go, but no more possible afterwards given the present ‘bugs’. 

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References
Vickaryous MK and Hall BK 2006. Homology of the reptilian coracoid and a reappraisal of the evolution and development of the amniote pectoral apparatus. J Anat. 2006 Mar; 208(3): 263–285. doi: 10.1111/j.1469-7580.2006.00542.x

wiki/Sternum