Does `dtruss -p PID` list system calls made before dtruss was run?
No

I've just run an experiment to test this. I wrote and ran this Ruby script:

puts "going to read foo"
puts File.read('/tmp/foo.txt')

sleep 5
puts "going to read bar"
puts File.read('/tmp/bar.txt')

sleep 5
puts "going to read baz"
puts File.read('/tmp/baz.txt')


After it started, I ran diagnose on its PID, as described above. I only saw one call to open:

20309/0x4a7118:     34082      41     34 open("/tmp/baz.txt\0", 0x1000000, 0x1B6)        = 7 0