Synapsid ontogeny: trends and patterns

Allometry vs isometry in the Synapsida
Yesterday’s post calling into question the conspecific identity of three of the four skulls attributed by Liu (2013) to Sinophoneus (Fig. 1) brings up the subject of allometric ontogeny among basal synapsids (non-mammals). If the small skulls here (Fig. 1) are indeed juveniles of the large skull of Sinophoneus, then the rostrum has to elongate and the skulls must become wider during maturation. This hypothesis has been accepted by Liu and Kammerer. It is called into question by phylogenetic analysis.

We can provide clues
to the allometry/isometry question by bringing in other examples of basal synapsid ontogeny. Happily there are several examples. From these we should be able to extrapolate growth patterns in the Synapsida.

Figure 1. The holotype of Sinophoneus and four specimens one of which was formerly attributed to Stenocybus to scale, from Liu 2013. The IGCAGS specimen is the holotype of Stenocybus. The smallest specimen is actually basal to the tiny dromasaurs. The IVPP V18120 is actually basal to dicynodonts and kin. The IVPP V12119 specimen nests between Sinophoneus and Deuterosaurus.

Figure 1. The holotype of Sinophoneus and four specimens one of which was formerly attributed to Stenocybus to scale, from Liu 2013. The IGCAGS specimen is the holotype of Stenocybus. The smallest specimen is actually basal to the tiny dromasaurs. The IVPP V18120 is actually basal to dicynodonts and kin. The IVPP V12119 specimen nests between Sinophoneus and Deuterosaurus.

The first example
comes from Dimetrodon (Fig. 2). Here the skull is no larger compared to the post-crania, but we don’t known what the shape of the skull is or whether it had a shorter rostrum and larger eyes. The limbs and tail are much longer relative to the torso. The sail is shorter. The pectoral girdle is smaller.

Figure 2. Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. This is the first reconstruction of this specimen that I am aware of. Note the smaller sail and longer legs and tail. Regressing the baby to egg size suggests the sail developed after hatching.

Figure 2. Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. The skull is not relatively larger in the hatchling, but the legs, tail and sail are relatively shorter in the adult.

The second example
comes from Heleosaurus (Fig. 3), a basal synapsid in the lineage of protodiapsids.

Figure 2. Heleosaurus ontogeny. Here the smaller skull, when scaled to the size of the larger skull, shows very little variation in skull proportions.

Figure 3. Heleosaurus ontogeny. Here the smaller skull, when scaled to the size of the larger skull, shows very little variation in skull proportions. I added the snout of the juvenile to the missing area in the adult noting that the angles match. Other Heleosaurus skulls are known that confirm these proportions. 

In Heleosaurus the skull proportions are the same in the juvenile (Fig. 2), documenting isometric growth.

The third example
comes from the cynodont, Massetognathus (Fig. 3). In this case the rostrum becomes relatively shorter during ontogeny as the temporal region enlarges with larger jaw muscles.

Figure 3. Massetognathus skulls showing a progressive reduction of the rostrum during ontogeny, just the opposite of many mammals. The orbit is reduced during growth.

Figure 3. Massetognathus skulls showing a progressive reduction of the rostrum during ontogeny, just the opposite of many mammals. The orbit is reduced during growth. Smaller skulls are to scale with PULR11 skull at right. Modified from Abdala and Giannini 2000.

In Massetognathus the length/width of the skull does not change appreciably. The orbits are relatively smaller in the adult. This skull documents allometric growth, but different from reptiles and mammals that elongate the rostrum during maturation. All this assumes that the smaller skulls are indeed juveniles and not just distinct species.

The IVPP V 18119 specimen
Comparing Sinophoneus to the IVPPP V 18119 (Fig. 1) specimen that nested with Sinophoneus documents an orbit that gets relatively smaller, if Sinophoneus is indeed the adult. The orbit itself does not grow appreciably. Both the rostrum and temporal region elongate if the IVPP V 18119 specimen is indeed a Sinophoneus juvenile.

Comparisons to Deuterosaurus
Since the IVPP V 18119 specimen also nested with Deuterosaurus it is worthwhile to compare these two skulls (Fig. 4). The resemblance is worthy of discussion.

Figure 4. Comparing the adult Deuterosaurus to the smaller, perhaps juvenile, IVPP V18119 specimen attributed to Sinophoneus by Liu 2013. Here there is very little difference in proportions. Essentially only the nasal and postfrontal bosses are distinct in Deuterosaurus.

Figure 4. Comparing the adult Deuterosaurus to the smaller, perhaps juvenile, IVPP V18119 specimen attributed to Sinophoneus by Liu 2013. Here there is very little difference in proportions. Essentially only the nasal and postfrontal bosses are distinct in Deuterosaurus. By comparison the skull of Sinophoneus is low and wide.

If the IVPP V 18119 specimen is a juvenile Deuterosaurus no elongation of the rostrum and temporal region is necessary. The orbit reduction is minimal. These skulls document very little allometry and more isometry. Then again if the IVPP V 18119 specimen is simply a transitional taxon between a sister to Sinophoneus and a sister to Deuterosaurus then that is what is represented by phylogenetic analysis.

Figure 6. Ventral views of the Sinophoneus and Stenocybus. If related the palate would have widen considerably. Perhaps these two are no closely related.

Figure 5. Ventral views of the Sinophoneus and Stenocybus. If related the palate would have widen considerably. Perhaps these two are no closely related.

The palate of Sinophoneus (Fig. 5) is quite broad. So are the jaw tips. Stenocybus has a narrow pointed palate, quite different from Sinophoneus. No other synapsids document such a broadening of the palate during maturation.

Can we include small skulls in phylogenetic analyses of the Synapsida? I think so. I hope so. Let’s not avoid it and consider the ramifications later.

Your comments are always welcome. On this subject several comments can be seen here.

References
Abdala F and Giannini NP 2000. Gomphodont cynodonts of the Chañares formation: The analysis of an ontogenetic sequence. Journal of Vertebrate Paleontology 20(3):501-506. 
Botha-Brink J and Modesto SP 2009. Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29(2):389-400.
Cheng Z and Li J 1997. A new genus of primitive dinocephalian – the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrata PalAsiatica 35 (1): 35-43. [in Chinese with English summary]
Kammerer CF 2011. Systematics of the Anteosauria (Therapsida: Dinocephalia), Journal of Systematic Palaeontology, 9: 2, 261 — 304, First published on: 13 December 2010 (iFirst)
Liu J 2013. Osteology, ontogeny, and phylogenetic position of Sinophoneus yumenensis(Therapsida, Dinocephalia) from the Middle Permian Dashankou Fauna of China, Journal of Vertebrate Paleontology, 33:6, 1394-1407, DOI:10.1080/02724634.2013.781505 
Sternberg CW 1942. The skeleton of an immature pelycosaur, Dimetrodon cf. grandis, from the Permian of Texas. Journal of Paleontology 16 (4): 485–486.

wiki/Stenocybus

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