Origin of tetrapod herbivory, effects on local plant diversity

Brocklehurst, Kammerer and Benson 2020
discuss the origin of tetrapod herbivory and its effects on local plant diversity. Plant diversity will not be covered here, but we can discuss basal tetrapod herbivory.

According to the authors,
“Time-series regression analysis supports a negative relationship of plant richness with herbivore richness but a positive relationship of plant richness with minimum herbivore body size. This is consistent with studies of present-day ecosystems… Thus far, there has been little discussion on how the origin and early evolution of herbivory, either in arthropods or tetrapods, affected pat- terns of community richness in plants.”

Brocklehurst, Kammerer and Benson 2020 report,
“The earliest tetrapod herbivores appear in the fossil record in the Pennsylvanian, although the precise time of origin for this behaviour is uncertain due to the difficulty of assessing diet in extinct organisms. Potentially herbivorous taxa appear in the Bashkirian (earliest Pennsylvanian)-aged Joggins Formation: pantylid microsaurs with robust palatal dentition, a possible adaptation for grinding plant material or crushing the thick exoskeletons of arthropods (or both). More probable herbivores are known from the Kasimovian stage later in the Pennsylvanian: specimens of the diadectid Desmatodon from the Conemaugh Group of Pennsylvania.”

The authors make no distinction
between reptiles and pre-reptiles (= amniotes and anamniotes) in their paper. And that’s fine. Neither do they put their herbivores into a phylogenetic context. And that’s fine. They don’t take a stand whether diadectids were reptiles or not. They are just concerned about herbivory… and that’s fine.

Reptile herbivory
is a subject touched on in 2012, following a placodont herbivory paper (Diedrich 2011). That old cladogram needs to be revisited again based on the large number of added taxa since then. Even so, many of the hypotheses advanced eight years ago still hold up today.

From 2012:

“The separation between plant-eaters and insect-eaters formed the basal split in the large reptile tree. The emergence of Limnoscelis and its kin and Lanthanosuchus and its kin from this clade bear further scrutiny. I’m sure there’s a story brewing there. Turtles also emerged from this clade of herbivores.

“Among charted lepidosauromorphs we don’t see any other herbivores until Iguana and even it supplements with insects when young. Let me know if I’m missing any others.

“On the archosauromorph side, Edaphosaurus is the first herbivore with several therapsid clades not far behind. Thereafter we don’t see any until the Placodontia (if they were indeed herbivores and not shell crushers), the Aetosaurs and the Phytodinosauria.”

Getting back to the present day:
In the large reptile tree (LRT, 1698+ taxa) herbivory seems to appear on one branch only at the basal dichotomy of reptiles immediately following Silvanerpeton. Many, and potentially all of the basal taxa in the new Lepidosauromorpha are herbivores. Most are universally accepted herbivores: captorhinids, diadectids + bolosaurids + procolophonids, pareiasaurids and caseasaurids. The taxa surrounding and interweaving with these obvious herbivore clades could also be herbivores based on phylogenetic bracketing. These include: Milleretta, Concordia, Cephalerpeton (Fig. 1) and even Limnoscelis.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH 77469, here in ghosted lines. Colors represent missing bones.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones.

No taxa in the basal Archosauromorpha
are obvious herbivores until the sailback synapsid, Edaphosaurus.and again in the therapsid clade Anomodontia. Insectivory seems to have evolved into carnivory most of the time.

Among microsaurs
Pantylus and Stegotretus seem to stand alone as likely herbivores among anamniotes (basal tetrapods) confirming Brocklehurst, Kammerer and Benson 2020.

According to Brocklehurst, Kammerer and Benon
(their figure 1) throughout most of the Carboniferous plants maintained their greatest diversity. That changed during the latest Carboniferous when plants experienced a large diversity reduction coeval with the arrival of tetrapod herbivores. The authors conclude, “We find that the early Permian diversification of herbivorous tetrapods constrained the α-diversity of plants for the rest of the Palaeozoic.”

A chronological tree of tetrapods
can be found here. As you’ll see, only a few, rare taxa are known before the Late Carboniferous. Thereafter, and especially in the Early Permian, fossils from a variety of clades are known, including herbivores. The question is: Did tetrapods suddenly flourish? Or did the world change and tetrapods suddenly find themselves in areas that tended to fossilize better?

In other words,
would figure 1 of Brocklehurst, Kammerer and Benson look about the same if just insectivores and carnivores were included? Perhaps their conclusions would have been stronger if insectivores and carnivores were also included, just to be sure it was indeed the herbivores responsible for a reduction in plant diversity and not the planet at the time.

Brocklehurst N, Kammerer CF and Benson RJ 2020 The origin of tetrapod herbivory: effects on local plant diversity. Proceedings of the Royal Society B 287: 20200124. http://dx.doi.org/10.1098/rspb.2020.0124
Diedrich CG 2011. Fossil middle triassic “sea cows” – placodont reptiles as macroalgae feeders along the north-western tethys coastline with pangaea and in the germanic basin. Natural Science 3 (1) 9-27 (2011)


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