Reconstructing the partial manus of Ceratosaurus and its bearing on Limusaurus

A recent paper
by Carrano and Choiniere 2016 excavated and described the metacarpus and forearm plus a few partial phalanges of the Ceratosaurus holotype  (USNM 4735).

FIguire 1. the partial manus of Ceratosaurus compared to that of Coelophysis, Dilophosaurus and Allosaurus.

FIguire 1. the partial manus of Ceratosaurus compared to that of Coelophysis, Dilophosaurus and Allosaurus. Restored areas are in gray.

Carrano and Choiniere reported,
“it is more parsimonious to identify the manus of Limusaurus as an autapomorphic condition instead of as primitive for Ceratosauria (Xu et al., 2009). This is particularly evidenced by the primitive metacarpal I in Ceratosaurus, nearly identical in morphology to that seen in other basal neotheropods and quite unlike that in Limusaurus.”

You may recall
that the putative digit 1 of Limusaurus is actually the reappearance of digit 0, a basal tetrapod digit medial to digit 1. We also looked at a possible digit 0 on a specimen of Coelophysis.

Although Ceratosaurus currently nests as a sister to Allosaurus
in the large reptile tree, the manus is more primitive (more like that of Dilophosaurus) with a smaller digit 1 and the retention of a vestigial digit and mc4. Limusaurus nests with the oviraptorid, Khaan.

Carrano and Choiniere correctly conclude:
“Therefore, extreme manus reduction occurred twice in ceratosaurs—once in Limusaurus (and possibly in closely related taxa) and once in derived abelisaurids—and differently in each.”

References
Carrano MT and Choiniere J 2016. New information on the forearm and manus of Ceratosaurus nasicornis Marsh, 1884 (Dinosauria, Theropoda), with implications for theropod forelimb evolution, Journal of Vertebrate Paleontology, DOI:10.1080/02724634.2015.1054497

SVP 18 – the pelycosaur Dimetrodon via Dr. Robert Bakker

Bakker et al (2015)
show evidence that Dimetrodon (Fig. 1) fed on aquatic prey as there were too few terrestrial reptilian herbivores to sustain their numbers.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

Figure 1. Dimetrodon, a sailback pelycosaur synapsid reptile of the Early Permian.

From the abstract
“In restorations, Dimetrodon often appear feeding upon large land herbivores, e.g., Diadectes and Edaphosaurus. 􀁄􀁑􀀃􀁄􀁏􀁗􀁈􀁕􀁑􀁄􀁗􀁌􀁙􀁈􀀃􀁙􀁌􀁈􀁚􀀏􀀃􀀲􀁏􀁖􀁒􀁑􀂶􀁖􀀃􀀤􀁔􀁘􀁄􀁗􀁌􀁆􀀃􀀩􀁒􀁒􀁇􀀃 Base Theory (AFBT) recognizes non-terrestrial prey as key for dimetrodont food webs. Over 45% of the bones are severely tooth-marked; ubiquitous shed Dimetrodon teeth are mingled with tooth-marked bones in every depositional unit. The CBB lacks any structures that indicate high current energy, so the hydraulic forces probably did not wash in bones from beyond the trough, though bloated whole carcasses could have floated in. There are 39 Dimetrodon, one each of the large herbivores Edaphosaurus and Diadectes, three of the large non-herbivore, non-apex carnivore Secodontosaurus, and three of the semi-terrestrial amphibian Eryops calculated form postcrania. Did benthic amphibians and fish fill the gap in prey? The benthic amphibian Diplocaulus is abundant in every bone-rich unit. Xenacanth sharks are very common in several layers; each shark carried a large, well ossified head spine. AFBT is corroborated: dimetrodonts fed intensively on aquatic prey at the CBB.”

Combine this with what we know of Spinosaurus, and finback reptiles appear to have been largely aquatic in habitat. That’s heresy joining the mainstream.

There is also a good Dimetrodon video (52 min.)
on YouTube featuring Dr. Bakker as he describes how the vast majority of Dimetrodon tails are missing, neatly cut and probably carried away for their meat (because that’s where the most of it is!) by other Dimetrodons.

References
Bakker RT et al. 2015. Dimetrodon and the earliest apex predators: The Craddock bone bed and George Ranch Facies show that aquatic prey, not herbivores, were key food sources. Journal of Vertebrate Paleontology abstracts.

Dimetrodon had serrated teeth! Why is this news?

Figure 1. The serrated tooth of Dimetrodon -- taken from a 2008 blog. Serrated teeth on Dimetrodon is old news, folks.

Figure 1. The serrated tooth of Dimetrodon — taken from a 2008 blog. Serrated teeth on Dimetrodon is old news, folks.

Try googling “Dimetrodon serrated” and you’ll get several pages of fresh-off-the-presses news about how exciting and important this new find is by Brink and Reisz (2014). Unfortunately, this is old news. Serrations on Dimetrodon teeth have been known for decades. The photo at left is from a 2008 blog.

What’s more exciting that finding serrations on a Dimetrodon tooth is witnessing the publicity machine whirling at Nature.com, the publisher of the article. It’s truly amazing.

Figure 2. Serrations on basal synapsids.

Figure 2. Serrations on basal synapsids. Note serrations appear at the same time on the much smaller Secodontosaurus with ghost lineages for therapsids predating them both.

Brian Switek writing for National Geographic online summarized the paper noting that the big Dimetrodon was eating the big prey items including: “pinheaded protomammals called caseids” and “amphibians called diadectids.” We trashed the caseid/synapsid connection here. And the amphibian/diadectid connection here. Such old data from a respected news outlet takes us back to tail-dragging dino days.

From the abstract
“Paleozoic sphenacodontid synapsids are the oldest known fully terrestrial apex predators. Dimetrodon and other sphenacodontids are the first terrestrial vertebrates to have strong heterodonty, massive skulls and well-developed labio-lingually compressed and recurved teeth with mesial and distal cutting edges (carinae). Here we reveal that the dentition of Dimetrodon and other sphenacodontids is diverse. Tooth morphology includes simple carinae with smooth cutting edges and elaborate enamel features, including the first occurrence of cusps and true denticles (ziphodonty) in the fossil record. A time-calibrated phylogenetic analysis indicates that changes in dental morphology occur in the absence of any significant changes in skull morphology, suggesting that the morphological change is associated with changes in feeding style and trophic interactions in these ecosystems. In addition, the available evidence indicates that ziphodonty evolved for the first time in the largest known species of the genus Dimetrodon and independently from the ziphodont teeth observed in some therapsids.”

What is intriguing is all the fuss about Dimetrodon grandis and D. limbatus having the oldest terrestrial serrations on the planet (that is, according to the headlines), when according to the Brink and Reisz charts, generic “therapsids” and Secodontosaurus had serrations earlier, at least according to their ghost lineages (Fig. 2).

What am I missing here?

Serrations are good for biting into big chunks of flesh, btw. Just ask any steak knife.

References
Brink KS and Reisz RR 2014. Hidden dental diversity in the oldest terrestrial apex predator DimetrodonNature Communications. doi: 10.1038/ncomms4269

Baby Dimetrodon – Chimaeras and Fakes – Part 1

Many have seen this cast (fig. 1) entitled, “Baby (or Juvenile) Dimetrodon.” It’s a common piece of plaster merchandise sold at fossil fairs, etc.

Is it a complete fake?
The specimen has not been illustrated in the literature (that I know of), but it has been described (Sternberg 1942) and the description is a perfect match. Parts (Fig. 1 in gray) have been added to the cast to make it more interesting and complete. Sternberg (1942) reported the specimen was originally at the Walker Museum in Chicago, but in 1953 most Walker paleo exhibits, perhaps including this one, were moved to the Field Museum.

Figure 1.  Sternberg 1942 described this specimen he attributed to a baby or juvenile Dimetrodon. Parts added by artisans are in gray.

Figure 1. Sternberg 1942 described this specimen he attributed to a baby or juvenile Dimetrodon. Parts added by artisans are in gray.

On the plus side
One of the more complete Permian fossils is this baby/juvenile Dimetrodon (Sternberg 1942, Figs. 1, 2), less than a quarter the size of an adult with a much shorter sail and much longer legs. If this is a juvenile Dimetrodon, these proportions change allometrically during growth. The mandible was slightly shorter, compared to the adult, indicating the skull was likewise not larger relative to the body.

Click to enlarge. Figure 1. Baby Dimetrodon (above) compared to adult (below) to the same scale and to different scales. 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. This is the first reconstruction of this specimen that I am aware of. Restored parts in light red. Note the smaller sail and longer legs and tail in the juvenile.  Regressing the baby to egg size suggests the sail developed after hatching. I’m curious about the rib length from front to back on the juvenile, different from the adult.

So, longer legs on a juvenile synapsid?
That’s not the pattern we see in mammals or Heleosaurus, a varanopid(?) protodiapsid in which adults have the longer legs. In Dimetrodon the juveniles didn’t have marginally longer legs. Juveniles had legs relatively twice as long as those on adults. Generally longer legs provide more speed to attack prey or avoid predators.

Is this really a baby Dimetrodon?
Or is it a different smaller species? Bakker(1982) suggested different habitats for Permian juveniles would help them avoid adult predation. Brinkman (1988)  cast doubt on Bakker’s idea by showing that the specimens found in floodplain and swamp sediments represented two different species, not adult and juvenile populations of the same species.

Do we need more tiny specimens and a few teenage specimens to help determine what the situation is here? Both sides make sense.

Sternberg (1942) wrote,
“The preservation of the bone is poor: It is probable that the bony elements were never well ossified.” He also wrote that three or four partial skeletons of Dimetrodon grandis were found in the same pocket, which lies in the breaks of Coffee Creek in Baylor County, Texas. If we assume those were adults, there goes Bakker’s and Brinkman’s hypotheses. Brinkman did not reference the Sternberg paper, but noted that poor ossification attended smaller Dimetrodon specimens.

Only parts are fake
Just because parts of this specimen have been added with restoration, doesn’t mean the rest of the skeleton is useless or should be labeled “a fake.” In this case, we should use what is real and avoid what is fake. The size and proportion relationships are still good data that make a good story.

References
Bakker RT 1982. Juvenile-Adult Habitat Shift in Permian Fossil Reptiles and Amphibians. Science 217 (4554): 53–55. doi:10.1126/science.217.4554.53.PMID 17739981.
Brinkman D 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology 8 (2): 172–180.
Sternberg CW 1942. The skeleton of an immature pelycosaur, Dimetrodon cf. grandis, from the Permian of Texas. Journal of Paleontology 16 (4): 485–486.

A paper written on fossil fakes is online here.