Astronomy vs. Paleontology

Having dealt with astronomy and paleontology for much of my life, I thought it would be a good time to compare and contrast the two.

In astronomy 
all the members of the Cosmos are available to anyone to observe with or without a telescope. All the specimens are complete with regard to their visual spectra. Interpretation is straightforward and typically not controversial.

In paleontology
all the undiscovered specimens are available to anyone who puts in the effort to find them and remove or expose them from the matrix, but some specimens cannot be excavated without a permit. Some of the discovered specimens are available for study in museums. A few discovered specimens are kept in desk drawers and offices awaiting description or redescription and are therefore unavailable. Privately held specimens cannot enter the literature, but some do. Complete specimens are relatively rare. Most to all specimens need to be reconstructed from in situ data to their in vivo state, but this is rarely done. Some bits and pieces can be misinterpreted and interpretations can be controversial. Sometimes its hard to tell a suture from a crack. Some bones are buried beneath others or leave only the faintest impressions and stains.

In astronomy
all of the visible specimens follow the law of physics and so are largely predictable and follow paradigms set down decades ago. Dark energy and dark matter remain the only enigmas. The age of the Universe and distances to various heavenly bodies appears to be universally agreed upon. Mistakes rarely if ever occur any more. No specimens need to be reconstructed: WYSIWYG.

In paleontology
most of the specimens fall readily into established clades and can be identified as to their diet and niche. However several specimens and clades have been and continue to be misidentified as to their nesting. Mistakes continue to be made largely due to taxon exclusion, sometimes by oversight, sometimes by refusal. Many determinations are made by opinion and by following tradition rather than by rigorous testing.

In astronomy
anyone can discover a member of the Cosmos, and announce it to the Astronomical Union. Time is often of the essence. The pros don’t mind if an amateur makes a discovery. Every discovery is celebrated.

In paleontology
if you discover something you have to write a paper, then submit it, then wait about six months for referees to review it, then go through the editorial process if accepted, then await its ultimate publication, often a year later. Time is never of the essence. Even so, anyone can make a contribution, if deemed acceptable, The pros don’t like amateurs making discoveries that they should be making. After all, something can only be discovered once. Some discoveries are shunned and ignored.

Let’s look at the sternum!

Everyone thinks they have a sternum.
But it’s not the same sternum that lizards have, or birds have or frogs have. Let’s take a closer look.

In the large reptile tree an ossified sternum appears about seven times:

  1. Rana the frog
  2. Palaeagama and the rib gliders + Megachirella and Pleurosaurus + Tritosauria (sans Jesairosaurus + Drepanosauridae) + Squamata (sans Eichstaettisaurussnakes) (sans ShinisaurusOphisaurus)
  3. Sphenodon and Kallimodon
  4. Petrolacosaurus + Araeoscelis
  5. Hovasaurus + Tangasaurus + Thadeosaurus
  6. LImusaurus through birds
  7. Haya and Heterodontosaurus

Note there are no synapsids
(including mammals) on this list. Note also the sternum is not present in basal tetrapods and basal amniotes. The sternum in fenestrasaurs, including pterosaurs is actually the sternal complex (clavicles + interclavicle + sternum). And finally, there does not appear to be a sternum in the mesosaur, Stereosternum.

Figure 1. 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.

Figure 1. 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. Click to enlarge. Image modified from Luo, Ji and Yuan 2007.

In mammals
what we call a sternum is actually a novel set of bones forming a ventral anchor for the ribs (as the sternum does in most tetrapods). The interclavicle is retained in basalmost mammals, but it too disappears in higher forms only to be replaced by these novel rib anchors.

I had no idea about this
until I found the Luo et al. 2007 reference. Thought I’d share it with you, especially if you need to get up to speed, like I did.

References
Luo Z-X,  Ji Q and Yuan C-X 2007. Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals. Nature 450, 93-97. doi:10.1038/nature06221

Pterosaur launch talk from 2012 on YouTube

Dr Mike Habib
gave a one-hour talk on pterosaurs and his hypothesis of forelimb takeoff back in 2012 when the idea was novel.. That talk was uploaded to YouTube here. In counterpoint back then we discussed Habib’s forelimb launch hypothesis for pterosaurs here and here. We’ll continue with that discussion today.

Habib reports
that he does not believe or is not aware of flightless pterosaurs, but I think he was aware of Jme-Sos 2028, which was in ReptileEvolution in 2011 and entered the literature in 2013. Habib does not believe in bipedal pterosaurs, despite bipedal tracks. He notes a tendency to produce giant flyers, but actually they quite rare with regard to taxon number, and were only present in the latest Cretaceous. Habib does not recognize tiny pterosaurs as adults and he does not believe in multi-modality (walking disconnected from flying), despite fossil evidence for disconnected hind and forelimbs. Habib did not discuss pterosaur origins.

Habib used CT scanning
for figuring out the inner and outer diameter of long pterosaur bones. The bone is thinner than in birds, about the proportions of a cardboard paper tube. Key to Habib’s hypothesis, he notes the forelimbs are stronger than the hind limbs in pterosaurs. He notes the hind limbs of pterosaurs are average-to-weak compared to birds. Furthermore, Habib reports that take-off in birds is ‘hindlimb’ driven with takeoff initiated 80-90% by leaping, the rest with a downbeat. Even in hummingbirds with their tiny legs and feet the ratio is 50-50.
Habib notes that initial lift is difficult in all flying creatures. The vampire bat uses its forelimbs to catapult itself 2 feet vertically before flapping. That is several times its standing height. He notes launch speed is related to wing loading (wing area/weight), which can increase substantially after a meal, which brings us to…
Quadrupedal launch in pterosaurs
As discussed several years ago at various posts (see above) unfortunately Dr. Habib ignores the literature on bipedal pterosaur tracks and the origin of pterosaurs from long-legged and bipedal fenestrasaur precursors. Late in the talk he gives credit to Dr. Padian, who championed bipedality among pterosaurs, but omly imagined bipedal ancestors, and had nothing to do with discovering fenestrasaurs. When you make all pterosaurs ungainly quadrupeds, shackled by a membrane that connects wing tips to ankles, you put pterosaurs at an unnatural and completely imagined disadvantage. Habib also imagined short manual digits that enabled digit 4 to act like grasshopper hind limbs to catapult them into the sky.
All pterosaurs were capable of bipedal locomotion.
They could balance with their feet beneath their armpits, like birds do. Quadrupedal ptero tracks were all produced by a few clades of beachcombing pterosaurs during their browsing mode. All of these had relatively small wing claws. Other pterosaurs had much larger trenchant manual claws, ill-suited for contact with the ground.
With regard to forelimb launch,
all of the animated pterosaurs that Dr. Habib approved appear to be helium filled as the first down flap comes a long time after the initial launch. Moreover, none of the animations show the pterosaur leaping several times its standing height, as in the vampire bat. Worse yet, the giant wing fingers, which initially are folded posteriorly during the forelimb leap need to be extended prior to or at the acme of the leap, but initially there is no airspace to do this. Based on the orientation of the ventral orientation of the forelimbs during launch and recoil, the wing finger has to extend ventrally in the plane of the wing, This is hazardous to the swinging wing tip if it contacts the launch pad. The ground gets in the way unless the pterosaur is high enough to avoid this. All pterosaur takeoff animations authorized by Dr. Habib appear to glaze over this point, as if the long wing finger had no mass or moment arm and the initial leap never experiences recoil in the ventral plane. Rather the wings imeediately pop out effortlessly. Even a lightweight fishing rod takes a little time and effort to get from one point to another. As a suitable analog, imagine doing a leaping pushup high enough to extend and produce a downflap with fishing rods rotating ventrally in both hands. Much better to flap and run from the start for maximum ground speed and thrust.
If a heavily muscled 6’ tall kangaroo
cannot initially leap its own height, or more, from a standing start, it seems unikely that a larger pterosaur can do this in the manner of tiny vampire bats. Size matters.
Large birds flap with great effort to get their mass off the ground or water. That seems to be a good model for large pterosaurs as well.
Quetzalcoatlus running like a lizard prior to takeoff.

Figure 10. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

New paper: the origin of snakes (Hsiang et al. 2015)

A new paper (Hsiang et al. 2015) on the origin of snakes presents an analytical reconstruction of the ancestor of crown snakes.

Unfortunately
the authors lament a “dearth of adequate paleontological data on early stem snakes.” On the other hand, the large reptile tree recovered an abundance of paleo data on early stem snakes and their ancestors. Note that nowhere in the following abstract are Jucraseps and her sisters mentioned. Nowhere in the cladogram are they shown. Rather the authors followed the paradigm of origins out of Varanoidea. So once again taxon exclusion raises its ugly head (intended snake metaphor).

From the Hsiang et al abstract:
Background The highly derived morphology and astounding diversity of snakes has long inspired debate regarding the ecological and evolutionary origin of both the snake total-group (Pan-Serpentes) and crown snakes (Serpentes). Although speculation abounds on the ecology, behavior, and provenance of the earliest snakes, a rigorous, clade-wide analysis of snake origins has yet to be attempted, in part due to a dearth of adequate paleontological data on early stem snakes. Here, we present the first comprehensive analytical reconstruction of the ancestor of crown snakes and the ancestor of the snake total-group, as inferred using multiple methods of ancestral state reconstruction. We use a combined-data approach that includes new information from the fossil record on extinct crown snakes, new data on the anatomy of the stem snakes Najash rionegrina, Dinilysia patagonica, and Coniophis precedens, and a deeper understanding of the distribution of phenotypic apomorphies among the major clades of fossil and Recent snakes. Additionally, we infer time-calibrated phylogenies using bothnew ‘tip-dating’ and traditional node-based approaches, providing new insights on temporal patterns in the early evolutionary history of snakes.

Results Comprehensive ancestral state reconstructions reveal that both the ancestor of crown snakes and the ancestor of total-group snakes were nocturnal, widely foraging, non-constricting stealth hunters. They likely consumed soft-bodied vertebrate and invertebrate prey that was subequal to head size, and occupied terrestrial settings in warm, well-watered, and well-vegetated environments. The snake total-group – approximated by the Coniophis node – is inferred to have originated on land during the middle Early Cretaceous (~128.5 Ma), with the crown-group following about 20 million years later, during the Albian stage. Our inferred divergence dates provide strong evidence for a major radiation of henophidian snake diversity in the wake of the Cretaceous-Paleogene (K-Pg) mass extinction, clarifying the pattern and timing of the extant snake radiation. Although the snake crown-group most likely arose on the supercontinent of Gondwana, our results suggest the possibility that the snake total-group originated on Laurasia.

Conclusions Our study provides new insights into when, where, and how  snakes originated, and presents the most complete picture of the early evolution of snakes to date. More broadly, we demonstrate the striking influence of including fossils and phenotypic data in combined analyses aimed at both phylogenetic topology inference and ancestral state reconstruction.

References
Hsiang AY, Field DJ, Webster TH, Behlke ADB, Davis MB, Racicot RA & Gauthier JA 2015. The origin of snakes: revealing the ecology, behavior, and evolutionary history of early snakes using genomics, phenomics, and the fossil record. BMC Evolutionary Biology May 2015, 15:87 DOI: 10.1186/s12862-015-0358-5
online

pdf

Field and Hsiang blog story

Purbicella, a basal scleroglossan from the Purbeck Limestone

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you're right! Here colorizing the bones helps identify sutures and paired elements. That's a right pterygoid covering much of the paired frontals. The teeth are blunt. 

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you’re right! Here colorizing the bones helps identify sutures and paired elements. That’s a right pterygoid covering much of the paired frontals. The teeth are blunt.

A few years ago
a rather complete lizard skull (BGS GSb581) was described (Evans et al. 2012) from the Purbeck Limestone (Late Jurassic to Early Cretaceous) of England. It was originally excavated more than a century ago and assigned to the genus, Paramacellodus. Evans et al. renamed it Purbicella. Their cladistic analysis nested Purbicella with Lacertoidea: (Lacertidae (including Acanthodactylus), Teiidae (including Tupinambus), Gymnophthalmidae (including Gymnophthalmus), and the burrowing Amphisbaenia (including Amphisbaena)), not Paramacellodus, which nested with skinks. Evans et al. based their nesting on a partial data matrix of Conrad (2008).

The large reptile tree nested Purbicella between Acanthodactylus and Liushusaurus. The large reptile tree recovered the above listed ‘lacertoid’ taxa as members of a paraphyletic clade, some preceding Purbicella in various clades and others succeeding it.

While Purbicella is Late Jurassic/Early Cretaceous, it must have had its origins much earlier, in the Late Carboniferous, because a descendant taxon, the TA1045 specimen, is Early Permian.

References
Conrad JL 2008. Phylogeny and systematics of Squamata (Reptilia) based on  morphology. Bulletin of the American Museum of Natural History 310:1–182.
Evans SE, Jones MEH and Matsumto R 2012. A new lizard skull from the Purbeck Limestone Group (Lower Cretaceous) of England. Bull. Soc. géol. France, 2012, t. 183(6):517-524.

 

Alveusdectes: a small, late-surviving diadectomorph – with procolophonid cheeks

Earlier we looked at the overlooked similarities of Diadectes and Procolophon (Fig. 1).

In the large reptile tree Procolophon nests with Diadectes, and both share a large otic notch, a trait Wiki says makes Diadectes an amphibian.

Figure 1. In the large reptile tree Procolophon nests with Diadectes, and both share a large otic notch, a trait Wiki says makes Diadectes an amphibian.

In the large reptile tree these two clades (procolophonids and diadectomorphs) nest together. No one has ever seen that before or since.

A new discovery
(Liu and Bever 2015) links these two clades closer together. Unfortunately, Liu and Bever used outdated cladograms. Taxon exclusion was the source of their errors. From their abstract: “Diadectomorpha is a clade of Late Palaeozoic vertebrates widely recognized as the sister group of crown-group Amniota* and the first tetrapod lineage to evolve high-fibre herbivory**. Despite their evolutionary importance, diadectomorphs are restricted stratigraphically and geographically, with all records being from the Upper Carboniferous and Lower Permian of North America and Germany. We describe a new diadectomorph, Alveusdectes fenestralis, based on a partial skull from the Upper Permian of China. The new species exhibits the derived mechanism for herbivory and is recovered phylogenetically as a deeply nested diadectid. Approximately 16 Myr younger than any other diadectomorph, Alveusdectes is the product of at least a 46 Myr ghost lineage. How much of this time was probably spent in Russia and/or central Asia will remain unclear until a specimen is described that subdivides this cryptic history, but the lineage assuredly crossed this region before entering the relatively isolated continent of North China. The discovery of Alveusdectes raises important questions regarding diadectomorph extinction dynamics including what, if any, ecological factors limited the diversity of this group in eastern Pangea. It also suggests that increased sampling in Asia will likely significantly affect our views of clade and faunal insularity leading up to the Permo-Triassic extinction.”

* This is an error.
Diadectomorpha are derived from Milleretta and kin in the large reptile tree.

** Another error.
Basalmost lepidosauromorphs were all herbivores.

In dorsal view
the skull of Alvuedectes has a strongly triangular appearance, similar to that of procolophonids. Most of the skull must be restored because it is missing. And it can be restored in at least two ways (Fig. 2).

Figure 2. Alvuesdectes (from Liu and Bever 2015) restored and compared to Diadectes and Procolophon. Note the triangular shape of the skull in dorsal view.

Figure 2. Alvuesdectes (from Liu and Bever 2015) restored and compared to Diadectes and Procolophon. Note the triangular shape of the skull in dorsal view. Click to enlarge.

Liu and Bever did not compare
their find to any procolophonids, only diadectomorphs. This is why the large reptile tree was created, to provide an umbrella study to provide taxa for more focused studies. It is unfortunate that Liu and Bever did not reference this study, which has been online for over four years. Procolophonids continued to the Late Triassic, which makes procolophonids the “last diadectomorphs.”

References
Liu J and Bever GS 2015. The last diadectomorph sheds light on Late Palaeozoic tetrapod biogeography. Biol. Lett.11: 20150100.

 

Ophiacodon and the origin of mammals: bone studies are supportive

A recent paper
by Shelton and Sander 2015 provides confirmation to the heretical hypothesis that Ophiacodon is a Therapsid/Mammal precursor, discussed here several years ago.

Figure 1. Varanosaurus, Ophiacodon, Cutleria, Biarmosuchus and Nikkasaurus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology.

Figure 1. Varanosaurus, Ophiacodon, Cutleria, Biarmosuchus and Nikkasaurus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology.

From the abstract: “The origin of mammalian endothermy has long been held to reside within the early therapsid groups. However, shared histological characteristics have been observed in the bone matrix and vascularity between Ophiacodontidae and the later therapsids (Synapsida). Historically, this coincidence has been explained as simply a reflection of the presumed aquatic lifestyle of Ophiacodon or even a sign of immaturity. Here we show, by histologically sampling an ontogenetic series of Ophiacodon humeri, as well as additional material, the existence of true fibrolamellar bone in the postcranial bones of a member of ‘Pelycosauria’. Our findings have reaffirmed what previous studies first described as fast growing tissue, and by proxy, have disproven that the highly vascularized cortex is simply a reflection of young age. This tissue demonstrates the classic histological characteristics of true fibrolamellar bone (FLB). The cortex consists of primary osteons in a woven bone matrix and remains highly vascularized throughout ontogeny providing evidence to fast skeletal growth. Overall, the FLB tissue we have described in Ophiacodon is more derived or “mammal-like” in terms of the osteonal development, bone matrix, and skeletal growth then what has been described thus far for any other pelycosaur taxa. Ophiacodon bone histology does not show well-developed Haversian tissue. With regards to the histological record, our results remain inconclusive as to the preferred ecology of Ophiacodon, but support the growing evidence for an aquatic lifestyle. Our findings have set the evolutionary origins of modern mammalian endothermy and high skeletal growth rates back approximately 20 M.Y. to the Early Permian, and by phylogenetic extension perhaps the Late Carboniferous.”

References
Shelton C and Sander PM 2015. Ophiacodon long bone histology: the earliest occurrence of FLB in the mammalian stem lineage. PeerJ PrePrints 3:e1262
doi: https://dx.doi.org/10.7287/peerj.preprints.1027v1 preprints